Neutral ammonium citrate extraction of biosolids phosphorus

Matching biosolids application rates to crop phosphorus (P) needs requires quantifying the P fertilizer replacement value of biosolids. Neutral ammonium citrate (NAC) extraction of P, used for assessing available P in mineral fertilizers, was evaluated for 35 different biosolids. Biosolids NAC-P was not statistically different (p = 0.05) from total P using strong acid digestion (EPA 3051-P). High P recovery by NAC was attributed to dissolution of P-containing iron (Fe)/aluminum (Al) oxides under the aggressive extracting conditions (0.88 M citrate at 65°C). Citrate effectively dissolves P-binding Fe/Al hydrous oxides, the very components that reduce phytoavailability when biosolids are land applied. Greenhouse studies with pasture grass (Paspalum notatum Flugge) grown in P-deficient soils amended with biosolids revealed P phytoavailability was not correlated (r² = 0.10) with biosolids NAC-P. Phytoavailability was inversely correlated (r² = 0.66) with biosolids total Al + Fe content. The NAC extraction, designed for commercial fertilizers, is inappropriate for quantifying biosolids phytoavailable P.