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Changes are reported in the chemical and biological composition of soil phosphorus (P) in a Black Chernozemic silt loam (Blaine Lake Association) and a Dark Brown Chernozemic sandy loam (Bradwell Association) during 60-90 years of cultivation. Cultivated and adjacent uncultivated soils were sampled, separated into particle size fractions by physical dispersion and the fractions subjected to a sequential chemical extraction to remove several forms of inorganic phosphorus (Pi) and organic phosphorus (Po).In the uncultivated Bradwell soil significant amounts (7%) of secondary (NaOH extractable) Pi forms were associated with high levels of labile (bicarbonate and resin extractable) Pi. These secondary Pi forms, which were concentrated in the finer particle size fractions ( < 2 pm), contributed to the P loss during cultivation of the coarse textured Bradwell soil, whereas all P loss in the Blaine Lake soils was due to Po losses alone. Sulphuric acid extractable P (thought to be mainly apatites) accumulated in both soils under cultivation, particularly in the coarse silt (50-5 pm) fraction. Labile P fractions were greatly reduced during cultivation, indicating a significant reduction in available P and P fertility of cultivated soils. This reduction in P fertility was closely tied to soil organic matter losses.

A sequential phosphorus (P) fractionation procedure was used to measure the changes in the labile and stable forms of inorganic and organic P following single broadcast P applications to Canadian Chernozemic soils under cereal cropping. Apg1o-limately half of the fertilizer residues remained in plant-available foims (resin, NaHCor). In a Black waskada clay loam 8 yr after the application of 200 and 400 kg P ha-', residual fertilizer p consisted of resin-p, 3\ab%; HCI_P, 25_3OVo;res_ idue-P, lU-l1Vo; NaOH-P, l}-l1Vo, NaHCOT-p, l\Vo; and aggregate protected p, 3Vo.The residues in a Dark Brown Sutherland clay 5 yr after the application of 160 kg P ha-t were: resin-P, 35Zo; NaOH-p , 3040Vo; Nai{COr-p, rcn;gCl_p , USE\; Hrso4-P, 5vo; and aggregate protected p, 5vo. The soils differed in the quantity of fertilizer recovered in inorganic HCl-extractable forms. In the Sutherland soil-the change from wheat-fallow to continuous wheat cropping produced a build-up of organic P which occurred with and without the addition of p fertilizer. Stewart 1975Williams et al. 1967). Experiments to evaluate fertilizer P residues using plant uptake or extractable P in soils measure only the labile, plantavailable soil P pool and do not document the fate of all the applied P. A more complete assessment of residual P in soils would include all soil P pools, as transformations among these pools may alter the long-term availability of fertilizer P residues.The purpose of the present work was to build upon previous studies which documented the effect that large applications of fertilizer P had on grain yield and labile inorganic P pools in Chernozemic soils (Bailey etal. 1977;Read et al. 1973 Read et al. ,1977Spratt et al. 1980 (Bailey et al. 1977;Read et al. 1973 Read et al. , 1977 0' 20' 40' 80 and I 60 kg P ha-' to a Sutherland soil. The field plan, yields and nutrient uptake of this study have been described previously (Wager et al' 1986). The location, classification and characteristics of the soils used in this study were obtained from Read et al. (1977) and Souster (1979 0-, 20-, 40-,80-and 160-kg P ha-' treatments was analyzed individually to allow for statistical interpretation. Phosphorus FractionationIn this procedure (Fig. l), labile inorganic P (P,) and organic P (P.) forms are removed first, followed by more stable P forms removed with stronger extracting agents (Hedley et al. 1982

In fertilized agricultural soils, both fertilizer and soil P are partly depleted by crop export, while residual fertilizer P reacts with the soil. As a result, P availability changes in complex ways that cannot be described by an available P pool. Our objective was to characterize the P availability in two long‐term field experiments on a Mollisol and an Alfisol (near Lethbridge and Breton, Canada), by using the soil solution P (Cp) and the time‐dependent isotopically exchangeable P (Et). The relationship of Cp to Et was further evaluated for short‐term reactions by incubating field samples for 1 d with increasing P rates. Both Cp and Et were related to changes in the soil P budget resulting from continuous cropping and wheat (Triticum aestivum L.)‐fallow rotations with and without fertilization, with lower Cp values in the more depleted soils. In the Alfisol, field variations in pH and texture affected both Et and Cp, obscuring relationships between the field P budgets and availability parameters. In the Mollisol, one single equation described the relationship between Et and Cp for both increases of Cp due to short‐term additions and decreases due to long‐term crop export, indicating that transformations of available P were fully reversible.

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