Agronomic and Environmental Soil Phosphorus Testing in Soils Receiving Liquid Swine Manure

Abstract

There is uncertainty concerning evaluation of bioavailable P in manured soils. This study assessed P availability in manured Iowa soils by measuring soil P with agronomic and environmental P tests and P uptake by corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Soil and plant samples were collected at the V5‐V6 growth stage from trials established at nine locations that received various rates of liquid swine (Sus scrofa) manure, and from farmers' fields that received other animal manures. Soil P was analyzed by the Bray‐P1 (BP), Olsen (OP), and Mehlich‐3 (M3P) agronomic tests, and by the Fe‐oxide impregnated filter paper (FeP), anion‐exchange resin membrane (RP), and water (WP) environmental tests. Soil P at a 15‐cm depth ranged from deficient to 15 times optimum levels for crops. Extracted P was highest for BP, M3P, and RP, intermediate for OP and FeP, and lowest for WP. Relationships between soil P extracted by the tests were linear, trends were similar for manured and unmanured plots, and correlation coefficients were ≥0.70 (correlations were poorest for WP). There was no conclusive evidence for differences between tests in detecting manure‐derived soil P at most sites. However, in some conditions BP, M3P, OP, and RP may extract proportionally more manured‐derived P than FeP and WP. Only the agronomic tests were significantly correlated (0.42 for M3P, 0.44 for BP, and 0.47 for OP) with plant P uptake across sites. Agronomic soil tests would predict plant P availability for crops better than environmental P tests in soils receiving liquid swine manure.