Influence of liming on soluble and labile P in fertilized soil

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Abstract
The influence of liming on soil P was investigated using samples from field plots which had been treated with different rates (0, 1.67, 3.33 and 5.00 tons/acre) of ground limestone 17 years previously. The plots, which were on established pasture, received annual applications of fertilizer P. Liming raised soil pH from 4.7 to 6.2 and increased the concentration of soil solution P from 0.29 to 0.67 μg/ml. Various indices of P availability (Morgan's, NaOH‐P, HCl‐P, and isotopically exchangeable P) were found to be poor indicators of the effect of liming on P solubility. Desorption of P in NaCl solutions was highly dependent on ionic strength, suggesting the presence of a more‐physically sorbed form of P. The P release characteristics of the soils were examined by sequential extraction with 0.2 M NaCl followed by extraction with distilled water. The total amount of P desorbed, which was taken as a measure of labile P, increased from 271 to 348 μg/g with liming. The results identify the more‐physically sorbed P with the labile P in soils. It appears that chemisorption of added P is reduced when soil pH is raised and, consequently, the amount of loosely‐bound, more‐physically sorbed P increases with liming.