Effective phosphorus management on organic farms is based on two complementary approaches – helping crops access the mineral and then recycling the nutrient back into the soil.
In recent research at the University of Manitoba’s Glenlea long-term rotation study, applying composted beef cattle manure to an organic forage-based crop rotation increased soil phosphorus levels and caused higher yields in subsequent grain crops.
Cathy Welsh of the U of M’s Soil Ecology Laboratory studied the effects of crop rotation and organic vs. conventional management on soil phosphorus in the Glenlea study. Established in 1992, the study compares the productivity and sustainability of annual and forage-based crop rotations under organic and conventional management.
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Welsh found that soil levels were affected by the management system and crop rotation. Soil phosphorus was lower in organically managed crop rotations than in conventionally managed rotations where phosphate fertilizers were used. Among organic crop rotations, soil phosphorus was lowest in a forage-based rotation (wheat, alfalfa, alfalfa, flax) and highest in an annual grain rotation (wheat, pea, wheat, flax). Soil phosphorus in a forage-based rotation that received one application of manure was intermediate between the other two rotations.
The organic forage system depleted the most plant-available forms of phosphorus, but also the forms that are only slightly available to plants.
Two major factors contributed to the difference in soil phosphorus between the annual and forage-based rotations. First, large amounts of phosphorus were removed from the forage-based rotation when alfalfa hay crops were harvested. A 2.5 ton per acre alfalfa hay crop removes about 2.5 times as much phosphorus as a 30 bushel per acre spring wheat crop. Secondly, wheat and flax yields were higher in the forage-based rotation than in the annual rotation because of the nitrogen supplied by the two-year alfalfa phase. Higher grain yields meant that more phosphorus was removed from the system.
In the fall of 2002, after phosphorus depletion was observed in the forage rotation, manure was applied to half of each plot in the forage rotation at a rate of 4.5 tons per acre. This manure application replaced almost half of the phosphorus removed from the organic forage system between 1992 and 2005, according to Welsh’s measurements. Adding manure also increased 2004 wheat yields by 32 percent in the organic forage rotation, likely because there were higher levels of available phosphorus, since soil nitrogen levels were already adequate.
Applying manure is an effective method of recycling phosphorus back into the system after it is exported as hay and consumed by cattle. Applying manure more frequently or allowing cattle to return nutrients themselves while grazing alfalfa stands would help to prevent phosphorus depletion and maintain crop yields in a forage-based system.
Recycling phosphorus through livestock can occur at the farm scale or at a regional level. Manure can be transported from livestock operations to grain or hay fields, or livestock can be temporarily moved to stockless farms for grazing and nutrient deposit, as long as requirements from organic certifying bodies are met.
Where manure is not available, alfalfa pellets and wood ash can be used as external sources of nutrients, although little is known about how well these products work as phosphorus fertilizers. Adding rock phosphate does little to increase levels of available phosphorus on acidic prairie soil.
While replacing exported phosphorus is crucial for long-term mineral management, there are also agronomic practices that can help plants access phosphorus and increase biological cycling of phosphorus through plants and back into the soil.
Legumes and buckwheat in rotation increase phosphorus availability by producing acids that break down the bonds that hold the mineral to other soil compounds. Fungal inoculants such as Jumpstart work in much the same way.
Naturally occurring mycorrhizal fungi associate with many crops, including flax, corn, legumes and cereals, and help the plant take up the phosphorus that is already available but out of reach. Welsh found that these beneficial fungi increased in organic rotations, both in numbers and diversity. Commercial inoculants such as Myke Pro can increase mycorrhizal colonization in crops when natural populations of these fungi are low.
While much work remains to be done to address phosphorus depletion on organic farms, practices such as manure application and promotion of natural phosphorus biocycling are valuable tools for phosphorus management.
Joanne Thiessen Martens is a research associate with the Organic Agriculture Centre of Canada working in collaboration with Dr. Martin Entz at the University of Manitoba. She welcomes comments at 204-474-6236 or by e-mail at j_thiessen_martens@umanitoba.ca.
