Least limiting water range for two conservation agriculture cropping systems in India

• Aeration was a plant-growth limiting factor beyond soil bulk density >1.55 Mg m−3.
• Brown manuring under zero tillage improved LLWR in the rice–wheat system.
• Permanent bed with residue had 14% higher LLWR than CT under cotton–wheat.
• Residue addition improved LLWR values in both surface soil layers.
• LLWR was not correlated with crop productivity, but with change in soil organic C.

Deterioration of the soil physical environment is a major reason for decreasing crop productivity in the western Indo-Gangetic Plains (IGP) of India. To address this problem, the least limiting water range (LLWR) for two conservation agricultural (CA) practices was quantified. Three year old rice (Oryza sativa)–wheat (Triticum aestivum) and cotton (Gossypiumhirsutam) – wheat CA systems located in New Delhi were evaluated. A novel method for computing the lower LLWR limit was developed by using a linear function to relate penetration resistance (PR) to gravimetric water content and soil bulk density (BD), rather than volumetric water content. During the third year, the 15–30 cm soil layer beneath the puddled transplanted rice, under conventionally tilled wheat (PTR–CTW) plots had PR values that exceeded 2 MPa, but under direct seeded rice with brown manuring, zero tilled wheat (DSR + BM–ZTW) plots had PR values of less than 1.5 MPa throughout the 0–60 cm profile. That said DSR + BM–ZTW plots also had significantly higher gain (over the initial soil) in total soil organic C content in the 0–30 cm soil layer. The 0–45 cm soil layer under permanent broad-beds with residue (PBB + R) had significantly lower PR than permanent narrow-beds with residue (PNB + R) and other plots [PBB; PNB; zero tillage with residue (ZT + R); conventional tillage (CT) and ZT] in the cotton–wheat system. Retaining crop residue resulted in lower BD and PR values in the 0–15 cm soil layer than removing them. The PBB + R plots had ∼12% higher LLWR than CT plots (LLWR = 10.1%) in the 15–30 cm layer. In the 0–15 cm soil layer, ZT + R, PBB + R and PNB + R had nearly 13, 24 and 11% higher mean (n = 24; 3 replications × 8 sampling events) LLWR values than ZT, PBB and PNB plots, respectively, confirming that crop residue retention improved LLWR. Sub-surface layers under ZT had significantly lower LLWR values than in the CT plots. Results also reveal that there were no significant relationships between the mean (of two years) grain yields with LLWR for all crops, indicating that LLWR was a poor indicator of crop productivity. Overall, among the treatments, PBB + R and DSR + BM–ZTW were the best management practices for improved soil physical environment under cotton–wheat and rice–wheat systems, respectively, and therefore could be adopted.