Potassium Availability as Influenced by Farmyard Manure Application under Continuous Soybean-Wheat Cropping in a Typic Haplaquept

The effect of 30 years of continuous cropping, fertilization and manuring on potassium (K) balance and availability in a sandy loam soil (Typic Haplaquept) was investigated under a rainfed soybean-wheat cropping. The total removal of K by the crops exceeded K applied to the soil in all the treatments showing a net negative K balance, ranging from 3.7 (in NK-treated plots) to 81.7 kg ha⁻¹yr⁻¹ (in N + FYM-treated plots). Continuous application of recommended doses of NPK + 10 t FYM (NPK + FYM) annually to soybean resulted in build-up of available K (+56 kg K ha⁻¹) in 0–45 cm soil depth over the initial soil value despite the highest average annual uptake of K by the system (150.8 kg ha⁻¹yr⁻¹), whereas, there was net depletion of available K (−80 kg K ha⁻¹) in 0–45 cm soil depth under the NPK-treated plots. The non-exchangeable K decreased substantially from 1274 to 1052 kg ha⁻¹ in the NPK treatment and to 986 kg ha⁻¹ in 0–15 cm soil layer in NPK + FYM treatment after 30 years of soybean-wheat cropping. The decrease in total soil K was significantly correlated to decrease in non-exchangeable soil K (R² = 0.53 **, P<0.01, n = 36). Hence, attention must be given to revise K recommendations for maintaining soil K pools.