Understanding Phosphorus In Soils Is Vital To Proper Management

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).

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.