Farmers are being told to take more responsibility for keeping crop nutrients out of ground and surface waters. That message is often aimed at nitrogen, and phosphorus is too often overlooked in this discussion.
Phosphorus in soils is almost entirely associated with soil particles. When soil particles are carried to a river or lake, phosphate will likely be contained in the sediment. When the sediment reaches a body of water, it can either bind or release soluble phosphate. In either case, sediment can be a potential source of phosphorus released into the aquatic system, causing increased algae growth and deterioration in water quality.
It is important to remember that phosphorus reacts differently in the soil. It isn't mobile like nitrogen and doesn't leach or convert in the soil to a more mobile form. Instead, phosphorus fixes in the soil and ties up quickly and becomes unavailable. It is this fixation 'problem' that makes managing phosphorus a challenge. It may also cause some to wonder why phosphorus is even a concern.
Anytime there is erosion and water detaches and moves soil particles off the landscape, it is also moving phosphorus that is fixed to it. These silt particles with phosphate molecules attached move down into streams, rivers and bodies of water and contribute to hypoxia.
Solutions are fewer and less obvious than with nitrogen, but there are some practical steps farmers can take. The thing to keep in mind is only a small amount of phosphorus is dissolved in soil water, and it is rarely a significant source of phosphorus runoff. It is the soil itself that is the issue.
Phosphate movement with soil is more with fine particles than coarse soil particles. When soil erosion occurs, more fine particles are removed from the landscape than coarse particles. Thus, fine-textured soils that are high in clay and silt content and soils that have been tilled, breaking down natural soil aggregates into fine particles, have a high potential to move phosphate.
The key to keeping phosphorus for the crop is keeping soil on the landscape. Practicing no-till and minimum till, using water-control structures like terraces and contour farming, and waterways and buffer strips help keep phosphorus in place.
Manures, compost and other animal wastes are good organic sources of phosphate because they aren't readily available until nutrients begin to mineralize the following spring and summer. The critical point here is to apply before soils become frozen so the liquid material enters the soil. Likewise, cover crops need time in the fall to be allowed to scavenge phosphate and other nutrients into their biomass as they grow and then mineralize them next spring and summer after the cover crop is terminated.
Deep placement of phosphate fertilizer also works as it places phosphate at a 4- to 8-inch depth in a concentrated band. This method keeps phosphate off the surface and concentrates it in a band in the soil so less ties up and more is available to the crop. Today deep placement is primarily limited a strip tillage when a band of fertilizer is placed in a zone 4 to 8 inches below the surface of the berm.
A new technique is to apply gypsum (calcium sulfate) when phosphate is applied. Gypsum is soluble when applied either as powdered ag gyp or pelletized gypsum. Calcium in gypsum ties up phosphate, trapping it until next spring and summer. As the calcium phosphate mineral weathers next season, the phosphate will be released back to the soil. In addition, you get the benefits of calcium on soil structure and sulfur for crop nutrition.
If you farm land that is erodible, you need to be thinking about how to keep phosphate in place and available for the crop. With huge rain events, some erosion is inevitable, even with the best conservation practices. However, start by adopting practices that protect the soil surface and reduce or eliminate soil erosion by keeping as much residue on the surface as possible. Then, add in cover crops or gypsum to temporarily trap phosphate until soil conditions warm up the next spring and phosphate is released for the crop.
If you have a question, e-mail Dr. Daniel Davidson at firstname.lastname@example.org
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