Investigation of inorganic fertilizers and manuring on copper fractions and their relationships with soybean-safflower yield and soil properties under long-term fertilizer experiment in vertisol

The present study investigates the long-term effects of integrated nutrient management on copper (Cu) fractions and their interactions with soil properties and crop yield in Vertisols under a soybean-safflower cropping system. Soil samples were collected after the 18th crop cycle from a long-term fertiliser experiment initiated in 2006-07 at VNMKV, Parbhani, Maharashtra. Various treatments combining inorganic fertilisers (NPK) and organic manures (FYM) were assessed for their impact on different Cu fractions, DTPA-Cu, water-soluble Cu, exchangeable Cu, reducible Cu, residual Cu and total Cu using sequential extraction techniques. The results revealed that balanced fertilisation with 100 % NPK combined with FYM at 5 t ha-1 significantly enhanced the concentrations of all Cu fractions, particularly residual and DTPA-extractable forms. This treatment also recorded the highest grain and straw yields for both soybean and safflower and the maximum Cu uptake. Positive and significant correlations were observed between all Cu fractions and key soil properties such as organic carbon, available N, P, K and S, whereas negative correlations were found with soil pH and CaCO3 content. Notably, DTPA-Cu, residual Cu and total Cu exhibited strong associations with crop yields, highlighting their role in Cu bioavailability and plant nutrition. In this study, all fractions of copper showed a positive and significant relationship with grain and straw yield of soybean and safflower at 1 % and 5 % levels. The study underscores the importance of integrating organic and inorganic nutrient sources to maintain soil health, improve Cu dynamics and sustain long-term crop productivity in vertisols.