Freezing significantly influences the amount and chemical form of extractable soil phosphorus and, therefore, has important consequences regarding plant nutrient availability. Much of the current evidence relies on field-based observations and tends to be rather contradictory. The present work reports the specific effects various freezing episodes have on phosphorus availability. Experimental treatments included the freezing rate and the time soil remained frozen together with the number of freezelthaw cycles. These laboratory incubations used two soils, an iron humus podzol and a peaty podzol, sub-samples of which were equilibrated at three different soil-moisture contents. Concentrations of phosphorus in soil solutions and acetic acid extracts (Ac-P) were measured. Freezing had the greatest effect on the organic (Op) horizon of the peaty podzol, where the amounts of total dissolved phosphorus (TDP) and Ac-P increased from 0.58 to 3.21 and from 11.8 to 19.3 mg P kg-l, respectively. TDP in soil solution was subdivided into three broad categories, dissolved (molybdate) reactive (MRP), organic (DOP) and condensed (DCP). The rather variable effect that freezelthaw episodes had on these fractions highlighted the problems associated with attributing the 'freezing effect' to any single process.

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The influence of soil acidity and phosphorus fertilization on phosphorus fractions and dissolved organic carbon (DOC) in soil solution was quantified experimentally in an iron humus podzol. Soil solution was isolated by centrifugation from top-and sub-soil samples. Total dissolved phosphorus (TDP), dissolved reactive phosphorus (DRP), dissolved organic phosphorus (DOP) and DOC increased as soil pH and P status increased. DOP was the fraction present at the highest concentration (0.080-0.464 mg P L -1) for the majority of samples. DOC and DOP concentrations which remained relatively constant down the soil profile were also highly correlated. Soluble organic P compounds may make a significant contribution to plant available soil P particularly for soils with low fertility levels. The relatively high DOP concentrations (ca 0.227mg P L -l) found throughout the soil profile have important consequences with regards to P leaching and plant nutrition.

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