Effects of increasing soil pH to near-neutral using lime on phosphorus saturation index and water-extractable phosphorus

Abstract

We studied the effects of liming to increase soil pH from acidic to near-neutral on the degree of phosphorus saturation (DPS), the P saturation index (PSI), Mehlich-3 P ( PM3), and water-extractable P ( Pw). Soils collected from a long-term highbush blueberry experiment were incubated at 22.5 °C for 93 days after CaCO3 amendment to increase pH values from 4.1, 4.8, and 5.5 to 6 and from 5.8 to 6.5. Liming decreased PM3 by 8%, 6%, 10%, and 11% with increasing initial soil pH. The PM3 concentrations of all the studied soils belonged to the very high class with critical DPS > 25%, which are associated with increased environmental risk of P loss with runoff. For soils with initial pH values of 5.8, Pw was 3.65 mg kg–1, in line with critical DPS > 25%, but decreased to 2.74 mg kg–1 with CaCO3 addition. In contrast, soils with initial pH < 5.5 had lower Pw concentrations and CaCO3 did not significantly decrease Pw at the end of the incubation averaging 1.02, 1.11, and 1.43 mg kg–1 for initial pH 4.1, 4.8, and 5.5, respectively. The low Pw concentrations of soils with initial pH < 5.5 were in line with low PSI (5.2%–6.1%), but did not reflect DPS values > 25%. It is possible that high exchangeable aluminum (Al) (AlM3 > 2500 mg kg–1) enhanced the fixation of phosphate ions from the soil solution, thus reducing Pw. Our results suggest that using PM3 as a sole indicator of environmental risk likely underestimates potential P losses compared with Pw.