Conservation Agriculture in a High Fertilizer Input Rice-Based Cropping System Improves Soil Fertility and Crop Productivity but Fails to Correct Negative K Balance

Potassium (K) depletion of soils in the intensively-cropped Ganges River Floodplains is a key challenge for maintaining crop productivity. Increased residue retention and decreased soil disturbance (i.e., Conservation Agriculture) can reverse negative K balance but this has not been investigated for high input systems such as the potato-maize-rice sequence. A two-year experiment was conducted with an annual potato-maize-rice rotation that involves a wide range of K input along with CA practices. Treatment factors were: (a) soil tillage (conventional = CT and strip planting = SP); (b) crop residue retention (low, LR = 15 cm and HR = 40 cm by height); and (c) K application (100% K (recommended level, RD), 50% or 75% K of RD (low level, LD), and 125 or 150% K of RD (high level, HD)). Soil samples were collected after harvesting rice in December 2018 and again in December 2020 after six crops to analyze physical and chemical properties. In 2020, soil pH, total soil organic carbon, total nitrogen, extractable phosphorus (P), sulfur (S), and potassium (K) all increased while soil bulk density decreased significantly due to CA practices. In 2018–19, rice equivalent yield (REY) was 7% higher in HR than LR, while SP-HD and SP-RD had 25% and 15% higher REY, respectively, than CT-RD. In 2019-20, SP-HR-HD achieved a 34% higher REY than CT-LR-RD. During the 2018-19 period, the K balances for the initial cropping cycle were negative, varying from − 60.7 to -89.8 kg ha ^-1 yr ^-1 for crop establishment methods, -61.3 to -89.9 kg ha ^-1 yr ^-1 for residue levels, and − 49.5 to -102 kg ha ^-1 yr ^-1 for K applications. In 2019-20, SP-HR-HD (24.3 kg ha ^-1 yr ^-1 ) showed a positive K balance while a highly negative balance was found in CT-LR-RD (-96 kg ha ^-1 yr ^-1 ). The K balance values were less negative for the SP and HR combination than for the CT and LR applications. In the high K input potato-maize-rice rotation, while strip planting and higher residue retention significantly reversed negative K balances, improved soil fertility and increased cropping system yield, additional K fertilizer was necessary to prevent soil K depletion.