Potassium balance as influenced by farmyard manure application under continuous soybean–wheat cropping system in a Typic Haplaquept

The effect of 30 years of continuous cropping, fertilization and manuring on the potassium (K) balances, the soil K pools and the non-exchangeable K release in a Typic Haplaquept soil from Almora, India under a rainfed soybean–wheat cropping system were investigated. The apparent K balance was measured as the difference between the total K added and that removed by the crops. The results showed that the total removal of K by the crops exceeded the amount of total K applied to the soil in all the treatments showing a net negative K balance. This ranged from 3.7 in the plots under NK to 81.7 kg ha− 1 year− 1 in the N + FYM treated plots. Continuous annual application of recommended doses of NPK + 10 t FYM (NPK + FYM) to soybean resulted in an accumulation (+ 56 kg K ha− 1) of exchangeable K (1 N NH4OAc extractable K) in the 0–45 cm soil depth over the study period, despite the highest average annual uptake of K by the system (150.8 kg ha− 1 year−1). However, there was a net depletion of exchangeable K (− 80 kg K ha− 1) in that soil depth under the NPK treated plots. The results also revealed that the content of non-exchangeable K decreased substantially from 3482 kg ha− 1 to 2677 and 2896 kg ha− 1 in the 0–45 cm soil layer after 30 years of cropping in the plots under NPK + FYM and NPK treatments, respectively. There was a significant decline in total soil K with the removal of non-exchangeable soil K in the surface (0–15 cm) soil layer (R2 = 0.526, P < 0.001, n = 36). Thus, long-term application of non-revised recommended fertilizer rates may threaten sustainability of the rainfed continuous soybean–wheat system.