Characterizing soil phosphorus supply on aggrading and degrading management regimes
Practical concerns regarding P fertility usually focus on crop maturity and quality. However, on highly degraded soils P supply can be so low as to limit grain yield. A study of soils treated with conservation and conventional tillage methods indicated that mineralizable fractions of organic N and S were enhanced under conservation managements. Increased soil organic matter quality on these managements may also increase organic P supply. However, assessing the importance of organic P using soil incubations is of little value because mineralized P is quickly fixed as insoluble forms. In this study, we investigate the supply of P on aggraded and degraded soils, using a sequential chemical fractionation which has been operationally linked to plant uptake and net P supply. We also quantify the size of these P fractions on severely degraded and manure amended soils in an attempt to link visual P deficiency and sufficiency to soil P levels. Total and organic P differ little among tillage regimes. Native grassland contains the least mass of P in the 10 em surface layer, which indicates the importance of fertilizer additions and mixing of mineral soil. Available P fractions are 1.5 to 2.5 times higher in the aggraded conservation managements, reflecting the input of fertilizer P. Trends in available Pi for cultivated soils confirm previous soil quality rankings based on N and S supply. The chemical fractionation of available inorganic P (Pi) clearly delineates the sufficiency of P supply to durum wheat plants. Severely degraded soils showing visual P deficiencies, have 88 % less available Pi than the aggraded manure amended soil, while total and organic P differed by less than 25 %. Therefore, sequential chemical fractionation is a more sensitive indicator of P supplying power than is total or organic P.
potential P supply
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