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Zinc in Rice-Wheat Irrigated Ecosystem

Mitali Mandal¹ and Dilip Kumar Das^2*

¹Department of Soil Science and Agricultural Chemistry, College of Agriculture, Orissa University of Agriculture and technology, India

²Department of Agricultural Chemistry and Soil Science, Faculty of Agriculture, Krishi Viswavidyalaya, India

Corresponding Author:

Dilip Kumar Das
Department of Agricultural Chemistry and Soil Science
Faculty of Agriculture, Krishi Viswavidyalaya, India
E-mail:dkdas1231@rediffmail.com

Received September 11, 2013; Accepted October 14, 2013; Published October 18, 2013

Citation: Mandal M, Das DK (2013) Zinc in Rice-Wheat Irrigated Ecosystem. J Rice Res 1:111. doi: 10.4172/2375-4338.1000111

Copyright: © 2013 Mandal M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Zinc has received a great deal of importance in crop production especially in a rice-wheat cropping system during the last few years, because of the report of widespread occurrence of its deficiency from different parts of the country limiting crop production. Significant response of rice-wheat to Zn fertilization has also been reported by different investigators from almost all the parts of India. The main reasons for the occurrence of Zn deficiency is the adoption of intensive cropping programme with high yielding varieties and modern agro techniques like use of high analysis chemical fertilizers, use of pesticides with the simultaneous very limited use of on-farm inputs like organic manures, crop residues etc.

Zinc is one of the important essential micronutrients for plants. The problem seems to be more acute for ricewheat as around half of the total rice-wheat area found to be severely affected by Zn deficiency. Zinc plays an important role in different plant metabolism processes like development of cell wall, respiration, photosynthesis, enzyme activity, auxin and protein synthesis, and other bio-chemical functions etc. amongst all the micronutrients Zn deficiency continues to be one of the key factors in determining the crop production in India and other countries of the World. Various factors associated with Zn deficiency are acid sandy soils low in total zinc, neutral or alkaline soils having higher amount of fine clay, silt and available phosphorus, organic soils etc.

Intensive cultivation and growing of exhaustive crops have made the soil deficient in macro as well as in
micronutrients. Now a day the use of only nitrogenous and phosphatic fertilizers also create nutrient imbalances particularly of Zn in soils. Although the requirement of micronutrients is very low, its effect on crop yield is, however, very significant.

The favourable rice-wheat environment is found mainly in the western part of the rice-wheat belt, where wheat is the dominant crop in Pakistan, the northwestern Indian states of Punjab and Haryana, and western Uttar Pradesh. The less favourable rice-wheat environment occurs in the eastern part of the Indo-Gangetic belt, where rice is the dominant crop in Bangladesh, West Bengal, the northern parts of Bihar and Uttar Pradesh and the Terai region of Nepal.

Geographically, the favourable rice-wheat environments in the Indo-Gangetic plains are located in the western part, where winter environmental conditions are suited for wheat, irrigation infra structure is good, marketing facilities are available, and both rice and wheat yields are high. The less favourable rice-wheat environments occur in the eastern part and are associated with partially irrigated and rain fed systems, and a shorter growing period for wheat. More detailed agroclimatic analysis would provide an in-depth understanding of the environmental constraints (e.g. Draught stress and flood proneness) for geographical targeting of varietal and crop management strategies in both the favourable and less favourable rice-wheat environments.

Keywords

Soil; Zinc in Rice; Zinc; Ecosystem

Evaluation of Micronutrient for Crop Production

Soil is the key source of food production and its resource plays a major role in determining crop productivity of an agro-ecosystem [1]. Many misconceptions exist regarding the role of soil in providing the different micronutrients especially Zn because of it wide spread deficiency in soils required for plants (Table 1) The usual practice is to rest the soil for restoring fertility and it is a viable technique for nitrogen only but not for the micronutrients as nitrogen finds addition through rainfall or N fixation by soil microorganisms. The magnitude of micronutrient requirement especially Zn for few cropping systems reasonably represent the amount of Zn removed at relatively high yield levels. Takkar [2] reported an average removal of Zn by major cropping systems (Table 2).

Table 1: Available Zn (mg/kg) in different agro-ecological sub-regions.

Table 2: Amount of Zinc removed by major intensified production system in India.

The responses of different crops to the application of Zn in India, is <200-7500; 200-500 and 200-500 kg/ha for cereals, pulses and oil seeds respectively. However, increased removal of Zn as a consequence of adoption of HYV and intensive cropping together with a shift towards high analysis NPK fertilizers caused decline in the level of Zn as discussed earlier, below that required for normal productivity of crops. Analysis of large number of soil samples throughout India for plant available Zn indicated that zinc deficiency (>45%) was most serious. Constraint to sustainable productivity in India and West Bengal in particular where more than 50% of the total cultivable areas are deficient in Zn [3]. Singh [4] studied the extent of multi micronutrient deficient areas in India and reported that deficiency of zinc (43%) was more prevalent than that of two nutrients namely Zn+Cu (2.7%) suggesting widespread deficiency of single micronutrient Zn. Therefore, the application of multi micronutrient mixtures should be avoided as their use would be uneconomical and also may lead to degradation of soil environment and hence sustainability.

Zinc in Soil

Crops feed differentially on various fractions and remove variable quantity of these not only from the surface soil layers but also from the sub-surface layers. It is imperative to have information about these changes in both soil layers in order to understand the causes for Zn deficiency under some cropping systems. Kanwar [5] reported that the critical limit of Zn in soil is low (<1.0 ppm); medium (1.0-2.5 ppm) and high (>2.5 ppm).

The critical limit of Zn in rice is given in Table 3.

Forms of zinc

Zinc occurs in soils in different chemical pools, which differ in their solubility and availability to plants [6]. Zinc is found to occur in soils as (i) Water soluble plus exchangeable, (ii) organically bound, (iii) amorphous iron oxide bound, (iv) crystalline iron oxide bound, (v) manganese oxide bound, (vi) carbonate and sulphide bound and (vii) residual forms [7]. Identification of these various forms of Zn has been found difficult because of the small amounts of Zn involved. Besides, the specific minerals controlling the solubility of Zn²⁺ soils are also not known. It has been pointed out that most of the Zn²⁺ in different soils is located in ferromagnesian minerals such as biotite and hornblende.

Table 3: Critical limits of Zn in rice.

Soil solution Zn: The oxidation state of Zn in soil is exclusively Zn2+. Several Zn hydrolysis species exist in solution with Zn²⁺ predominating at soil reactions below pH 7.7. Above this pH, Zn OH+ becomes the most abundant species.

Organically bound Zn: It is evident that organic matter in soil occurs in various forms including water soluble and solid forms with various intensities of decomposition. Zinc is attached with these organic materials resulting Zn less available to the plants by insoluble chelation reaction, causing resistance to exchange etc. Das (1996) reported that the higher recovery of organic matter bound Zn may be due to higher rate conversion of amorphous sesquioxide bound Zn and subsequent low recovery of organic fractions of Zn at the later period of submergence may also be due to greater microbial immobilization as well as formation of insoluble complexes with soil organic matter. This fraction of Zn can be extracted from soils with the help of an extractant 0.7 (M) NaOCl (pH 8.5).

Amorphous and crystalline iron oxide and manganese oxide bound Zn: oxides of Fe and Mn particularly of amorphous iron oxide have a significant effect on micronutrient particularly zinc reactions in soils because all these oxides commonly occur as coatings or concretions and as discrete particles of colloidal dimensions and their strong affinity for metal ions including zinc. Another mechanism which operates at lower pH and resulting in non-specific adsorption of Zn2+ is shown below:

Residual Zn: The residue zinc is the major fraction of soil Zn. Prasad et al. [8] reported that the amount of Zn recovered in water soluble plus exchangeable, organically bound, oxide bound Zn recorded a progressive decrease with an increase in residual forms upon submergence. About 50 percent of the added Zn was transformed into the residual form.

Transformation of zinc

Zinc is relatively immobile in most soils. Zinc undergoes transformation in soils by various mechanisms like sorption by clays, hydrous oxides, organic matter etc. which affect the availability of Zn in soils and hence growth and nutrition of plants. Shuman [9] reported that removal of either organic matter or Mn-oxides decreased Zn sorption, but that Fe-oxide removal increased sorption.

Organic matter also plays an important role in controlling the availability of Zn in soils and hence its uptake by the plants. It is evident that Zn forms stable complexes with soil organic matter. The humic and fulvic acid fractions of organic matter are prominent in Zn adsorption. The following reaction mechanism between organic matter and Zn has been identified:

Mechanism I: Immobilization by high molecular weight organic substances such as lignin.

Mechanism II: Solubilization and mobilization by short chain organic acids and bases.

Mechanism III: Complexation by initially soluble organic substances, which then form insoluble salts.

Das [6] reported that the availability of Zn decreases due to submergence, which may be attributed to the following reasons:

(i) Formation of insoluble franklinite (ZnFe₂O₄) compound in submerged soil.

(ii) Formation of very insoluble compounds of Zn as sphalerite (ZnS) under intense reducing conditions.

(iii) Formation of insoluble compounds of Zn as ZnCO₃ at the later period of soil submergence due to high partial pressure of CO₂ arising from the decomposition of organic matter

(iv) Formation of Zn(OH)₂ at a relatively higher pH which decreases the availability of Zn.

(v) Adsorption of soluble Zn2+ by oxides, hydrous oxides, organic matter, carbonates, sulphates, clay minerals etc. decreases the availability of Zn.

(vi) Formation of various other insoluble zinc compounds which decreases the availability of Zn in submerged soils. e.g. high phosphatic fertilizer induces the decreased availability of Zn.

Factors affecting zinc availability

Various factors affect the Zn availability of which some of them are discussed here.

Soil reaction (pH): soil reaction may modify the uptake of zinc by influencing the activities of soil micro-organisms and changing the ability of the plant to absorb or transport to the tops, the stability of soluble and insoluble organic complexes of Zn, the solubility of antagonistic ions, any rhizosphere effects etc. Zinc deficiencies are generally found in soils having pH more than 6.0. The solubility of native as well as applied Zn in soils is highly pH dependent and its solubility decreases by a factor of 102 for each unit increase in soil pH. The activity of Zn –pH relationship has been described as follows:

Organic matter: The presence of organic matter in the soil very oftenly promotes the availability of zinc presumably by complexing the substances that fix Zn. The contribution of organic matter to micronutrient particularly Zn binding is highest when the predominant clay minerals is kaolinite and lowest when it is montmorillonite. Das and Mandal [10] suggested that the amount of DTPA extractable Zn has been found to be increased more in the treatment receiving organic matter (well rotten FYM) 14 and 28 days before puddling or nonpuddling the soil.

Lime: It has been reported by Nair and Mehta [11] that the availability of Zn has been found to be decreased with an increase in the amount of lime to the soil. Therefore, Zn deficiencies are very common in calcareous soils. Liming of acid soils, especially in soils of low Zn will reduce uptake of Zn2+. This depressive action is usually attributed to the effect that increasing pH has on lowering Zn2+solubility.

Partial pressure of CO₂: When alkaline and calcareous soils are submerged, the solubility as well as availability of Zn will be increased resulting from the lowering of soil pH due to release of sufficient amount of CO₂ during decomposition of organic matter.

Redox potential (Eh): Zinc does not change its oxidation state even at low redox potential, but soil submergence resulted in a decrease in Zn concentration in the soil solution.

Nutrient interactions: Synergistic and antagonistic.

Transformation of zinc under submerged condition

Zinc deficiency is very common in lowland rice particularly where fertilizer responsive high yielding varieties of the crop are grown. To alleviate such deficiency, Zn is applied to the soil in the form of ZnSO₂. 7H₂O. The recovery of applied Zn by rice is, however, very low, which may be due to its transformation to different chemical forms. Application of bulky organic manure is a common practice followed by the rice farmers of South-East Asian countries. Decomposition of such manure under anaerobic environment in the lowland rice field is likely to influence the transformation of the applied Zn to different forms in soil and consequently the Zn nutrition of rice.

Laboratory and green house experiment were conducted by Mandal et al. [12] with two soils viz. laterite and alluvial collected from the surface layer (0-15 cm) of typical lowland rice fields situated in the district of Nadia and Midnapore in West Bengal, a major rice growing state in India. The collected laterite and alluvial soils were having pH, 6.08 and 6.80; organic carbon, 0.55 and 0.73%; CEC, 5.84 and 19.15 me/100 g; clay, 16.0 and 31.0; free Fe₂O₃, 1.43 and 1.00 and total Zn, 110.0 and 140.0 mg/kg, respectively. Two levels of organic matter were added (0 and 0.5% of the soil weight). The soils were kept under submergence for 20 days; thereafter they were treated with three levels of Zn (0, 5 and 10 ppm) as ZnSO₄.7H₂O and basal application of N, P and K at 100, 60 and 60 kg/ha in the form of urea, SSP and MOP respectively.

Two 25-day-old rice seedlings (var. IR-24) were allowed to grow in each pot. Five gram portion of the prepared soil was placed in a 100 ml polyethylene centrifuge tube successively extracted with 20 ml of neutral normal OAC and 0.05 (M) Cu(OAC)₂ solution to obtain water soluble plus exchangeable and organic complexed forms of Zn respectively. The soil residue after Cu(OAC)₂ extraction was suspended in 20 ml of 0.2 (M) (NH₄)₂C₂O₄ (pH 3.0) solution and dithionite citrate system buffered with NaHC₃ to get Zn bound to amorphous and crystalline sesquioxides respectively [13].

Effect of incubation period on zinc concentration

A laboratory experiment was conducted by Phogat et al. [14] with two surface soils (0-0.2 m) of typical wetland rice field situated in Kaul (S₂) and Mojukhera (S₂), a major rice growing area of Haryana state. Some properties of these soils (S₁=Aquatic Ustochrept; S₂=Typic Natrustalf) are: pH, 8.25 and 9.15; EC, 0.50 and 0.41 dS/m; organic C, 0.67 and 0.32%; CEC, 18.2 and 11.9 c mol (p+) kg-1; clay, 32.0 and 14.1%; DTPA extractable Zn, 0.48 and 0.36 mg/kg, respectively. With time of incubation, the recovery of Zn in WE, OM and CFeOx decreased with a concomitant increase in MnOx and A FeOx fractions under all the treatments. Generally, the magnitude of decrease or increase in these fractions may be attributed to higher CEC, organic carbon, free Fe and Mn contents of the soil.

The effect of different Zn fractions on Zn uptake by rice was examined by simple correlations and multivariate regression analysis. All Zn fractions except the residual mineral Zn showed significant positive correlation with each other and with Zn uptake by rice. The uptake can be predicted by employing pH (X₁), exchangeable-Zn (X₂), complexed-Zn (X₃), AMS-Zn (X₄), CRS-Zn (X₅) and the RM-Zn (X₆) as independent variables in the multiple regression equation. Zn uptake=82.71+23.64. X₁+88.14. X₂+1.77. X₃+7.71. X₄ – 10.18 X₅ – 0.56 X₆ (R²=.60).

Interaction of zinc

A green house experiment was conducted by Chatterjee et al. [15] in the University farm at Kalyani, West Bengal. The soil had a pH, 7.9; organic C, 5g/kg; CEC, 34.5 c mol (p⁺) kg^-1; clay, 43.7%; NH₄OAC extractable K, 23.0 mg/kg and DTPA extractable Zn, 1.8 mg/kg. Two levels of nitrogen 0 (N₀) and 40 (N₄₀) mg/kg, three levels of potassium i.e. 0 (K₀), 20 (K₂₀) and 40 (K₄₀) mg/kg and three levels of Zn i.e. 0 (Zn₀), 5 (Zn₅) and 10 (Zn₁₀) mg/kg in the form of (NH₄)₂SO₄, KCl and ZnSO₄, respectively were applied. Zinc was extracted by 0.005 (M) DTPA solution following the method of Lindsay and Norvell [16] and analysed by Atomic Absorption Spectrophotometer.

The results (Table 4) show that application of N either alone or in combination with K or Zn or both significantly increased the dry matter yield of rice plants. The increase was highest when it was applied along with K and Zn at their highest levels (K₄₀Zn₁₀). Similarly, application of K alone or in combination with N significantly increased the dry matter yield, but it failed to do so in combination with Zn when N was not applied. Application of Zn alone caused a decrease in the dry matter yield. This may be attributed to the imbalance in nutrients because of insufficient N and K in the growth medium. On the contrary, the application of Zn when combined with N and K significantly increased the dry matter yield, the magnitude of increase with 10g/kg level being 22% over control, no application of Zn. The uptake of Zn by rice plants followed a similar pattern to that of dry matter yield. Application of N, K and Zn caused a significant increase in the uptake of Zn. This was associated with increased dry matter yield with N and K and increased Zn concentration with Zn and K application.

Table 4: Effect of nitrogen, potassium and zinc application on dry matter yield (g/pot) and Zn uptake mg/pot) of rice.

Synergistic Zn vs N [17]

                 Zn vs Fe [18]

                 Zn vs Mo [19]

                Zn vs B [20]

                Zn vs S [21]

Antagonistic Zn vs P [22]

                   Zn vs Ca [23]

                  Zn vs S [24]

                  Zn vs Mn [25]

                 Zn vs Cd [26-28]

Effect on earthworm

The increased metal concentration in soil may disturb soil ecosystem by affecting the population and activity of soil organisms. Four species of earthworms are generally seen in rice field soil (Drawida willsi Michaelsen, Ocnerodrilus occidentalis Eisen, Lamptio mauritii Kinberg and Glyphidrillus tuberosus Spephenson). Among these D. willsi constitutes 70% of the total biomass and is widely distributed in tropical agro ecosystem of India [29].

A field experiment was undertaken by Panda and Sahu [30] in irrigated rice near Katapali, Orissa. Four study plots (plot I, II, III, IV) were taken each of 10m x 10m in size. The experiment was carried out from February to May, 1994. The average rainfall (mm), air temperature (°C), relative humidity (%), soil temperature (°C) and soil moisture (g%) during the study period were 11.8, 25.1, 63.7, 24.4 and 16.9, respectively. The soil was a sandy loam having pH, 6.8; organic matter, 25g/kg; available N, 2.2g/kg; C:N ratio, 11:4; and Zn content, 30.00 mg/kg. Considering the volume of soil in upper 20 cm, ZnCl₂ was applied at 0.20, 0.80 and 1.60 kg per 10 m² to bring the soil Zn level up to 50, 200 and 400 mg/kg in plots II, III and IV, respectively. The highest concentration was chosen to mimic the level mostly found in contaminated agricultural soils of India [31].

The plot I served as control and no application of ZnCl₂ was made. Kalinga –90 variety of rice with crop duration of about 100-120 days was cropped in plots I-IV. Handsorting and wet sieving methods were used for the extraction of worms and cocoons. Three different age and size classes of D. willsi were distinguished for analysis for population dynamics and reproductive biology. These were juvenile (non-clitellate,<cm); immature (non-clitellate,>2<4 cm) and adults (>4 cm) [32]. Rate of reproduction was assessed on the basis of number of cocoons produced per adult worm at each sampling occasion [33]. Earthworm mortality was determined on the basis of consecutive changes in numbers utilizing the formula: E=N_t1 – (N_t2–V_t2), where E=death rate, N_t=density in fortnight, V_t=number of newly recruited individuals.

The population dynamics and rate of reproduction of D. willsi in different concentrations of zinc applied rice plots (plots I, II, III and IV) are shown in Table 5. The total population density (numbers per m2) ranged from 80 to 337 in 0 mg/kg (plot I), 77-310 in 50 mg/kg (plot II), 53-186 in 200 mg/kg (plot III) and 37-110 in 400 mg/kg (plot IV) Zn applied plots during February to April, 1994. Average worm population during the same period was 207, 179, 104 and 66 in plots I, II, III and IV, indicating about 14, 50 and 68 percent decrease in worm population in Zn applied plots (plots II, III and IV, respectively) compared to control plot (plot I).

Table 5: Population dynamics (numbers per m2 per fortnight) and rate of reproduction of D. willsi earthworm in Zn applied rice field.

The population mortality was 18, 20, 34 and 43 percent in plots I, II, III and IV, respectively. Zinc is required in trace amounts for normal physiological functions of living organisms including earthworms [34]. But when present at elevated level in soils, zinc may cause harmful effect on earthworms [35]. These results indicated significant increase in population mortality and decline in reproduction when zinc applied in soil exceeded a level of 200 mg/kg. Such effects of zinc on earthworms have been reported by Van Gestel et al. [35]. The drop in reproduction and increase in population mortality in D. willsi at elevated concentrations of Zn (>200 mg/kg soil) was reflected sharp decline in total population (50 and 68% decrease in 200 and 400 mg/kg zinc applied plots, respectively). This may not only hamper the positive contribution of earthworms towards soil structure and fertility, but also lead to eradication of earthworm population in highly polluted agricultural lands.

Zinc in plant

Species and varieties of plants differ in their susceptibility to Zn deficiency. Cultivars differ in their ability to take up Zn, which may be caused by differences in Zn translocation and utilization, differential accumulations of nutrients that interact with Zn, and differences in plant roots to exploit for soil Zn.

Zinc deficiency: Lowland rice is more vulnerable to Zn deficiency. Therefore, growing rice on permanently wet soils is frequently an early causality of Zn deficiency. Symptoms of Zn deficiency appear on the lower leaves while have a chlorotic mid rib, particularly towards the base. The leaves develop characteristic brown rusty spots, which coalesce and form continuous brown areas. In the case of acute deficiency, the whole leaf turns brown and dries and plants may succumb. An uneven stand of rice and stunted plants with brown rusty appearance are indicative of Zn deficiency.

“Khaira” disease is another name for Zn deficiency in rice. The “Khaira” disease first described from terai soils of the Nainital district in U.P. was reproduced by the authors in rice variety, “IR-8” in refined sand culture at Lucknow. In this variety, the first symptoms of Zn deficiency appear in 3 to 4 wk old seedlings when the young leaves develop reddish-brown pigmentation. The pigmentation appears first in the middle of the leaves, then intensifies and spreads over the entire lamina. The affected tissue becomes papery and necrotic and under conditions of severe deficiency, the entire mass of leaves collapsed and further growth of the plant is arrested.

Zinc deficiency appears as fading of the middle of the lower half of the lamina on the older leaves of wheat. The first symptoms of Zn deficiency are, however, seen on the leaf next to the youngest. Following the commencement of the fading of the lamina, the affected leaves develop minute reddish brown spots which tend to coalesce forming reddish-brown lesions. These lesions later turn brown and limp. The leaves present a withered look. In severe Zn deficiency, the whole plant may be affected. Its maturity is delayed. Grain formation and ultimate yield are markedly reduced.

The wheat variety “Kalyansona” is very susceptible to Zn deficiency. Soon after the emergence of the seedlings, growth is depressed and characteristic Zn deficiency symptoms appear in three to four week old plants. Symptoms first appear in third leaf in base of the lamina. The lesions soon enlarge and coalesce and the affected tissue becomes discolored. The discolouration proceeds from the base to the apex of a leaf, leaving a narrow green band along the leaf margins. The white necrotic areas later become dry and papery. The leaf apices are seldom affected.

Zinc toxicity: Young leaves show mild chlorosis between the veins followed by the development of a reddish-brown coloration in the areas between the veins, which later become dry and papery. The coloration often starts from near the base of a leaf and spreads towards the leaf apex. The affected leaves show the rolling of the leaf margins. Roots turn brown and often necrotic. The range of toxicity varies widely. Cereals are generally resistant.

The Zn depletion in soil depends on the cropping sequence as well as on the fertility level. It appears that the maize based cropping sequence depletes the maximum micronutrients from soil, especially Zn. In order to maintain high crop productivity, periodic application of zinc fertilizer is necessary, based on the soil test data.

Zinc response to rice: The application of Zn to soil (0-48 kg/ha Zn) increased yields by 11.62 to 33.36% with the highest mean yield being given by 12 kg/ha Zn. Foliar applied Zn (0-4 kg/ha Zn) increased yields by 9.25 to 21.96 percent with the highest mean yields being given by 2-4 kg/ha. Das and Mandal [36] found that the application of zinc sulphates to the acid latosol of West Bengal at 20 kg/ha gave the highest average rice yield (4.12 t/ha) with the benefit cost ratio of 1.52. The significant positive correlation was also obtained between the yield of rice and grain Zn concentration (r=0.95**). However, the effect of ZnSO₄ application on the yield of rice and net economic return is shown in Table 6. Application of Zn-EDTA can increase more rice yield over ZnSO₄ (Figure 1).

Table 6: Effect of zinc sulphate on the yield (t/ha) and net economic return (Rs/ha) in acid latosol of West Bengal during the year 1984.

Naik and Das [37] conducted field experiments on rice cv. IET 4094 in an Aeric Endoaquept (pH7.2) during wet seasons of 2005-06 and 2006-07 to study the relative performance of chelated Zn (Zn- EDTA) and ZnSO₄ of on the growth and yield of rice. The results show that the DTPA extractable Zn concentration in soil and total Zn content in dry matter of rice increased initially upto 28 days of crop growth when Zn was applied as a single basal source, being greater with chelated Zn compared with ZnSO₄ application. The highest mean Zn uptake by rice grain and straw was found to be 209.2 and 133.8 g ha^-1 with the simultaneous highest increase in the yield of grain and straw was 5.5 and 7.3 t ha^-1 in the treatment 1 kg Zn ha^-1 as Zn-EDTA at basal was applied respectively. From the economic analysis, it was recorded that the highest cost-benefit ratio was 1.71 with the basal application of 0.5 kg Zn ha^-1 as Zn-EDTA. With regards to modes of application of ZnSO₄, split application of 10 and 20 kg Zn ha^-1 resulted in a higher cost-benefit ratio of 1.32 and 1.21 respectively, over the corresponding basal applications. However, basal application of 1 kg Zn ha^-1 as Zn-EDTA resulted in a higher cost-benefit ratio of 1.69 over its corresponding split application.

Plots of Bray’s percent yield against DTPA-extractable Zn in soil and also plant Zn concentration by Singh et al. [38] indicated 1.2 mg/ kg in soil and 35.95 mg/kg in rice plants as the critical limits, below which rice plants might respond to Zn application. Table 7 showed that the application of Zn in addition to recommended NPK at 2.5 ppm as ZnSO₄.7H₂O significantly increased the total dry matter yield of ADT 38 rice ranged from 13.14 (S₅) to 27.55 (S₉) g/pot. The yield in NPK treated control (Zn – 0 ppm) plot in all soils ranged from 11.51 to 25.86 g/pot.

Table 7: Effect of Zn on total dry matter yield (g/pot) [period of growth �7 weeks after transplanting, Variety-ADT 38 Rice] pot experiment.

A field experiment was conducted by Ram et al. [39] during rainy season (kharif) of 1992 and 1993 at Faizabad on a partially reclaimed sodic soil. The surface soil (0-15 cm) had pH 9.2, EC of saturation extract 7.8 ds/m, exchangeable Na-40%, organic carbon- 0.22%, CaCO₃ – 2.6%, Available Zn- 0.3 ppm and silty loam texture. There were total 9 treatments viz. T₁, control; T₂, 40 kg ZnSO₄/ha basal; T₃, 40 kg ZnSO₄/ ha top dressing; T₄, 1 spray of 0.5% ZnSO₄ solution; T₅, 2 sprays of 0.5% ZnSO₄ solution; T₆, 3 sprays of 0.5% ZnSO₄ solution; T₇, 20 kg ZnSO₄/ ha basal+1 spray of 0.5% ZnSO₄ solution; T₈, 20 kg ZnSO₄/ha basal+2 sprays of 0.5% ZnSO₄ solution; T₉, 20 kg ZnSO₄/ha basal+3 sprays of 0.5% ZnSO₄ solution.

Treatments T₂, T₅, T₆, T₈ and T₉ increased the plant height, panicle length and filled grains/panicle significantly compared with the control (Table 8). Maximum filled grains/panicle was observed with T₉, but it was at par with T₂, T₅, T₆ and T₈ treatments. Zinc application had no significant impact on 1000-grain weight. The ZnSO₄ application, irrespective of the method and rate, increased grain and straw yields significantly compared with the control. Foliar spray of ZnSO₄ twice at 20 days after transplanting at an interval of 10 days (T₅) produced grain and straw at par with those obtained from T₂, T₅, T₆, T₈ and T₉. Hence foliar application of ZnSO₄ twice is economically superior to soil application of ZnSO₄, as it leads to a net saving of 30 kg ZnSO₄/ha. The increase in yield may be attributed to the better supply of Zn, which plays specific role in various metabolic activities. It was concluded that foliar application of 0.5% aqueous solution of ZnSO₄ twice 20 days after transplanting rice at an interval of 10 days was superior to its soil application in sodic soil.

Table 8: Effect of rate and method of Zn application on yield, yield attributes, content, uptake and recovery of applied Zn by rice in a sodic soil of Uttar Pradesh.

A field experiment was laid out in two seasons by Tandon [40] at the experimental farm of Agriculture University, Kanpur, India. The soil had a pH of 7.1 and available 0.93 ppm Zn and 0.81 ppm Fe. There were 36 treatments comprising combinations of three levels each of ZnSO₄ and FeSO₄ (0, 20 and 40 kg/ha) and four rice varieties (IR-20, Jaya, Pusa 2-21 and IET-1444). A split plot design with varieties in the main plots and Fe x Zn treatments in the subplots was adopted. N, P and K as basal manures were supplied in the ratio of 120:60:60.

A perusal of the yield data (Table 9) revealed that on overall mean basis, Jaya and Pusa 2-21 gave significantly the highest yield, IET-1444 yielded significantly lower than Jaya while IR-20 gave a significantly lower yield than the above mentioned three varieties. Increasing doses of both iron and zinc individually increased grain yield per hectare quite significantly. Interaction studies revealed that IR-20, which was the lowest yielder among the four varieties, did not respond to zinc and in case of iron, application of 20 and 40 kg FeSO₄/ha gave almost significantly higher yield than no iron treatment.

Table 9: Effect of different treatments on grain yield at maturity (kg/ha).

Zinc response to wheat: Ten on-farm experiments on a Haplaquept (pH, 6.80-7.35; organic carbon, 0.52-0.71%) were conducted by Maiti et al. [41] in ten sites of Dhantala, in the district of Nadia, West Bengal during rabi season. Wheat crop (cv. UP-262) was grown on a Haplaquept having the following treatments. Zinc was applied in the form of Zn-EDTA (Chelamin) at 0.5 kg+suitable carrier per hectare. Each on-farm field experiment was divided into seven subplots receiving the following treatments namely T₁ (P+K+Zn), T₂ (N+K+Zn), T₃ (N+P+Zn), T₄ (N+P+K), T₅ (with organic matter only), T₆ (FFP, Farmer’s Fertilizer Practice) and T₇ (absolute control).

The result (Table 10) show that although the yield of straw and grain was recorded higher in the treatment T₄ where recommended levels of N, P and K fertilizer without Zn but showed non-significant with that of T₃ treatment when N, P and Zn was applied together. The result further pointed out that the application of FYM showed a higher yield of wheat as compared to the treatment (T₁) where P, K and Zn were applied together. Therefore, there is possibility of increasing yield of wheat with the use of FYM maintaining a proper soil health and sustainability of crop production. The uptake of Zn was recorded highest (66.49g/ha) in the T₂ where N and K along with Zn was applied which was followed by the treatment T₃ (63.59 g/ha) when N and P along with Zn was applied. Such highest uptake of Zn might be due to synergistic effects among N, K and Zn as against combined application of N and P along with Zn where antagonistic effect between Zn and P exhibited a lower uptake of Zn [42].

Table 10: Influence of different treatments on the yield and uptake of Zn by wheat (UP-262).

A field experiment was conducted by Shukla and Yadav [43] during 1993-94 and 1994-95 at Kumarganj, Faizabad. The soil was silty loam. The treatments were M₁, P and Zn at sowing or planting; M₂, P at sowing or planting and Zn at 20 days; M₃, Zn at sowing or planting and P at 20 days; M₄, P at first ploughing and Zn at sowing or planting; and M₅, P as basal and Zn spraying (0.5%) at 20 days; and 3 frequency of P and Zn application; R, P and Zn application to rice crop; W, P and Zn application to wheat crop; and R+W, P and Zn application to rice and wheat (both crops) in subplots with 3 replications. Rice variety ‘Sarju 52’ and wheat ‘Malviya 234’ were sown.

The highest grain yield of rice, panicles/m² and grains/panicle were recorded under M₄ treatment. Lowest value of panicle/m², grains/panicle and grain yield was recorded under M₃ treatment. The application of P and Zn to both rice and wheat crops (R+W) was significantly superior to the treatment in which P and Zn were applied to rice crop only (R). This (R+W) treatment also gave the highest values of yield components which were found significantly higher over the treatment in which P and Zn were applied to wheat crop only (Table 11). The yield of rice, uptake of N, P, K and Zn by rice was lower in the treatment where recommended levels of NPK were applied. The results suggested that there is a good response of applied Zn in increasing rice yield and subsequent uptake of N, P, K and Zn by rice. The highest mean yield of wheat (58.01 q/ha) was recorded in the treatment where recommended levels of N, P, K and Zn was applied which was closely followed by the treatment where N, K and Zn were applied [44].

Table 11: Effect of time, method and frequency of P and Zn application on yield attributes and grain yield of rice.

The lowest values of ears/m², grains/ear were recorded in M₃ where Zn was applied at planting or sowing and P was applied 20 days after planting or sowing. The highest yield was obtained under M₄, whereas lowest yield was recorded under M3. The lowest value of yield attributes and yield under M₃ was due to under utilization of P by wheat crop. The highest number of ears/m² and filled grains/ear were recorded with the application of P and Zn to both crops (R+W) in both the years (Table 12). The values under this treatment were significantly higher than P applied to rice crop (R) only in rice-wheat cropping system and were at par with P and Zn applied to wheat (W) crop only. P and Zn application in wheat had beneficial effect on growth of wheat as evident by higher number of tillers/plant.

Table 12: Effect of time, method and frequency of P and Zn application on yield attributes and grain yield of wheat.

The results of the green house experiment (Table 13) show that the concentration of Zn in shoots and roots increased with the increase in Zn application (0, 5 and 10 ppm Zn) while it decreased with the increase in P application. Similar inverse relationship between Zn concentration in plants and levels of P application in soil was also observed by Haldar and Mandal [45].

Table 13: Effect of P and Zn application on Zn concentration (ppm) in shoots and roots of rice.

The results (Table 14) show that although the dry matter yield of both shoot and root increased due to P application, the uptake of Zn by the shoot declined while that in roots increased which suggests that the decrease in Zn concentration in shoots is not possible due to a dilution effect. It may, therefore, be attributed partly to retardation of its translocation from root to shoot and partly to the decrease in its absorption by plants owing to its decreased availability in soil resulting from P application.

Table 14: Effect of P and Zn application on the uptake (�µg/pot) of Zn by shoots and roots of rice.

Influence of zinc on enzyme activity: Superoxide dismutase (SOD) and catalase are the important enzymes that have a protective role against the oxygen toxicity in plant cells and are expected to control the level of lipid peroxidation [46]. Superoxide radical (O₂^-) and hydrogen peroxide, the potent agents of oxygen toxicity, are highly destructive to certain functional groups present in biomolecules. Illuminated chloroplast produces O₂^- by light dependent univalent reduction of oxygen. Chloroplast contain SOD that catalyses the dismutation of O₂^- into H₂O₂ which in turn is removed by catalase. The possible control of SOD and catalase on lipid peroxidation during senescence and under salinity has been suggested [47,48].

Rice var. 4-14 was grown in a green house experiment by Seethambaram and Das [49] under natural photoperiod (day temperature, 28-35°C; night temperature, 20-25°C). Zinc at varying concentration (0, 0.025, 0.05 and 0.1 ppm) was given from 8^th day onwards after germination. The measurements were made at 15^th, 25^th and 35^th day of the growth. SOD (EC 1.15.1.1) was assayed by measuring its ability to inhibit the photochemical reduction of nitro blue tetrazolium (NBT). Log A₅₆₀ was plotted as a function of the volume of enzyme extract used in the reaction mixture [50].

From the resultant graph, the volume of enzyme extract corresponding to 50% inhibition of the reaction was read and was considered as one enzyme unit. Catalase (EC 1.11.1.6) was assayed spectrophotometrically following the decrease in absorbance at 230 nm. The reaction mixture contained 10 mM phosphate buffer (pH, 7.0), 2mM H₂O₂ and the enzyme extract [51]. Lipid peroxidation in the leaf tissue was measured in terms of malondialdehyde (MDA, lipid peroxidation) content determined by the thiobarbituric acid (TBA) reaction [47]. Leaf sample (0.25 g) was homogenized in 5 ml 0.1% TCA. The homogenate was centrifuged at 10,000 g for 5 minute. To 1 ml aliquot of the supernatant, 4 ml of 20% TCA containing 0.5% TBA was added. The mixture was heated at 95°C for 30 minute and then quickly cooled in an ice-bath. After centrifugation at 10,000 g for 10 minute the absorbance of the supernatant at 532 nm was read and the value for non-specific absorption at 600 nm was subtracted. The concentration of MDA was calculated using its extinction coefficient of 155/mM/cm.

SOD and catalase: The activities of this enzyme were not affected by zinc nutrition in rice at 15^th day of the growth. However, decreases in activities of these enzymes were observed in zinc deficient plants of rice at 25^th day of the growth. With increasing age, the activities of these enzymes were further declined by zinc stress. At 35^th day of the growth the activity of SOD was reduced by 70% in rice plant that received 0 ppm Zn as compared to those of 0.1 ppm Zn. Under similar conditions, the activity of catalase was also suppressed by 71% in rice [49].

Lipid peroxidation increased in Zn deficient plants of rice (Table 15). Though the marginal increase in lipid peroxidation was noticed at 15^th day in Zn deficient plants of rice, the enhancement of lipid peroxidation was found to be more with increasing age in the plants grown at lower levels of zinc. The enhancement in lipid peroxidation was observed to be 6.5 times in rice in 0 ppm Zn plants as compared to the plants at 0.1 ppm Zn at the 35^th day of the growth. At 0.1 ppm Zn level, age of the plant had no significant influence on lipid peroxidation in rice. The results (Table 16) show that the Zn deficiency had no effect on the lipid peroxidation in the 1^stand 2^nd leaves (from top) of rice but it was significantly higher in 3^rd and 4^th leaves of the plants that received low levels of Zn.

Table 15: Changes in the lipid peroxidation (MDA content, n moles/g fresh wt.) as influenced by Zn nutrition and growth stage in rice.

Table 16: Lipid peroxidation (MDA content, n moles/g fresh wt) of different leaves of rice at 35^th day of growth in plants grown at different leaves of zinc.

SOD of the chloroplasts is the soluble Cu-Zn and cyanide-sensitive enzymes. Lipid peroxidation requires O₂ uptake and involves the production of superoxide radical (O₂^-). The reduction of SOD and catalase may possibly result in the accumulation of O₂^- and H₂O₂ under Zn deficiency. Two other highly reactive chemical sp. Singlet oxygen (*O₂) and hydroxyl free radical (OH) may be produced by an interaction between O₂^- and H₂O₂ [52]. Development of bronzing in rice under Zn deficiency may be due to the increased lipid peroxidation. Further, the enhancements of lipid peroxidation only in the 3^rd and 4^th leaves where brown spots appear confirm this observation.

Influence of Zinc on Sterility

Field experiments were conducted by Jeyaraman and Ramiah [53] in a split plot design at Agricultural college and Research Institute farm, Madurai, during kharif and rabi seasons of 1985-86 to study the effect of zinc coated fertilizers on rice. Nine treatments comprised of different sources and methods of Zn application in. The soil was deficient in average Zn status. The rice varieties IR 50, IR 20 were used as test varieties for kharif and rabi seasons, respectively. A basal dose of 100 kg N+50 kg P₂O₅+50 kg K₂O/ha was applied. All treatments received a common dose of 50 kg N/ha and 50 kg K₂O/ha as topdressing in two equal splits at tillering and panicle initiation stage. Zincated DAP was applied at 100 kg/ha to supply 2, 4 and 6 kg of Zn/ha. For root dipping, root of seedlings were dipped in 2% ZnO suspension for a period of 20 minutes just before transplanting. In foliar spray, 2% ZnSO₄ was applied in two stages.

The results indicated that the percentage spikelet sterility was more in second crop compared to first crop (Table 17). Application of Zn had significant effect in reducing the spikelet sterility both the seasons. This is in conformity with the results obtained by Gill and Hardip Singh [54]. Among the different grades and forms of zincated DAP, application of 6% zincated DAP ZnO recorded significantly the lowest spikelet sterility of 12.0 and 15.4% during first and second season respectively. This might be due to increased concentration of Zn in this grade, which in turn increased the available status of Zn at all stages of crop growth [55]. Root dipping in 2% ZnO was comparable with the soil application of 6% zincated DAP with ZnSO₄ in reducing the spikelet sterility. Foliar spray of 2% ZnSO₄ also recorded lower spikelet sterility percentage over no Zn and was comparable with the soil application of 2% ZnSO₄ zincated DAP with ZnSO₄. This decreased spikelet sterility subsequently increased the grain yield during both the seasons.

Table 17: Effect of Zn application on spikelet sterility and grain yield of rice.

Management of Zinc

Since, the availability of Zn is controlled by a multitude of factors like submergence, organic matter, nutrient interaction etc. its management in the field level should be done very judiciously.

Green manuring

A field experiment was conducted by Duhan and Singh [56] during 1991-92 at research farm of CCS Haryana agricultural University, Hisar. The experimental soil was sandy loam in texture, having pH (1:2), 7.7; EC (1:2), 0.72 ds/m, available N, 80.5 mg/kg and DTPA extractable Zn as 0.62 mg/kg. Three GM crops viz. dhaincha, moong and sunhemp were buried into soil at the age of 50 days after sowing. Rice var-PR 106 was transplanted 7 days after green manure incorporation. The treatments consisted of four GM sources, viz. fallow, dhaincha, moong and sunhemp in main plots and four levels of nitrogen (0, 40, 80 and 120 kg N/ha) applied to subplots.

There were altogether 8 treatments replicated 3 times in a splitplot design. Rice crop as well as GM including fallow were fertilized with basal dose of other nutrients (P, K and Zn) according to the recommended package of practices. The amount of dry matter added to soil through sunhemp, dhaincha and moong was 5.11, 4.89 and 3.21 t/ha, respectively. The samples of GM were analyzed for various micronutrients and contents of Zn, Cu, Mn and Fe in dhaincha were 39.4, 6.7, 192.0 and 274.0 mg/kg, in moong 42.0, 5.9, 178.0 and 268.0 mg/kg and in sunhemp 29.9, 6.3, 165.0 and 382.0 mg/kg, respectively.

These GM crops were also analyzed for N content and total N supplied to soil by dhaincha, moong and sunhemp were 111, 97 and 119 kg/ha, respectively. In rice crop, half dose of N and full dose of P, K and ZnS were applied at the time of transplanting and remaining half of N was applied after 21 days of rice transplanting. In rice, submergence condition (5 cm standing water) was maintained up to crop maturity. The rice was milled and husk yield was obtained by subtracting the grain yield from the total weight of rice grain+husk. After recording the grain, straw and husk yield the samples of each were analyzed for Zn by atomic absorption spectrophotometer.

Application of GM significantly increased the mean rice grain, husk and straw yields over control (No GM) (Table 18). Alzard and Becker [57] also observed the increase in rice grain yield with the GM application over control and N at 10 g m^-2. Among the different GM crops, sunhemp recorded the highest mean rice grain yield (4.41 t/ ha) followed by moong (4.26 t/ha) and least by dhaincha (4.1 t/ha). However, Hundal et al. [58] reported the highest rice grain yield by cowpea followed by dhaincha and sunhemp. The possible reason for highest rice grain yield with sunhemp may be the highest amount dry matter supplied by sunhemp. Although moong supplied the least dry matter to the soil, it registered higher rice grain yield than dhaincha possibly due to easy and timely decomposition/mineralization of moong green manure as compared to dhaincha.

Table 18: Effect of green manures and N levels on rice yield, content and uptake by rice.

Application of GM significantly increased the Zn content in rice grain and straw over fallow treatment. Swarup [59] also reported that the GM increased the concentration of Zn in rice crop. Data further revealed that there was no significant effect of GM and N application on the contents of Zn in rice husk. In all, N application increased the Zn content in rice grain and straw from 18.4 to 22.4 and 22.4 to 26.6 mg/kg, respectively over control. Application of GM and N increased the Zn uptake by rice. The effect of different green manures on rice yield and nutrient uptake was in order: sunhemp>moong>dhaincha.

Green leaf manuring

One field experiment was conducted by Savitri et al. [60] in Typic Ustochrept and Typic Haplustalf soils with rice (cv. IR 20) as test crop. The green leaf manures studied were Gliricidia maculata and Sunhemp (Crotolaria juncea) at 6.25 t/ha. ZnSO₄ was applied at two levels of 12.5 and 25 kg/ha. The design of experiment used was randomized block design with 3 replications. Total micronutrient content and soil micronutrient content of different green manures and weeds are given in Table 19.

Table 19: Effect of different green manures and weeds on availability of Zn.

From the above table (Table 20) it has been observed that the Zn use efficiency varied from 2.2 to 9.4% in Ustochrept and 3.1 to 7.1% in Haplustalf. The physiological efficiency of Zn was increased by the incorporation of green leaf manure in coarse textured soil (Haplustalf) compared to fine textured soil (Ustochrept).

Table 20: Effect of green manures and ZnSO4 application on rice (IR 20).

Gypsum and fym application

Sodic soils formed under the influence of sodium carbonate occur extensively in the Indo-Gangetic plains of Northern India and in several parts of the world. Reclamation of sodic soils basically requires the replacement of exchangeable Na by Ca, which can be supplied by adding gypsum or any other suitable amendment. These soils are generally deficient in zinc and crops respond favourable to Zn fertilization. The use of organic manures along with fertilizers not only helps in maintaining favourable physico-chemical characteristics and fertility in soils but also increases the crop yield markedly. Application of organic manures to one crop exhibits residual effect on the succeeding one. Application of gypsum and subsequent leaching of sodic soil brings down the soil pH and transforms nutrients into forms of varying solubility. It has also been reported that solubility of Zn increases with decrease in soil pH [61].

Green house experiments were conducted by Sachdev and Deb [62] on a zinc deficient sodic soil from village Mitapur, Delhi on three crops (wheat, rice and wheat). The soil had a pH, 9.95; EC, 0.45 m mhos cm^-1; organic C, 0.39%; CEC, 9.8 m eq/100g soil and DTPA extractable Zn, 0.45 ppm. The treatments consisted of three levels of FYM (0, 5 and 10 t/ha) and three levels of gypsum viz. 0, 13.8 and 27.6 q/ha, respectively corresponding to gypsum requirement (GR) of 0, 1 and 2. The treatments were replicated thrice in a completely randomized block design. Zinc was applied at 10 kg/ha in the form of ZnSO₄. 7H₂O labeled with Zn65 at the rate of 2 mci/g Zn. A basal application of N, P₂O₅ and K₂O at 120, 60 and 60 kg/ha respectively was given to all the plots through analytical reagent grade salts. Wheat crop was grown first with application of gypsum, FYM and Zn and the residual effect was evaluated in the succeeding rice and wheat crops. To subsequent rice and wheat, N, P and K were applied at recommended doses. Zn was determined by Atomic Absorption Spectrophotometer and Zn₆₅ by single channel analyzer using thallium activated NaI crystal as the detector.

The dry matter yield of first wheat crop increased with the application of both FYM and gypsum (Table 21). The second crop (rice) showed a significant increase in dry matter yield due to FYM application while gypsum had no effect. The dry matter yield of wheat grown after rice in sequence was significantly influenced by both gypsum and FYM applied to the first wheat crop. Singh et al. [63] also reported that at low levels of gypsum, plant growth was poor owing to toxicity of Na and/or deficiency of Ca. Farm yard manure (FYM) proved effective in all the crops but gypsum was effective only in wheat after rice.

Table 21: Average effect of gypsum and FYM on dry matter yield (g/pot) on wheat, rice and wheat crops.

Addition of gypsum decreased the Zn content in the first crop of wheat in both grain and straw, but in rice grain, there was an increase in Zn content with gypsum. The Zn content in both grain and straw increased in wheat grown after rice with the application of gypsum to the first crop of wheat. Application of FYM in sodic soil showed a marked increase in Zn concentration in all the 3 crops (Table 22). Similar results have been reported by Hans and Gupta [64]. Increase in Zn content by the application of FYM may be due to the production of complexing agents, which from stable organomatallic complexes with Zn [65].

Table 22: Zn content (ppm) in grain and straw of crops as affected by gypsum and FYM.

The percent Zndff in grain was found to decrease with the application of FYM to the first wheat crop but gypsum had no effect. It was only 12 percent in wheat grain while the subsequent rice grain absorbed quite higher amount (41.0 to 64.0%) of fertilizer Zn applied to previous wheat crop (Table 23). The subsequent wheat crop utilized only 14.0 to 18.0 percent of the Zn derived from fertilizer. In the straw of all the crops, the fraction of fertilizer Zn was quite low (7.0 to 13.0% in wheat) while it was only 3.0 percent in rice straw. The application of FYM significantly affected the percent Zndff in grain of all the three crops.

Table 23: Effect of gypsum and FYM on Zndff (%).

The data on utilization of applied fertilizer Zn65 by the three crops are given in Table 24. It is evident from the data that the application of FYM significantly affected the fertilizer zinc utilization by all the three crops. However, overall recovery of fertilizer zinc in the first wheat crop was of the order of 0.30 to 0.54 percent only as observed in many field and pot experiments [66]. In the second crop (rice), although no fresh zinc was applied but it utilized a substantially higher amount of residual fertilizer Zn, the range being 0.99 to 1.25 percent. This may be attributed to the improvement in soil conditions due to the application of gypsum and FYM to the soil prior to wheat.

Table 24: Effect of gypsum and FYM on % utilization of applied Zn by wheat and subsequent two crops.

Application of gypsum decreased DTPA extractable Zn after the harvest of each crop (Table 25). Application of FYM significantly increased the availability of Zn after the harvest of each crop over the treatment where no FYM was applied. According to Swarup [67] Zn is known to form stable complexes of different stability with organic ligands, which decrease their susceptibility to adsorption, fixation or precipitation reactions in soil. Application of FYM might have resulted in the formation of such metal organic complexes of higher extractability. Moreover, the submergence of sodic soil in the presence of organic matter (FYM) resulted in greater decrease of pH, which could make a substantial contribution to Zn supply.

Table 25: Effect of gypsum and FYM on available Zn (ppm) in soil after crops of wheat, rice and wheat.

Application of blue green algae

Bio-fertilizers such as blue green algae, azolla etc. are important supplementary sources in rice cultivation. Additionally, BGA bring about changes in chemical and electro-chemical properties of the soil which modify the oxidation-reduction status of the growing soils, the chelating capacity of the soil organic matter etc. which in turn, may bring about changes in the availability of different micronutrients like Fe, Mn, Cu and Zn in soils.

A field experiment was conducted on a silty clay loam soil (Haplustalf) in the University Farm at Kalyani, West Bengal during kharif 1989 in a factorial randomized block design with five main treatments. A soil-based mass culture containing a mixture of Anabaena, Nostoc, Cylindrospermum and Tolypothrix sp were applied to each plot (size: 5 m×4 m) at 12 kg/ha. The soil characteristics of the experimental plot was: pH, 8.30; Organic C, 6.2 g/kg; CEC, 24.5 c mol (p+)/kg; total N, 1.1 g/kg and DTPA extractable Zn, 6.8 mg/kg soil, respectively.

The results (Table 26a) also show that the algal inoculation brought about an increase in the extractable Zn content in the soil and the magnitude of such increase in the extractable Zn content being about 22.8, 4.4 and 19.2 percent at 15, 30 and 45 DAT, respectively than that of uninoculated one. It is known that BGA during their growth release various extra cellular organic compounds which can chelate heavy metals like Zn. Some of these chelates are water-soluble and are mostly extractable; this may explain the initial significant increase in the amount of Zn due to BGA inoculation.

Table 26 (a): Influence of BGA on the periodical changes in DTPA extractable Zn (mg/kg soil) in submerged rice soil.

Application of biogas slurry

A field experiment was started by Singh et al. [68] during kharif 1993 in a Zn-deficient highly calcareous sandy loam soil (Calciorthent) at the Rajendra Agricultural University Research Farm, Pusa (Bihar) to evaluate the optimum ratio of Zn and organic manures for ricewheat cropping system. The experimental soil has the following characteristics: pH, 8.8; EC, 0.34 ds/m; organic C, 3.4g/kg; free CaCO₃, 0.33 mg/kg; DTPA Zn, 0.52 mg/kg. Sixteen treatment combinations (Table 26b) consisting of three levels of Zn (0, 2.5 and 5.0kg Zn/ha) and six levels of dry biogas slurry (BGS) as a source of organic manure alone or in combination to give Zn:BGS ratio of 1: 250, 1:500, 1:1000 and 1:2000 were replicated thrice in a randomized block design.

Table 26(b): Effect of Zn mixed with biogas slurry on zinc use efficiency after four crops.

The required quantity of BGS alone or amended with Zn for each plot of 10 m² was incubated for 20 days in polythene bags at field capacity at room temperature. The incubated material was applied in respective treatment plots and mixed with surface soil before transplanting rice as first test crop under rice-wheat cropping system. Two cycles of ricewheat system i.e. four crops were grown till maturity. Rice var. sita and wheat var. HP 1102 were taken as test crops. The N, P₂O₅ and K₂O at 110, 60 and 40 kg/ha were applied as urea, SSP and MOP to each crop but the Zn and BGS treatments were applied only to first crop to record direct as well as residual effects. The biogas slurry used had the following composition on dry weight basis: organic C, 35.3%; total N, 0.93%; total P, 0.45%; total K, 0.55%; total Zn, Fe, Cu and Mn 105, 2504, 50 and 241 mg/kg respectively and DTPA extractable Zn, Fe, Cu and Mn as 48, 95, 5.2 and 44 mg/kg, respectively.

The data in Table 26b suggest that the ZnUE of BGS was increased at higher Zn levels. At lower Zn levels, though BGS has increased the ZnUE but the effect of all the BGS levels appeared to be identical. However, at higher Zn level, i.e. 5 kg Zn/ha, the increasing rate of BGS continuously increased the ZnUE from 9.5 to 15.0 percent. The overall results suggest that Zn: BGS ratio of 1:500 was optimum in increasing ZnUE.

The available Zn content in post-harvest soil of each crop has been reported in Table 27. The available Zn in soil after wheat was found to be comparatively higher than that after rice. This may be due to high soil water regime of rice field, causing decreased Zn availability in soil. Also higher temperature at the harvesting time of wheat might have increased Zn availability in soil due to microbial activity. The residual effect was observed even after the 4^th crop where Zn and BGS were applied together of course, the effect of Zn:BGS ratio of 1:1000 appears to be superior at both Zn levels with respect to their residual value but the effect of Zn:BGS ratio of 1:500 was significantly higher than Zn alone at both levels. The depletion of Zn in soil with subsequent crops was apparent under similar i.e. rice-to-rice and wheat-to-wheat. The depletion was more marked where the initial build up was higher. Though, wider Zn: BGS ratio was superior but the ratio of 1:500 appears to be optimum for best utilization of native as well as applied Zn.

Table 27: Effect of zinc and biogas slurry on available Zn content (mg/kg) under rice-wheat cropping system after harvest of crops.

Cultural practice

A green house experiment was conducted by Das [69] to study the effect of soil moisture regimes, puddling and time of application of organic matter on the uptake of Zn by rice and its yield. Variation in soil moisture regimes in rice cultivation is common, and may influence the nutrient uptake by rice and its yield. Puddling decreases Eh and increases the soil pH and EC of the soil solution thus creating a reduced condition [70]. Puddling helps to maintain high amount of Zn in roots under waterlogged and saturated moisture regimes [71]. Puddling slows down the decomposition of organic matter leading to an accumulation of toxic organic compounds, CO₂, CH₄, H₂S, thus affecting the growth and yield of rice [72].

Time of application of organic matter also influences the yield and uptake of rice. Das and Mandal [36] conducted a green house experiment taking cultivated surface soils from the districts of Murshidabad situated in the Gangetic alluvial region of West Bengal having pH, 7.60; CEC, 16.70 me/100g; organic C, 0.38%; DTPA extractable Zn, 0.45 mg/ kg respectively. The organic matter (well decomposed FYM) having 31.5 mg/kg Zn was used for the experiment. Two levels of moisture regimes viz. (i) saturated (S) and (ii) waterlogged (W); two levels of puddling (i) puddling (ii) non puddling and five stages of organic matter application (i) T₁, 28 days before puddling or non-puddling (ii) T₂, 14 days before puddling or non-puddling (iii) T₃, zero days before puddling or non=puddling (iv) T₄, 28 days after puddling or nonpuddling (v) T₅, control or no application of organic matter were used for the experiment.

The organic matter was applied at 1% by weight of the soil. In all there were 20 treatment combinations (2 moisture regimes x 2 tillage treatments x 5 stages of organic matter application). Each treatment combination was replicated three times. Waterlogged moisture regime was maintained at 5 ± 0.5 cm water head above the soil surface. Two, 20-day-old rice seedlings (cv. IR 579) were transplanted in each pot and allowed to grow till harvest.

The average yield of straw, grain and root was significantly higher in plants grown under waterlogged soil moisture regimes as compared to saturated condition (Table 28). Similar to yield, uptake of Zn by root, straw and grain was more in plants grown under waterlogged condition than in those grown under saturated condition. Ghosh et al. [73] reported that the redox potential value (Eh) under waterlogged condition falls much low (-190 mv to –300 mv) as compared to saturated condition (-70 mv to –90 mv) indicating more reducing environment under the former moisture regime. The higher yield and uptake by rice in waterlogged condition may be ascribed due to favourable chemical environment of the root medium leading to higher root proliferation and nutrient adsorption by the crop due to healthy reducing conditions.

Table 28: Effect of moisture regimes, puddling and time of application of organic matter on the dry matter yield and uptake of Zn by root, straw and grain of rice.

Although unpuddled condition increased the yield of root, straw, grain and uptake of Zn to that of puddle condition, the data did not differ statistically. Unpuddled condition increased nutrient uptake and yield, possibly because of less resistance to root penetration which favour root proliferation and production of favourable environment by preventing the accumulation of toxic substances such as CO₂, CH₄, H₂S, acetic acid, lactic acid due to enhanced rate of decomposition of native and applied organic matter.

Application of organic matter at different stages showed a significant effect on the yield of root and straw, whereas it had no marked effect on grain yield (Table 28). The highest yield of root, straw, grain and highest uptake of Zn was recorded when organic matter was applied just before puddling or non-puddling.

Correction of zinc deficiency

Zinc deficiency can be corrected by either soil application or foliar spray of Zn through zinc sulphate (ZnSO₄.7H₂O). As an emergency treatment spray application is done. Higher yield is observed in case of soil application as compared to foliar spray.

Soil application: Zinc sulphate (ZnSO₄.7H₂O) containing around 22% Zn is the most commonly used Zn fertilizer. It is suggested to use 5-10 kg actual Zn per hectare. This represents about 25 to 50 kg zinc sulphate material. Low Zn levels (25 kg ZnSO₄ ha^-1) are recommended for sandy soils and high Zn levels (50 kg ZnSO₄ ha^-1) for heavy texture clayey soils, and in areas where rice is grown on saline-alkali and permanently wet soils. Because Zn generally does not move far in the soil, it is important to place it where the roots can get it. The successful methods of application are to broadcast zinc sulphate and to plough it in or to drill it in the soil below and on a side of the seed. For Zn treatment to be fully effective, it is essential to apply it prior to sowing or transplanting of the crops. Annual soil applications of zinc fertilizer are not necessary. The application rates suggested above should provide ample Zn for at least 2-4 years.

Foliar spray: In case Zn deficiency is diagnosed after sowing or transplantation of a crop, it is preferentially cured by foliar sprays. Zinc spray should consist of 5 g zinc sulphate and 2.5 g lime per liter of water. To drench thoroughly, the foliage of one-hectare crop, about 500 liters of spray solution are adequate. Two to four, weekly sprays can cure the deficiency. Except for tree crops, sprays have not given as good a control of Zn deficiency as soil application.

Das [69] reported that the application of zinc sulphate at 20 kg/ ha to acid latosol of West Bengal gave the highest average rice yield (4.12 t/ha) with benefit-cost ratio of 1.52. He also suggested that the application of 20 kg/ha ZnSO₄ gave 25.60% additional yield over control, no application of Zn, while 15 and 25 kg/ha ZnSO₄ showed the yield increase of only 8.53 and 20.12% respectively.

A field experiment was conducted by Agarwal and Bhan [74] with seven levels of sequence for three consecutive years from 1990-91 to 1992-93 in randomized block design with four replications at C.S.Azad University of Agriculture and Technology, Kanpur. The soil of the experimental field was sandy loam in texture with pH, 7.2; EC, 0.52 m mhos/cm; organic C, 0.42%; available P, 12.0 kg/ha; available potash, 115kg/ha and available Zn in traces. Recommended dose of 120 kg N, 60 kg P₂O₅ and 60 kg K₂O per hectare was given to each rice and wheat crops and Zn sulphate was given as per treatment as soil application before transplanting or sowing of the crops. Rice variety ‘Sarjoo 52’ and wheat variety ‘HD 2285’ were tested in all years of experimentation.

Three years results indicated that in each year significant increase in the grain yield of rice and wheat crops were obtained with the application of ZnSO₄ at 25 kg/ha over control (Table 29). Highest mean grain yield of rice (4305 kg/ha) and wheat (4417 kg/ha) were recorded with application 25 kg/ha of ZnSO₄ as soil application along with recommended dose of NPK in each rice and wheat crops giving a difference of 622 kg/ha (17%) and 619 kg/ha (16%) of rice and wheat respectively over control (no ZnSO₄). Application of 25 kg ZnSO₄ in rice and 12.5 kg/ha ZnSO₄ in wheat gave 530 kg/ha (14%) higher grain yield of rice and 354 kg/ha (9%) higher yield of wheat over control. The increase in yield due to Zn application to the soil could possibly be due to the enhanced synthesis of carbohydrates and proteins. It also plays an important role in photosynthesis and enhances the uptake of nitrogen.

Table 29: Effect of ZnSO₄ on the yield of Rice-wheat (kg/ha).

Crop Sustainability Indicators

Yield

Crop yields determinate agricultural production and therefore, this is an important SI. Several studies on rice-wheat cropping system at the experimental centers reported the yield decline in rice [75]. Of the 7 long-term rice-wheat experiments examined by Ladha et al. [1], none had a significant decline in wheat yield, but rice yields at Pantnagar declined at a rate of 2.3% per year, while the decline at Ludhiana was 2.7% per year. Such results question the sustainability of the rice-wheat cropping system and call for ameliorative measures if this cropping system is to continue.

Factor productivity

Factor productivity is the ratio of output and input in a production system. When only one input such as fertilizer N is taken into consideration, it is termed as partial factor productivity (PFP) and the input is indicated by a subscript. For example, PFPn is referred to as PFP for nitrogen. Yadav [76] studied PFPn from the field experimental data for 16 years from 4 research centers (Pantnagar, Faizabad, Sabour and Rewa) and observed that there was a decline in PFPn in rice but not in wheat (Table 30).

Table 30: Partial factor productivity (PFPn) (kg grain / kg N) for rice and wheat at start and after 16 years of rice-wheat cropping.

Kumar et al. [77] on the other hand studied total factor productivity (TFP) in 3 states (Punjab, Haryana and Uttar Pradesh) and found that TFP during 1985-92 was lower than that in 1976-85; as a matter of fact, it was negative in Uttar Pradesh (Table 31). Farmers in these 3 states have increased their fertilizer application rates over the years to obtain the same yield and this indicates a general feeling of reduced PFP due to fertilizers.

Table 31: Trend in indices of total factor productivity (TFP) in rice-wheat cropping system in different states of India.

Summary and Conclusions

Summary

Zinc is receiving great importance in rice-wheat cropping system because of its widespread deficiency throughout the country caused by adoption of intensive cropping programme and modern agrotechniques. Various factors associated with Zn deficiency are acid sandy soils low in total Zn, neutral or alkaline soils having higher amount of fine clay, silt and available P, organic soils etc. Intensive cultivation and growing of exhaustive crops have made the soil deficient in macro as well as in micronutrient. Now a days, use of only nitrogenous and phosphatic fertilizers also create nutrient imbalances particularly of Zn in soils.

Analysis of large number of soil samples throughout India for plant, available Zn indicated that Zn deficiency (>45%) was most serious. and deficiency of single micronutrient Zn was more prevalent than that of two micronutrients. This indicated that application of multinutrient mixture is uneconomical and may lead to degradation of soil environment and hence sustainability. Lowland rice is more vulnerable to Zn deficiency. An uneven stand of rice and stunted plants with brown rusty appearance are indicative of Zn deficiency widely known as “Khaira” disease. In wheat Zn deficiency appears as fading of the middle of the lower half of the lamina on the older leaves followed by formation of reddish brown lesions and withering of leaves together with delayed maturity and reduced yield. Zinc depletion in soil depends on the cropping sequence as well as on fertility level.

Zinc occurs in soils in different chemical pools which differ in their solubility and availability to plants. Zinc is relatively immobile in most soils and undergoes transformation in soils by various mechanisms like sorption by clays, hydrous oxides, organic matter etc. which affect the availability of Zn in soils and hence growth and nutrition of plants. Zinc availability decrease in submerged condition which may be attributed to the formation of insoluble franklinite (ZnFe₂O₄); sphalerite (ZnS); ZnCO₃, Zn(OH)₂, Zn₃(PO₄)₂ and adsorption by oxides, hydrous oxides, organic matter, carbonates, sulphates, clay minerals etc. Soil reaction, organic matter, lime, partial pressure of CO₂, redox potential and nutrient interactions are some of the factors affecting Zn availability. Soil application of 12 kg/ha and foliar application of 2-4kg Zn/ha gave the highest mean yield of rice. In case of rice, the critical limits for Zn in soil and plant are 1.2 mg/kg and 35.95 mg/kg, respectively.

Foliar application of 0.5% aqueous solution of ZnSO₄ twice 20 days after transplanting rice at an interval of 10 days was superior to its soil application in sodic soil. Application of green manure and N increased the Zn uptake by rice. The effect of different green manures on rice yield and nutrient uptake was in order: sunhemp>moong>dhaincha.

In lowland rice, the recovery of applied Zn by rice is very low due to its transformation to different chemical forms. Application of organic matter increased the percent utilization of applied Zn, possibly due to increased availability of Zn with the presence of applied organic matter caused by substantial increase in the content of water soluble plus exchangeable, organic complexed and amorphous sesquioxides bound fraction of native soil Zn with a concomitant decrease in that of crystalline sesquioxides bound fraction.

The four fractions of soil Zn could retain 47.5 – 53.7% of the applied Zn in soil in presence of added organic matter as compared to 27.4 – 46.4% in its absence, indicating that organic matter application helped in retaining a higher portion of applied Zn in these fractions in soils which might be useful to the succeeding crops. All Zn fractions except the residual mineral Zn showed significant positive correlation with each other and with Zn uptake by rice. The Zn uptake was recorded highest when N and K (along with Zn) was applied followed by application of Zn in combination with N and P which may be due to antagonistic interaction between N and P and synergistic interaction between N and K.

The dry matter yield of both shoot and root increased due to P application but the uptake of Zn by the shoot declined while that in root increased, which suggests that the decrease in Zn concentration in shoots is not possible due to a dilution effect. It may be due to retardation of its translocation form root to shoot and partly to the decrease in its absorption by plants owing to its decreased availability in soil resulting from P application. Application of Zn when combined with N and K significantly increased the dry matter yield and uptake of Zn. Increasing doses of both iron and zinc individually increased grain yield per hectare quite significantly.

In wheat-rice-wheat cropping sequence, FYM proved effective in increasing dry matter yield of all crops but gypsum was effective only in wheat. Application of FYM in sodic soil showed a marked increase in Zn content in all three crops, while addition of gypsum decreased Zn content in grain and straw of first wheat crop. But gypsum application to the first wheat crop, increased Zn content in grain of succeeding rice and Zn content in grain and straw of second wheat crop after rice. The percent Zn derived from fertilizer in grain was found to decrease with the application of FYM to the first wheat crop but gypsum had no effect.

Application of FYM significantly affected the fertilizer zinc utilization by all the three crops. The overall recovery of fertilizer Zn in first wheat crop was low but the second crop (rice) utilized a substantially higher amount of residual fertilizer zinc which may be attributed to the improvement in soil conditions due to the application of gypsum and FYM to the soil prior to wheat. The DTPA-extractable Zn after harvest of each crop was decreased by gypsum application and increased by FYM application.

The extracellular organic compounds released by BGA from water soluble and extractable chelates with heavy metals like zinc and hence instrumental in increasing extractable Zn content in the soil. High concentration of zinc (>200 mg/kg soil) in soil caused sharp decline in total population of earthworm due to their significant drop in reproduction and increase in population mortality.

Biogas slurry ratio of 1:500 was optimum in increasing zinc use efficiency and utilization native as well as applied Zn. With increasing age, the activities of superoxide dismutase (SOD) and catalase were declined by zinc stress while lipid peroxidation increased significantly in 3rd and 4th leaves of Zn deficient plants of rice. Lipid peroxidation requires O₂ uptake and involves the production of superoxide radical (O₂^-). The reduction of SOD and catalase may possibly result in the accumulation of O₂^- and H₂O₂ under Zn deficiency. Development of bronzing in rice under Zn deficiency may be due to the increased lipid peroxidation which is further confirmed by the enhancements of lipid peroxidation only in the 3rd and 4th leaves where brown spots appear.

Zinc application significantly reduced the spikelet sterility and increased grain yield of rice crop in both kharif and rabi seasons. The physiological efficiency of Zn was increased by the incorporation of green leaf manure in coarse textured soil (Haplustalf) compared to fine textured soil (Ustochrept). Application of ZnSO₄ at 25 kg/ha increased grain yield of both rice and wheat possibly due to the enhanced synthesis of carbohydrates and protein influence on photosynthesis and enhanced uptake of nitrogen. Application of ZnSO₄ at 25 to 50 kg/ ha once in 2-4 years is sufficient. Higher dose should be applied in heavy textured clayey soils, saline-alkali soil and permanently wet soil. The ZnSO₄ should be ploughed down after broadcast or drilled in the soil below and side of the seed. Two to four weekly foliar sprays of ZnSO₄ (5 g ZnSO₄+2.5 g lime/l water) can cause Zn deficiency in standing crops. The synergistic relationship of Zn was observed with N, Fe, Mo, B while P, Ca, Mn, Cd shows antagonistic with Zn. Sustainability indicators like yield and factor productivity shows a declining trend in rice.

Conclusion

Deficiency of Zn has been found in most Indian soils, and it is associated with specific soils, soil properties and cropping systems especially in rice-wheat crop sequence. Zinc deficiency has been identified on the basis of limited number of soil, plant analysis and biological responses to its application. Zinc, the most limiting micronutrient elements in rice-wheat cropping sequence, exists in soils in its various fractions namely, water-soluble plus exchangeable, organic complexed, amorphous sesquioxides and crystalline sesquioxides bound etc. which have a great significance in plant nutrition.

The application of P and Zn to both rice and wheat was significantly superior in relation to production than that of the treatments where P and Zn was applied only in rice crop. The soil or foliar application of Zn either as ZnSO₄ or Zn-EDTA in addition to recommended NPK always recorded higher yield in rice-wheat sequence preferably when Zn- EDTA was applied. The application of on-farm inputs (organic matter, crop residues, green manures etc) in addition to off-farm inputs (NPK fertilizers) significantly increased the crop production in a rice-wheat sequence with the simultaneous increase in physiological efficiency of Zn as well as content in rice and wheat crops.

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