Soil Phosphorus Sources and Their Relative Water Solubility and Extractability

Abstract

ABSTRACT Soil testing is a tool used to predict crop response to residual soil phosphorus (P). It is commonly perceived that high soil test P indicates high potential for off-site movement and impact on surface water quality. However, P source potentially confounds the relation between soil test P and soil P solubility. This study evaluated how soil test P (Mehlich-1 and Mehlich-3) and total P (TP) related to water-soluble P (WSP) for manure-(n = 120) and fertilizer-amended (n = 120) soils, and soils forming in phosphate-rich parent material (phosphatic soils; n = 60). Results document marked differences in slopes of WSP as a function of soil test P and TP for the three P sources. Manure-amended soils showed the highest regression coefficients (R 2 = 0.89, 0.89 and 0.92) and slopes for WSP but phosphatic soils, in contrast, showed no tendency for increased WSP with increasing soil test P or TP despite having the highest range of TP content. Fertilizer-amended soils had lowest values for all P measures. Manure-applied P accumulated to much higher levels in these sandy soils than P applied as inorganic P suggesting the latter is either quickly recovered by crops or lost. Phosphate minerals in naturally phosphatic soils do not likely constitute the same extractable P pool as for agriculturally amended soils. Hence, soil test extractions may not be well-calibrated for phosphatic soils. Standard soil test P values may not be a reliable indicator of high P leaching loss potential for naturally phosphatic soils, to the extent that WSP is related to leaching risk.