Phosphorus Sources and Management in Organic Production Systems

Abstract

<jats:p>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. Increases in organic matter will be accompanied by an increase in the organic P pool. Furthermore, management of cover crops and potentially enhanced arbuscular mycorrhizal fungi colonization from organic production practices 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. Phosphorus fertilization in organic production systems entails balancing the P inputs with crop removal through selection and management of both nitrogen (N) and P inputs. Organic production systems that rely on manure or composts for meeting crop N demand will likely have a P surplus; therefore, P deficiencies will not be an issue. Systems using other N sources may have a P deficit, therefore requiring P supplementation for optimal plant growth. In such situations, maintenance P applications equal to crop removal should be made based on soil test recommendations. Primary organically approved P sources are phosphate rock (PR), manure, and compost. Phosphate rock is most effective at supplying P in soils with low pH (less than 5.5) and low calcium concentrations. Phosphate rock applications made to soils with pH greater than 5.5 may not be effective because of reduced PR solubility. Manure- and compost-based P has high plant availability, ranging from 70% to 100% available. Use of manures and composts requires extra considerations to reduce the risk of P loss from P sources to surface waters. Best management practices (BMPs) for reducing source P losses are incorporation of the manures or composts and timing applications to correspond to periods of low runoff risk based on climatic conditions. Organic production systems that use manures and composts as their primary N source should focus on minimizing P buildup in the soils and use of management practices that reduce the risks of P loss to surface waters. Evaluation of P loss risk with a P index will assist in identification of soil and management factors likely to contribute to high P loss as well as BMPs that can decrease P loss risks. BMPs should focus on controlling both particulate and dissolved P losses.</jats:p>

Legumes have a greater effect on rhizosphere properties (pH, organic acids and enzyme activity) but a smaller impact on soil P compared to other cover crops

“…This suggests that MF provided limited P benefits to these species, consistent with mixed results reported in Richardson et al (2011) but in contrast to many studies that report significant MF benefits for plant P acquisition (Frossard et al 1995;Nelson and Janke 2007). The lack of P benefits for mycorrhized cereals in this study could be due to a lower MF dependence for P acquisition in the cereals studied here (e.g., wheat) vs. other mycorrhized crops such as maize or soybean (Ryan and Kirkegaard 2012).…”

“…Cover crops -crops that are returned to the soil rather than harvested -influence soil P dynamics by taking up P at times when fields would otherwise be fallow, and potentially by increasing soil P mobilization (EichlerLoebermann et al 2008;Nelson and Janke 2007). Because cover crops are not harvested, tradeoffs between yield and allocation to belowground processes will be less important than for harvested crops.…”

Many shades of gray—The context-dependent performance of organic agriculture

“…Because of the low N:P ratios of many organic inputs, organic farmers often overfertilize for P while trying to match crop N requirements ( 65 ). High-value organic horticultural systems, which often apply large quantities of external organic matter to avoid N limitation, typically have the highest P surplus, whereas organic arable systems, which rely more on BNF (biological N fixation) for N management, often have a P deficit ( 63 – 65 ).…”

Advances and Trends in Organic Fruit and Vegetable Farming Research

“…Indeed, the imbalanced nutrient composition of many organic amendments relative to plant nutrient uptake may result in excess P content in the soil over time and an increased risk of P contamination of water, which may lead to major environmental burdens (Nelson and Janke 2007;Rosen and Allan 2007). For example, imbalanced ratios between N, P, and K (42:11:47) were observed for tomato as compared with the crop demand (37:6:57) (Voogt et al 2011).…”