Effect of long-term differential fertilization on depth-distribution of forms of sulphur and their relationship with sulphur nutrition of wheat under maize-wheat sequence

The available, water soluble and heat soluble sulphur (S) contents over the profile depth up to 120 cm were investigated in a long-term experiment, which was continuously cropped for 22 years with maize-wheat sequence. The minimum amount of S was found in water soluble form. All the forms of S decreased with increasing soil depth but this effect was more noticeable up to 45–60 cm soil depth. The increasing level of N, P and K affected depth-wise distribution of S. The content of all the forms of S increased with application of 40 kg P₂O₅ ha⁻¹ but the trend reversed with the application of 80 kg P₂O₅ ha^−l, irrespective of N levels. Application of 40 kg K₂O ha⁻¹ at all the levels of N and P resulted in a decline in all the forms of S. The accumulation of sulphur was lower in the NPK treated plots than N- and NP-treated plots. The amount of water soluble S in lower soil depth was found to be very low. Regression equations were used to predict S uptake by wheat using S status in different soil layers as independent variables. Multiple regression analysis indicated that the coefficient of determination (R²) value between S uptake and available S improved significantly when its status up to 60–90 cm soil depth was included. Heat and water soluble S contributed significantly towards S uptake by wheat when accumulation of forms of S in the soil profile up to depth of 60–90 cm included in the regression analysis.