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Nutrient Management on Organic Farms

Organic farmers must have a crop production plan, including rotations, that documents the need for various fertilizer amendments and includes soil testing and any deficiency observations. Managing nutrients on organic farms is more complicated than in conventional cropping systems. Organic farmers chose not to use the common fertilizer products such as urea, ammonium nitrate, ammonium sulphate, triple phosphate, MAP, DAP, or muriate of potash. There are concerns for the environment from the use of highly soluble fertilizers, and for the environmental impacts and high-energy consumption in their manufacturing.

Organic farmers recycle nutrients from livestock (manure) or crop operations (culls, trimmings) or to use naturally mined fertilizer inputs. They also need to reduce losses of nutrients from their farm system. This both protects the environment and conserves the nutrients for crop use. One of the keys to conserving nutrients is to have a healthy organic matter content in the soil. Only allowable substances and practices can be used on organic farms in the 36 months prior to the harvest of certified organic crops. Municipal wastes such as biosolids are not allowed for use on organic farms.

Livestock manure is the most economical and effective source of nutrients for organic farms. The most frequently recommended source of nutrients is composted manure. Manure must be composted for 6 months. Both the Canadian and USDA organic standards have some requirements for temperatures and C:N ratios to enhance the composting process. Raw manure cannot be used within 120 days prior to the harvest of crops where the edible portion of the crop is in contact with the soil or 90 days for other crops. Raw manure can be used post-harvest where it is followed by a cover crop to capture nutrients and reduce losses.

Legumes are also key to maintaining nitrogen levels. Legumes such as clover or alfalfa have the great capacity to produce nitrogen (45-100 kg/ha). Including a full year for forage production increases the amount of nitrogen production compared to a plowdown crop that only grows for a few months. Beans and pea crops are legumes and can supply lower amounts of nitrogen to the system (0-30 kg/ha), but in many cases these pulses are poor at supplying adequate amounts of nitrogen for future crops. Clover and alfalfa crops also have a larger root mass to improve soil structure.

Crop rotation is a key to managing nutrients as well as controlling pests. In most cases it is not feasible to apply compost every year. Some crops need higher levels of nutrients and they are usually grown in the year following a compost application. Crops with lower fertility requirements are grown in subsequent years. Similarly crops requiring higher nitrogen levels are grown following legume crops or cover crops. Cover crops are used to retain nutrients in the post harvest periods. Increasing the cover crops also helps to increase soil organic matter and to stimulate the soil flora and fauna (all of the critters in the soil) and the various nutrient cycles.

Other permitted off-farm nutrient products include (always check with your certification body before using to confirm status):

• Lime - both dolomitic and calcitic
  
• Potassium sulphate - The one that is allowed for use in organic agriculture is from a natural source and is not altered
  
• Sulphate of potash magnesia - The K-mag "Natural" and Sul-Po-Mag "Natural" are the products allowed for organic agriculture
  
• Rock phosphate
  
• Bone meal - is permitted but there are some concerns, especially in Europe due to the risk of BSE contamination

There are numerous other products that can be used to complement a good crop rotation and the use of compost where the need is documented. Organic Associations in Atlantic Canada have a website that lists many of these with more information on each.

Learn more

• Atlantic Canadian Organic Regional Network