How to Avoid Soil Phosphorus Build-up From Manure

By Chryseis Modderman

Farmers understand that animal waste is not waste at all, but a valuable (though smelly) nutrient source for plants. And aside from nutrients, it adds organic matter to the soil which, over time, improves water holding capacity and infiltration. You might say it’s some good … “stuff”.

However, applying manure is not all rainbows and butterflies, and there are some limitations that make manure complicated. One of which is that the nutrient ratios are fixed. Unlike commercial fertilizers that can be mixed and adjusted to fit crop nutrient needs, manure is what it is. This is a problem because applying the necessary amount of one nutrient with manure inevitably over- or under-applies another nutrient; and over-application of nutrients can lead to runoff and nutrient pollution of waterways.

For example, application of turkey manure often results in over-application of phosphorus. When turkey manure is applied at a rate to supply necessary nitrogen, phosphorus is over-applied. In some instances, turkey manure applied to meet the nitrogen needs of corn supplies over five times the phosphate needed, and fields that receive turkey litter each year often show high levels of phosphorus build-up.

You might be thinking, “Why is phosphorus buildup such a big deal? It’s not very mobile in the soil like nitrate, so why is it a problem if my soils have extra phosphorus?” Well, you are correct in that phosphorus is fairly immobile compared to nitrate, but the idea of “banking” extra phosphorus is problematic when it never gets used. Continuously adding more phosphorus to soil will eventually lead to phosphorus runoff in either a dissolved or particulate form, which is not only an environmental threat but also a waste of valuable nutrient.

Excess phosphorus of just 20 to 50 ppb (that’s parts per billion, not million) in freshwater, such as a lake, can set off a chain of events that lead to low oxygen states, fish kills, and loss of habitat for aquatic life. And even though there are many other contributors to phosphorus pollution besides agriculture, manure managers still need to do their part to be good stewards of the environment.

Follow these tips to minimize excess phosphorus buildup in soil

1. Apply manure at a phosphorus-based rate: To prevent phosphorus buildup in soil, apply manure at a rate that fits the phosphorus needs of the crop. Of course, this will probably under-apply nitrogen, so supplemental commercial nitrogen fertilizer will be needed to fulfill the crop’s nitrogen needs.
    
2. Apply manure less-than-annually at a nitrogen-based rate: Another method to prevent phosphorus buildup is to apply at a rate that meets the nitrogen needs of the crop, and then refrain from manure applications in following years until the excess phosphorus has been depleted by crop uptake. This method works best with rotations that include crops with adequate phosphorus uptake. Otherwise, it might take many years before manure could be applied again. For example, in some pasture systems, turkey manure applied using this method would receive 15 years’ worth of phosphorus. And some regions have regulations stating that no more than 5-years-worth of phosphorus can be applied at a time from manure.
    
3. Use feed containing phytase: Grains and oil seeds contain a type of phosphorus called phytate that must be broken down by the enzyme phytase to be digested by livestock. Poultry and swine typically have low levels of natural phytase, so the enzyme is often added to their feed. Since phytase makes the phytate in turkey feed digestible, supplemental phosphorus is often unnecessary to meet turkey nutrition needs. That means that turkey manure from phytase-fed turkeys will contain less phosphorus than manure from turkeys that received no phytase and, therefore, needed supplemental phosphorus.

Managing manure can be tricky from both the livestock and crop side, and preventing phosphorus buildup in soils from turkey manure is no exception. By using the above information and tips, you will be better prepared to minimize phosphorus buildup while retaining the benefits of manure. Happy spreading!