Is There Enough Soil Phosphorus on Organic Farms?

by Melissa Arcand

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Organic farming practices aim to maximize cycling of soil nutrients on the farm and to minimize external inputs. While this approach to soil management can help to reduce nutrient loading, it can also pose challenges. In particular, concerns have been raised that organic farming practices lead to the depletion of soil phosphorus (P).

University of Guelph researchers recently conducted whole-farm nutrient budgets for 15 organic dairy farms in Ontario. At the time of the study there were a total of 46 certified operations in the province. The amounts of P leaving the farm in milk, animals, and crops were measured and compared to the amounts of P brought on the farm in mineral supplements, feed, and other inputs.

The researchers found that the average P budget showed a surplus of 1 kg P ha-1 yr-1, which contrasts considerably to surpluses of 37 kg P ha–1 yr–1 on confinement based dairy operations in the US. Manure handling and leaching weren’t considered in the equation and therefore a total loss of soil P was still possible among the organic dairy farms.

In the same study, 225 soil samples were collected from fields across the 15 organic dairy farms. The soil test P levels were low (<10 ppm) for roughly 50% of the 225 fields, suggesting that P may limit crop growth on these fields.

Data on the soil P status in other sectors of organic agriculture in Ontario are lacking. However, researchers in other parts of the country and abroad have found similar deficiencies in plant-available P. On the prairies where export grain production predominates, researchers found that 170 fields sampled on 14 organic farms were low in plant-available P, particularly for farms managed organically for a long time (over 30 years).

Phosphorus is a finite soil resource that is found in minerals, soil microbes and organic matter. Unlike N it cannot be fixed from the atmosphere by legumes or other crops. Depending on management, Ontario soils contain between 1000 to 3500 kg of total P per hectare in the plow layer. While this may seem like ample amounts of P, less than 1% of this total can be accessed by crops.

Under Canadian certification standards, organic farmers cannot use chemically manufactured P fertilizers, which directly supply readily available P to crops. Instead, they rely on the decomposition of organic materials produced on the farm, such as manures and crop residues, to cycle P back into the soil which may eventually become available for crop uptake depending on how much P and C is contained in the materials.

Recycling nutrients on the farm cannot solve the P deficit if more P is taken off the farm than is put back. This will continue to be a challenge for soil management on organic farms and needs to be addressed by considering potential sources of external P. However, small P surpluses and low soil test P levels may not be so grim to the growth of crops.

The reliance on organic materials may result in a shift toward more nutrients held in organic forms than in inorganic forms. This means that more P can be held within the soil microbes and organic matter, which may be released over time through mineralization. Otherwise, P can be strongly bound, both chemically and physically, to calcium and clay.

Studies have shown that microbial activity is high on organic farms relative to farms that supply readily soluble P to soils. The cycling of P through soil microbes can be rapid on organic farms even when soils are not recently amended with fresh organic materials. This suggests that there may be greater potential for the supply of plant-available P on organic farms that rely on inputs of organic materials.

Soil tests that detect plant-available P might not be appropriate for predicting the supply of P to crops from organic materials. Despite low soil test P values on the prairies, the farmers found that they still maintained acceptable yields. Further understanding of soil biology is the key to understanding how P may be supplied to crops in a farming system reliant on inputs of organic materials.

References
Entz, M.H., Guilford, R., Gulden, R. 2001. Crop yield and soil nutrient status on 14 organic farms in the eastern portion of the northern Great Plains. Canadian Journal of Plant Science 81, 351-354.

Martin, R., Lynch, D., Frick, B., and vanStraaten, P. 2006. Phosphorus status on Canadian organic farms. Journal of the Science of Food and Agriculture 87:2737–2740 (2007)

Oberson, A., Fardeau, J.C., Besson, J.M., Sticher, H. 1993. Soil phosphorus dynamics in cropping systems managed according to conventional and biological agricultural methods. Biology and Fertility of Soils 16, 111-117.

Roberts, C, Lynch, D.H., and Voroney, R.P. 2007. Farm nutrient status in relation to farm management and productivity across fifteen organic dairy farms in Ontario. Submitted to Canadian Journal of Soil Science.


Melissa Arcand is a Consultant for the Organic Agriculture Centre of Canada. Please send comments or questions by phone to 902-893-7256 or by email to oacc@nsac.ca.


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Posted July 2007