Phosphorus is one of 17 chemical elements that all plants need, and it’s one of the nutrients that we sometimes need to add to soils in fairly high quantities. Phosphorus levels in soils depend on the ancestry of the soil and how it’s been managed during its farming history.
Phosphorus availability in soils is very strongly associated with pH. At low pHs, phosphorus tends to bind up with iron and aluminum in soils and becomes unavailable to plants. At high pHs, phosphorus can bind to calcium and magnesium and that also decreases it’s availability. Acidic soils bind up phosphorus worse than alkaline soils do. Phosphorus is most readily available to plants at pHs of at least 6.5. If the soil pH drops below 6.0, phosphorus becomes very unavailable. Applying lime to very acidic soils is always a good idea, and one of the benefits of that is to help phosphorus become more available even without adding it as a supplement.
There are several methods used by various soil testing labs for determining how much phosphorus soils can supply to the crops we grow in them. When we interpret the results of soil tests, we need to consider the method used by the laboratory that does the test. The numbers that the various tests provide don’t really mean anything on their own until they are correlated to how well crops grow at the various levels. Some of the common extractions for phosphorus are the weak and strong Bray solutions (Bray-1 and Bray-2), Mehlich-1 and Mehlich-2, the Morgan and modified Morgan extractions, and the Olsen or sodium bicarbonate test. Some tests are only appropriate for soils with specific characteristics. Examples of these tests are the Olsen test, which is only appropriate for high pH soils with high carbonate content, or the Mehlich-1 test, which does a great job of extracting phosphorus from soils with a low cation exchange capacity (like many coastal plain soils of the southeastern U.S.).
The Bray-1 and Mehlich-3 tests are common in many parts of the country and the results from these tests follow similar patterns. The lab we use for the CROPP Agronomy Program (Midwest Labs) uses the Bray-1 and Bray-2 phosphorus tests for most samples, but the Olsen test for soils with high pHs. The Morgan and modified Morgan tests are used by many labs in the northeastern United States and these results are harder to correlate with numbers like we’d see from the Bray-1 test.
Whatever lab you’re using, it’s important to understand how crop growth responds to the phosphorus level reported on your soil test. For our program, we use a target range of 25 to 50 parts per million (ppm) of Bray-1 phosphorus as our target fertility range for most agronomic crops. These numbers represent a sort of middle ground between levels that universities in various states have established as high phosphorus levels for field crops and the levels where environmental regulations usually kick in.
The phosphorus levels recommended by some consultants are sometimes much higher than what the university recommendations are. Each farmer has to decide for himself or herself what level to aim for, but here is some information that you should consider when you’re making this decision. Target levels recommended by universities are usually based on a combination of crop values, yield goals, and the price of conventional fertilizer. These factors are all different with organic cropping systems, but there are still principles we can use to help us sort through how much phosphorus we really need in our soils.
If we identify a phosphorus deficiency in soils, there are several things we can do to correct that. The most common sources of phosphorus for organic cropping systems are manure and rock phosphates. For farms that have a livestock or poultry enterprise, the manure these animals produce should be the first place we look when we need to add phosphorus to soils. If we don’t have enough of a supply of manure to meet the phosphorus we need, or if we need only phosphorus and not the potassium that also comes with manure, we can use rock phosphates. There are different sources and forms of rock phosphate, but they share the characteristics of being fairly slow to dissolve in soils. Growing legume crops on soils where we’ve applied rock phosphate can help speed up this process by the crops removing calcium, which helps the rock phosphate to dissolve faster. Another technique that can work well is to mix the rock phosphate into manure by applying it to bedded packs, in gutters, or mixing it into manure or compost piles. The phosphate part of the fertilizer also binds with ammonium in manure to help keep it from evaporating, and that means more nitrogen from the manure will actually make it back to the land.
Manure is a great source of phosphorus, but we often focus on manure as a nitrogen source for crops like corn. If we use manure as the only source of nitrogen for growing corn, we will continue to raise phosphorus levels over time because manure provides nutrients at different proportions than what crops need to grow. This can be helpful for soils that are low in phosphorus, but if we already have high phosphorus levels it can eventually cause problems.
When soil fertility levels for almost any nutrient go up, so do crop yields. If we add phosphorus to a soil that’s very deficient, we see dramatic yield responses as the amendment becomes available. If we continue to add more phosphorus, the yields generally continue to increase, but at a lower rate. Eventually we reach a point where we need to add larger and larger amounts of phosphorus for smaller and smaller increases in yield. If we’re growing crops that have very high value and the source of phosphorus is relatively inexpensive, it’s tempting to try to achieve the highest yields we can. But that might not be a good idea.
Phosphorus in soils tends to bind very tightly with mineral compounds of iron, aluminum, calcium, and magnesium in soils, and phosphorus associated with organic matter is usually pretty well protected against runoff or leaching losses. Because of this, soil scientists used to believe that we could build up extremely high levels of phosphorus in soils without any danger of environmental damage. We’ve learned over the past 20 years or so that this isn’t true. When we have soils with very high phosphorus levels, runoff waters can carry significant amounts of phosphorus to surface waters. This elevated phosphorus level in the water makes its way to rivers, lakes, and eventually the ocean, where it causes an explosive growth of algae. These algae eventually die and decompose, which depletes the oxygen level in the water, causing the “dead zones” we hear about so often in places like the Gulf of Mexico, Chesapeake Bay, and even in the Great Lakes.
As phosphorus levels in soils continue to rise to extremely high concentrations, the soil gets to a point where it just can’t hold any more and the phosphorus begins to leach out of the profile and end up in groundwater. This can cause problems in well water, and where groundwater seeps into surface waters, it can cause problems in lakes, rivers, and the ocean just like surface runoff.
Because of the potential for environmental problems like these, nutrient management planning regulations often focus on phosphorus levels in soils. This is good for all of us to keep in mind, but it’s especially important for farmers who have livestock and need to spread manure. When soil test phosphorus levels reach a threshold for the area where you farm, you might be prohibited from being able to apply manure to that field. We should aim for phosphorus levels that give good crop yields and still allow us some flexibility with where we can apply manure if we need to.
Phosphorus levels in soil tend to be very stable without active management to change them. If you have higher levels of phosphorus than you want, the best way to bring them down over time is to harvest crops that will draw down the reserves. Various crops remove phosphorus to different degrees. A 4 t/a dry matter yield of typical forage removes 40 to 60 lb of phosphate (P2O5) per acre, while a 20 t/a yield of corn silage at 65% dry matter removes about 72 lb of phosphate per acre. A 150 bu/a crop of corn grain removes around 57 lb/a of phosphate per acre. Small grains remove varying amounts of phosphorus depending on species and yield. A 60 bu/a yield of oats would remove around 17 lb of phosphate, and if we removed a 1.5 t/a straw crop along with the grain it would remove another 14 lb of phosphate. A 60 bu/a yield of wheat would remove 30 lb of phosphate, and a 1.5 t/a yield of straw would take an additional 9 lb of phosphate.
The Bray-1 phosphorus target ranges we use in the coop’s soil testing program are well above the levels used by universities that research soil test levels, but below the thresholds established for most nutrient management programs where restrictions come into play. Except for produce crops, you should be able to attain high crop yields at Bray-1 phosphorus levels between 25 and 50 ppm. If your soil test levels are low, it’s worthwhile to work on bringing them up, but remember that more is not always better, so be careful about building up phosphorus levels beyond the point of good stewardship.
