Soil Acidity and Nutrient Stratification As Affected By Long-Term Tillage and Nitrogen Fertilization.

This study examined the long-term effects of N fertilizer management (N rates of 0, 22, 45 and 67 kg N ha^-1) and tillage intensity [conventional tillage (CT), reduced tillage (RT) and No-till (NT)] on soil nutrient distribution and acidity at different soil depths in dryland wheat-sorghum-fallow cropping system. Soil available nutrients and acidity were significantly (P < 0.0001) affected by depth × N application rate × tillage interaction. Averaged across N rates and tillage practices, extractable phosphorus (P), potassium (K), and magnesium (Ca) concentrations decreased with increasing soil depth.  Regardless of tillage practice, residual soil nitrate (NO₃) –N concentration at the top 0 to 60 cm increased with increasing N fertilizer application rate. At 0 to 60 cm depth, NO₃-N concentration was significantly greater in the CT than RT and NT systems, particularly at higher N application rates. No significant NO₃-N leaching occurred beyond 60 cm, except at 67 Kg N ha^-1 in the CT system where a significant increase in NO₃-N occurred at deeper soil depths.  After 40-yr of tillage and N fertilizer application, there was a marked decline in pH at the soil surface (0 to 15 cm) with increasing N application regardless of tillage intensity. Average across N rates, soil surface pH was lower with NT compared to CT and RT systems. Based on our findings, growers adopting NT need to monitor changes in soil surface chemistry and take necessary corrective measures to maintain satisfactory pH levels for nutrient availability and crop productivity.