Farm Table says:
Organically produced fruit and vegetables are among the fastest growing agricultural markets. With greater demand for organically grown produce, more farmers are considering organic production options. Furthermore, there is an increasing interest in maintaining optimal production in an organic system, which involves appropriate nutrient management.
The objectives of this review were to summarize the current state of our knowledge concerning effects of organic production systems on phosphorus (P) availability, describe P availability in common organically accepted P sources, and review best management practices that can reduce environmental risks associated with P management in organic systems. Organic production systems seek to improve soil organic matter and biological diversity, which may impact P cycling and P uptake by crops.
Phosphorus is an essential element for plant growth and is involved in many plant metabolic functions.
• an essential component of adenosine diphosphate and adenosine triphosphate—organic molecules that are used for energy storage and transfer.
• a structural component of nucleotides, phospholipids, phosphoproteins, and coenzymes.
Organic agriculture effects on soil properties and phosphorus availability:
• The primary means by which organic production practices would influence P availability would be through alteration of soil organic matter content, increased P availability from degradation of cover crop residues, and increased arbuscular mycorrhizal fungi (AMF) colonization resulting from lack of soluble P fertilizer applications.
Nitrogen source effects on phosphorus:
• Both high P application rates and high soil test P can increase the risk of environmental damage resulting from P losses to surface waters
• Organic producers using manures and composts as a major N source will likely have high soil test P and their P management strategies should focus on reducing P losses to surface waters.
• On the other hand, organic producers who use manures and composts sparingly or not at all will likely have greater crop P removal than P inputs and may need supplemental P sources to sustain adequate crop production.
• Soil testing and knowledge of management history will be the best tools for indicating if P is a limiting nutrient in the production system.
Phosphorus sources of organic agriculture:
• Common P sources include rock phosphate, manure, and compost, all of which are frequently used in research studies.
• Bone meal and guano are among the less commonly cited P sources but can have high P contents (ranging from 7% to 12% and 1% to 9%, respectively).
Environmental issues associated with phosphorus management:
• Phosphorus losses from agricultural systems generally occur through surface runoff and erosion followed by transport to streams and rivers in concentrated flow processes
• Any surface application of manure or compost should be regarded as an increased risk of P loss.
Organic production systems seek to improve soil organic matter and biological diversity, which may impact P cycling and P uptake by crops.
• Increases in organic matter will be accompanied by increases in organic P. Management of cover crops and potentially enhanced AMF colonization can increase the availability of soil P pool (both organic and inorganic) by stimulating microbial activity and release of root exudates. This can help compensate for low soil P, but will not supersede the need to replace P removed by the harvested crop.