Nutrient budgeting for phosphorus and potassium in a long-term fertilizer trial

Influence of N, P and K application through inorganic and organic fertilizers on P and K removal in crop plants, changes in soil fertility status and their balance in the soil-plant (maize-wheat-cowpea fodder) studied for the first 22 years of a long-term experiment at Punjab Agricultural University farm, Ludhiana, India. The results showed that P uptake by wheat was about 1.5 times that of maize, whereas K uptake by wheat was only 1.1 times that of maize. The apparent P recovery by both maize and wheat depended on the rates of N, P and K application. Fertilizer rates greater than the recommended (150% NPK) resulted in lower P recoveries. At optimum level of N, P and K application (100% NPK) the mean P recovery (for 22 years) was 30.3% (+/-5.47) in wheat as compared to 20% (+/-11.35) in maize. The apparent P recovery in maize declined as the number of cropping years progressed. In 100% NPK plots, it declined from 45.6% in 1973 to 12.5% in 1992. The decline in P recovery was due to the accumulation of plant available P in the soil which increased from the initially low status to high-very high due to continuous application of P fertilizer. The application of FYM in conjunction with 100% NPK led to significantly greater accumulation of available P as compared to 100% NPK treatment alone despite the higher amount of total P removal in the former treatment than that from the latter. A trifle build-up in available K was observed in K amended plots notwithstanding the negative balance of K based on the approach of input-output relationship. The release of K from non-exchangeable form contributed towards K uptake by the crops. The results suggested the need for modifying the existing K fertilizer recommendations to compensate for gradual loss of native soil K fertility.