Evaluation of Soil Test Phosphorus Fertilizer Use in Maize-Wheat Cropping Sequence in a Typic Ustochrept.

Maize and wheat crops are highly responsive to phosphorus (P). Due to low P efficiency of northwestern India soils, farmers have over applied P fertilizers over many decades to improve crop growth and increase profitability. As a result, a considerable part of cultivated land of this region qualifies for ‘high P’ (9-20 mg kg^-1) and ‘Very high P’ (> 20 mg kg^-1) as per regional soil test based classification for phosphorus. Depletion of P sources and its increasing fertilizer subsidy expenditure call for its economization. The objective of this study was to work out the effect of different levels of P application on soil test phosphorus, critical concentrations  and the relationship between maize and wheat grain yields at three sites: PAU-1, farmers field, PAU-2 (artificially created with graded levels of fertilizer P application) having different initial soil P levels . The initial soil test P values at PAU-1 were 28 mg kg^-1 and three basal fertilizer levels of 0, 13 and 26 kg P ha^-1 were evaluated for 11 years. PAU-2, having three initial STP levels of 8.5, 17 and 25 mg kg^-1 and farmer’s field, typical of long term over use of P with initial soil test-P of 26 mg kg^-1 with five basal fertilizer levels of 0, 13, 26, 40 and 52 kg P ha^-1 at both sites were evaluated for 3 years to test the results of PAU-1 long term experiment. Results at PAU-1 indicated a gradual, linear and a less steep decline in soil test- P during 11 years of study. On average, P declined 2.4 kg P ha^-1 yr^-1 during 11 years. Similar P trends at farmers field for without P fertilizer plots were noticed. P rates of 26 kg P ha^-1 showed STP buildup and highest buildup occurred with 53 kg P ha^-1 at all three sites. At PAU-1, maize and wheat grain yields revealed a significant (P<0.05) impact of soil test P in 6 of 11 years of experiment in no P plots. Our results showed that sustaining maize and wheat yields under high P soils, 13 kg P ha^-1 may be applied annually and for very high P soils, the P application can be omitted until the soil test level declined below the critical concentration of 17 mg kg^-1. Critical concentrations approved by various statistical methods across three sites were 20.0 to 21.0 mg P kg^-1 for maize and 21.0 to 25.5 mg P kg^-1 for wheat crop. However, by graphical Cate Nelson (CN) was 17 mg P kg^-1 for both crops. The results of 18 short-term experiments were in agreement with graphical CN. Therefore, the present study signifies a considerably useful departure from the existing guidelines for fertilizer P application, which recommend using 20 kg P ha^-1 under high P environment and no P addition when soil test phosphorus exceeds 20 mg kg^-1.