Value of soil zinc balances in predicting fertilizer zinc requirement for cotton-wheat cropping system in irrigated Aridisols
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- Rafique, E., Rashid, A. & Mahmood-ul-Hassan, M. Plant Soil (2012) 361: 43. doi:10.1007/s11104-012-1434-x
The cotton (Gossypium hirsutum L.)-wheat (Triticum aestivum L.) cropping system practised in >3 million ha of irrigated Aridisols in Pakistan is pivotal to its national economy and food security. Thus, the prevalent stagnation in its productivity is a matter of serious concern. Widespread deficiency of zinc (Zn) in these low organic matter alluvial calcareous soils is amongst the suspected constraints. Therefore, studying the impact of improved nutrient management strategies and crop residue recycling on crop productivity and soil Zn balances was thought imperative.
A 5-year permanent layout field experiment was conducted on two predominant soil series of the cotton-wheat belt [i.e., Awagat (coarse loamy mixed, hyperthermic Fluventic Camborthid) and Shahpur (fine silty mixed, hyperthermic Fluventic Camborthid)] to compare the impact of (1) Farmers’ fertilizer use (FFU); (2) Recommended fertilizer use (RFU); and (3) Integrated nutrient management (INM) on crop productivity and apparent soil Zn balances. The nutrient management strategies were compared with and without cotton-wheat residue recycling, in flat-bed sown and raised-bed sown cotton systems.
Under the FFU treatment, Zn deficiency occurred both in wheat and cotton. Overall lowest mean yields (Mg ha-1), obtained with FFU, were: seed cotton—Awagat, 2.19; Shahpur, 2.45; wheat grain—Awagat, 3.03; Shahpur, 3.94. Yield increases with RFU were: cotton, 24 % in Awagat and 18 % in Shahpur soil; wheat, 37 % in Awagat and 24 % in Shahpur soil (P ≤ 0.05). With INM, crop yields were slightly higher than with RFU. Also, cotton yields were ~10 % greater on raised beds than on flat beds. Crop residue also increased yield of both crops, up to 10 %. Zinc uptake patterns of both crops were closely related to their yields. Fertilizer Zn use efficiency by the cotton-wheat system, in both soils, was quite low, i.e., 1.78–2.36 % of the annually applied 5 kg Zn ha-1. Thus, ~98 % of the applied Zn was retained (fixed) in the soils. Though Zn input from organic sources (i.e., crop residue and farm yard manure) was inadequate to meet crop requirements, Zn use efficiency from organic sources was much greater, i.e., 13–24 %. As Zn uptakes by the cropping system were quite low (i.e., 62–123 g Zn ha-1 by cotton; 74–170 g Zn ha-1 by wheat) compared with Zn inputs (i.e., 1.12–1.79 kg Zn ha-1 year-1), all nutrient management treatments, including FFU, resulted in positive apparent Zn balances in both soils.
Thus, unlike nutrient balances for macronutrients, apparent Zn balances have little significance in predicting fertilizer Zn need of the cropping system. Despite positive soil Zn balances, even without using fertilizer Zn, prevalence of Zn deficiency in cotton and wheat crops may be attributed to high Zn fixation in calcareous soils rather than low total Zn content in the soils. In this scenario, soil testing and plant analysis remain the reliable approaches for diagnosing Zn deficiency problem.