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Understanding Phosphorus in Soils Is Vital to Proper Management

February 05, 2009

Phosphorus is one of the key nutrients that can cause algal blooms and related water quality problems in lakes, rivers, and estuaries worldwide. Phosphorus entering waters originates from a variety of sources. Agricultural land receiving long term applications of organic by-products such as animal manure is one of the major contributors. Such soils often become enriched with P, leading to elevated P loss through erosion and runoff. Information on the chemical characteristics of P in these soils is essential to improving our understanding of how P behaves in soils and how it is transported in runoff to devise better management practices that protect water quality.

A group of scientists in the USA and Australia have identified the chemical forms of P, using 31P nuclear magnetic resonance (NMR) spectroscopy, in soils receiving organic by-products for at least eight years (treated) as compared with soils not receiving P application (untreated). Results from the study were published in the January-February issue of the Soil Science Society of America Journal.
Regardless of the type of organic materials applied (dairy, swine, poultry, or spent mushroom compost), orthophosphate (inorganic P) was the single dominant P form, more so in treated soils (79-93% of total P) than in untreated soils (33-71%). Orthophosphate was also the only P form that differed dramatically between paired soils, three to five times greater in treated than untreated soils. Other P forms included condensed inorganic P and various organically bound P groups; however, their amounts were relatively small and differences between each paired soils were insignificant.

Surprisingly, the study revealed no evidence of phytate-P accumulation in any of the soils receiving organic wastes. Phytate is an organic storage form of P that is known to be present in animal manures, in particularly large proportion (up to 80% of total P) in poultry manure. Phytate-P is generally considered to be recalcitrant in the agroecosystem because of its chemical structure. However, the lack of phytate-P accumulation in several soils receiving poultry manure in this study indicates that manure-derived phytate-P may not be biologically and environmentally benign.

Zhengxia Dou, the lead author, stated "in terms of potential P loss in the long run, organic materials containing large amounts of phytate-P such as poultry manure may not differ from other material containing mainly inorganic P". Andrew Sharpley, a collaborating scientist, further explained "when the soils' P sorption capacity was nearly saturated after years of manure application, phytate or other organic P forms could be exposed to breakdown and potential loss". Therefore, it is important to strive towards balancing P inputs with outputs and to prevent P from building up in soils to which manure is applied.

The Soil Science Society of America (SSSA)