Phosphorus (P) is needed by plants and often found in the soil — the challenge is having an adequate and constant supply of phosphorus in a form that plants can use. Canadian scientists have been studying the P cycle on farms, particularly organic farms, for several years. A recently published paper summarizes many of the findings.
First, the researchers discovered that “Crop yields could be maintained at lower than recommended soil test P concentrations.” One way to do this is by increasing the availability of the P already in the soil. Farmers can improve a crop’s access to P by ensuring there are high levels of soil organic matter and by supporting arbuscular mycorrhizal fungi (AMF). AMF form symbiotic relationships with many plant species, particularly legumes, and enable the plant roots to access more water and nutrients including P. These helpful fungi are harmed by many common farming practices including tillage, application of high rates of soluble P fertilizers, leaving soil bare and monoculture. Farmers can protect AMF by avoiding these destructive practices and also using diverse crop rotations that include green manures, particularly mixes of cover crops that include legumes.
As mined sources of rock phosphate are being depleted, soil scientists are seeking other sources of phosphorus. Struvite might be the answer. In traditional mixed farms, P was removed from the fields when crops were harvested but much was later returned in the form of livestock bedding mixed with urine (a source of P) and manure.
Struvite is created by precipitation and crystallization of liquids containing urine or other high-P materials. The result is a high-phosphorus substance that can be used as a fertilizer. The original source of the material affects the quality of the struvite. For example, struvite can be made from municipal wastewater but this may contain undesirable chemicals that have also been flushed down the drain. Another alternative is to produce struvite from livestock urine and manure or food processing wastes.
The researchers suggest a multi-faceted approach to P availability is needed. For example, plant breeders might select for crops that form symbiotic relationships with AMF or are more efficient at using P. Many of the practices recommended to improve the availability of P (e.g., complex crop rotations, protecting soil organic matter, keeping soil covered) are generally good soil management practices with many other benefits.
Source: Schneider, KD, JR Thiessen Martens, F Zvomuya, DK Reid, TD Fraser, DH Lynch, IP O’Halloran and HF Wilson. Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales. Journal of Environmental Quality. Vol. 48 No. 5, p. 1247-1264. https://dl.sciencesocieties.org/publications/jeq/abstracts/48/5/1247