Using Pi soil test to estimate available phosphorus in biosolids-amended soil*1

Abstract

The Pi soil test uses iron (Fe)-oxide coated filter paper strips as an ion sink. This method has been used successfully to assess phosphorus (P) availability in a variety of soils. However, information on its effectiveness in biosolids-amended soils is limited. This study was conducted to determine the potential of Pi test in assessing P-availability in a biosolids-amended acid soil, and compare the assessment with chemical extractants, such as the Mehlich I and a salt solution (1 M KCl). Soil and plant samples were analyzed for P from a field study where bahiagrass (Paspalum notatum Flugge) was grown in a sandy Spodosol, amended with two Class A biosolids at three rates. Biosolids supplied significant amounts of extractable P to the soil, particularly at higher application rates, and the effects persisted even 3.5 y after initial application. Plant P uptake decreased over time, but varied significantly with biosolids application rates. Best correlations between plant and extractable soil P were obtained using the Pi test, followed by Mehlich I. Though P extracted by 1 M KCl explained 90% of the variation in plant P uptake in the first cropping season, the correlation decreased in subsequent seasons, as soluble P leached out of the root zone. The Pi test and the Mehlich I methods extracted significant portions of P from the same sources as KCl, presumably the soluble, “readily available” P pool. In addition, these methods extracted P from other “slow-release” sources not available for extraction by KCl, but to which the plants responded. Results suggest that the Pi test can serve as an analytically satisfactory, theoretically reasonable, and practically acceptable procedure for assessing available P in soils amended with biosolids. *Contribution of the Agric. Exp. Stn. Journal Series No. R-07613.