Keys to Phosphorus Management are Cycling and Recycling

By Joanne Thiessen-Martens

Effective phosphorus (P) management on organic farms is based on two complementary approaches – recycling exported nutrients back into the system and helping crops access soil P and then cycling it 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 P levels and caused higher yields in following grain crops.

Cathy Welsh, a graduate student of the Department of Soil Science’s Soil Ecology Laboratory, studied the effects of crop rotation and organic vs. conventional management on soil P dynamics in the Glenlea Study. Established in 1992, the Glenlea Study compares the productivity and sustainability of annual and forage-based crop rotations under organic and conventional management.

Welsh found that P available to crops (soil test P) was affected by both management system and crop rotation. Available soil P was lower in organically managed crop rotations than in conventionally managed rotations where phosphate fertilizers were used. Among organic crop rotations, available soil P was lowest in a forage-based rotation (wheat – alfalfa – alfalfa – flax), and highest in an annual grain rotation (wheat – pea – wheat – flax). Available soil P in a forage-based rotation that received one application of composted manure was intermediate between the other two rotations.

The organic forage system depleted not only the most plant-available forms of P, but also the forms that are only slightly available to plants. Welsh’s findings on the forms of phosphorus in the soil were discussed in more detail in a previous article in this column (date).

Two major factors contributed to the difference in available soil P between the annual and forage-based rotations. First, large amounts of P were removed from the forage-based rotation when alfalfa hay crops were harvested. A 2.5 ton/ac alfalfa hay crop removes about 2.5 times as much phosphorus as a 30 bu/ac spring wheat crop! Second, wheat and flax yields were higher in the forage-based rotation than in the annual rotation because of the nitrogen supplied by the 2-year alfalfa phase. Higher grain yields meant that more P was removed from the system.

In the fall of 2002, after available P depletion was observed in the forage rotation, composted manure was applied to half of each plot in the forage rotation at a rate of 4.5 ton/ac. This manure application replaced almost half of the total P removed from the organic forage system between 1992 and 2005, according to Welsh’s measurements of soil P. Adding manure also increased wheat yields by 32% in the organic forage rotation in 2004, likely due to higher levels of available P, since soil test N levels were already adequate.

Applying composted manure is an effective method of adding phosphorus to the system or, more accurately, recycling P back into the system after being 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 P 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 deposition, 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 P fertilizers. Adding rock phosphate does little to increase levels of available P on our high pH prairie soils.

While replacing exported P is crucial for long-term P management, there are also agronomic practices that can help plants access phosphorus and increase biological cycling, or biocycling, of P through plants and back into the soil.

Legumes and buckwheat in rotation increase P availability by producing acids that break down the bonds that hold P to other soil compounds. Fungal inoculants such as Jumpstart work in much the same way, by attempting to increase P release in soil by fungal activity.

Naturally-occurring mycorrhizal fungi associate with many crops, including flax, corn, legumes and cereals, and help the plant take up the P that is already available but out of reach. Welsh found that these beneficial fungi increased in organic rotations, both in numbers and diversity. Commercial mycorrhizal inoculants such as Myke Pro may be able to 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 and Extension Associate with the Organic Agriculture Centre of Canada working in collaboration with Dr. Martin Entz at the University of Manitoba. She welcomes your comments at 204-474-6236 or by email at j_thiessen_martens@umanitoba.ca. The production of this article was supported in part by funding from Agriculture and Agri-Food Canada.

Posted August 2008