Sustained Productivity in High Phosphorus Soils through Fertility Management in Cowpea-Amaranthus Cropping Sequence

Nutrient-related stresses are becoming increasingly widespread in many soils due to the indiscriminate use of high-analysis fertilizers, often leading to low crop productivity. In Kerala, the southernmost state in India, the regular use of phosphatic fertilizers in cropping has resulted in a high phosphorus (P) status in soils. Nutrient interactions with the high P content reveal reduced uptake of other nutrients especially zinc (Zn), ultimately resulting in declined yields. An experiment was carried out at the College of Agriculture, Vellayani, Thiruvananthapuram during 2017–2018 to optimize the levels of P, potassium (K) and Zn for nutrient management in grain cowpea in a high P soil and to assess the residual effect of the fertilization on succeeding amaranthus crop. Foliar application of ZnSO₄ @ 0.025% twice (at branching and flowering) along with soil application of K₂O @ 10 kg ha⁻¹ without additional P recorded significantly higher grain yields (1.29 t ha⁻¹) in cowpea. Significant changes were recorded in soil chemical properties with the legume cropping but the interaction of P*K*Zn effects could not bring about significant variations in the yield of amaranthus raised after the harvest of pods of cowpea and incorporation of haulm. Residual effect of soil application of Zn along with K₂O @ 20 kg ha⁻¹ in grain cowpea resulted in the maximum amaranthus yield (2.61 t ha⁻¹). However, foliar application of ZnSO₄ and soil application of K₂O @ 10 kg ha⁻¹, skipping P was economically more profitable in grain cowpea - amaranthus cropping system in the high P soil.