Impact of Phosphorus Source on Soil Test Phosphorus Increase.

Soil test phosphorus (STP) can increase over time with repeated P applications, exacerbating P loss risk from agricultural fields. Better predictions of rate of change of STP after P applications would aid in setting limits on P applications to land. Here we investigated change in Morgan STP after 28 d soil incubations on 14 soil types from across New York treated with different P sources: (1) ammonium polyphosphate (APP), (2) granular monoammonium phosphate (MAP); (3) dissolved MAP; (4) raw dairy manure; (5) separated liquid dairy manure; (6) separated dairy solids. In addition, change in STP with addition of manure treated with aluminum compounds (Al₂(SO₄)₃ and AlCl₃) was investigated. Soil type, P source, and aluminum compounds influenced the change in STP_Morgan but relative differences were similar across soil types. Across all soil types, granular MAP raised STP_Morgan more than APP (0.80 vs 0.60 mg P_Morgan kg^-1 mg P_added^-1, respectively), both of which increased STP_Morgan more than all other P sources. Treatment of manure with Al₂(SO₄)₃ or AlCl₃ resulted in a 58% smaller increase in STP_Morgan mg P_added^-1. Based on these results, New York dairy farms that maintain whole-farm mass balances <13 kg P ha^-1 (feasible P balance) increase STP_Morgan levels from 4.5 mg kg^-1 (agronomic critical value) to 40 mg kg^-1 (current Phosphorus Index P application cutoff), after seven granular MAP and 36 liquid manure applications, assuming no losses. Nutrient management plans should incorporate regular soil testing and should credit farmers that minimize STP_Morgan increase over time.