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

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 ( P M3 ), and water-extractable P ( P w ). Soils collected from a long-term highbush blueberry experiment were incubated at 22.5 °C for 93 days after CaCO 3 amendment to increase pH values from 4.1, 4.8, and 5.5 to 6 and from 5.8 to 6.5. Liming decreased P M3 by 8%, 6%, 10%, and 11% with increasing initial soil pH. The P M3 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, P w was 3.65 mg kg –1 , in line with critical DPS > 25%, but decreased to 2.74 mg kg –1 with CaCO 3 addition. In contrast, soils with initial pH < 5.5 had lower P w concentrations and CaCO 3 did not significantly decrease P w 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 P w 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) (Al M3 > 2500 mg kg –1 ) enhanced the fixation of phosphate ions from the soil solution, thus reducing P w . Our results suggest that using P M3 as a sole indicator of environmental risk likely underestimates potential P losses compared with P w .