Relationship Between Soil Test Phosphorus and Phosphorus Release to Solution

Continued fertilizer applications in excess of those required for optimum plant growth can increase soil phosphorus (P) concentration and the potential for P movement to surface waters, which can contribute to freshwater eutrophication. Although soil test methods were developed for soil fertility assessment and fertilizer recommendations, they are frequently used for environmental risk assessment because of a lack of consensus on what constitutes a technically defensible environmental soil P test. Several studies have found soil test P (STP) is related to the concentration or release of P into soil solution-overland, subsurface, or drainage flow-by two linear relationships of significantly different slopes (P < 0.05) on either side of a change point for a limited number of soils. Thus, we investigated the existence and behavior of a change point in soil P release for a wide range of variously managed soils from the United Kingdom, New Zealand, and the United States. The soils varied in pH (3.0-8.2), organic C (1-172 g kg−1), and P (2-173 mg kg−1 as 0.5 M NaHCO3 extractable P (Olsen P) and 21-553 mg kg−1 as Mehlich-3 P). Soil P release was determined by CaCl2 extraction (5:1 solution to soil ratio for 30 min). For all soils, CaCl2-P increased with STP as either Olsen or Mehlich-3 P (representing a quantity/intensity relationship typical of sorption-desorption isotherms). Statistically significant (P < 0.05) change points for Olsen P occurred in most soils (20-112 mg P kg−1) and for Mehlich-3 P for the United States soils (120-190 mg P kg−1). Soil P release (CaCl2-P) increased at a greater rate per unit STP increase above these change points than below. Where no change point was detected, it was found that sampled soils were either of low or high P saturation and, thus, were grouped below or above the change point. The change point could be estimated to within 40% of the determined value with a minimum of eight randomly selected samples (4 on either side of the change point).