Factors Affecting Change in Soil Test Phosphorus Following Manure and Fertilizer Application

In many states, Morgan (M) or Mehlich‐3 (M3) extractions are the basis for P fertility recommendations and P runoff risk assessment. The impact of timing of sampling, P source, and extractable Al on agronomic soil test P (STP) and P release to the environment are not well understood. Incubation studies were conducted to determine: (i) changes in M‐P, M3‐P, and 0.01 M CaCl 2 –P with time following Ca(H 2 PO 4 ) 2 addition; (ii) the efficiency of liquid dairy manure, NH 4 HPO 4 , and Ca(H 2 PO 4 ) 2 in raising M‐P and M3‐P levels; and (iii) the degree to which initial P and extractable Al impact the efficiency of applied P. Twenty‐eight noncalcareous soils were incubated at 23°C, in the dark, at field capacity moisture content. Extractable P decreased over 60 d, with a greater proportion of P being lost from the more labile pools (CaCl 2 –P > M‐P > M3‐P). The increase in STP per unit P added (ΔP‐M and ΔP‐M3) was affected by P source: NH 4 HPO 4 raised P levels more efficiently than Ca(H 2 PO 4 ) 2 or liquid dairy manure. There was a positive linear relationship between ΔP and initial STP. Extractable Al was inversely related to ΔP‐M and ΔP‐M3 and followed exponential decay functions. Extractable Al was positively and linearly related to the amount of P required to raise M‐P and M3‐P levels 1 mg kg −1 , with Morgan Al as the most accurate predictor of ΔP. Our results suggest that the New York P index should include guidance on sampling time and account for the higher P sorption capacity of soils inherently high in extractable Al.