Soil Phosphorus

What soil tests are used to determine if phosphorus (P) fertilizer should be applied?

There are three tests that can potentially be used to determine the likelihood of a crop yield response to adding P fertilizer. The Mehlich-3 test is now the most widely used phosphorus test by commercial labs. The Bray P1 test was previously the most common phosphorus test used. This test is not used in alkaline soils and should only be used in soils with a pH of less than 7.3. Another test that may be used is the Olsen test, which can be used to test P concentration in soils with high pH and relative carbonate content. Each of these tests is used in the estimate of the likelihood that applying additional fertilizer at a particular soil test concentration will increase yields. The soil test value above which nutrient additions are unlikely to economically increase yield is referred to as the critical soil test value (CSTV). Soil samples for testing should always be collected at the same time of year and under similar moisture conditions.

What influences phosphorus concentrations in Illinois soil?

Soils in Illinois do have some inherent concentration of plant-available phosphorus (previously referred to as their P-supplying power), but these regional differences can be hard to determine as they are influenced by soil type, the parent material of that soil, and by drainage. Generally, crops will be more able to take advantage of this inherent phosphorus in soils with good drainage, where roots are easily able to spread. 

Phosphorus is lost from fields by erosion, and from soil as dissolved phosphate via surface run-off or subsurface leaching, or removal by the crop. Corn and soybeans take up large amounts of phosphorus from the soil relative to dissolved losses, which end up in the grain and are removed from the field with harvest. Corn removes 0.37 lbs. P2O5 per bushel, soybeans remove 0.75 lbs. P2O5 per bushel, and wheat removes 0.47 lbs. P2O5 per bushel. These removal rates are 75th percentile values, meaning that three-quarters of the measured values were below these and one-quarter were above (Table 1). P2O5 the standard compound name used in the United States on fertilizer bags, but this is not a compound that actually exists in soils. Growers should be mindful of this distinction when comparing the results of their soil test (shown in units of P) to what commercial fertilizer is labeled as (in units of P2O5).

How do I decide how much phosphorus to apply?

The amount of phosphorus that needs to be applied depends on the crop you are going to plant, the results of your soil test, and the soil type of your farm. Table 2 below shows the phosphorus recommendations of different land-grant universities in the Midwest. These recommendations vary state-by-state due to the factors previously mentioned. University of Illinois phosphorus recommendations are an ongoing research area. Maintenance fertilizer is the amount that should be added to replace what is removed by harvesting the crop. On average for a corn-soybean rotation in the Midwest, 16-18 lbs. of P2O5are required to increase post-harvest soil test P by 1 ppm, but some research has indicated that the amount of P2O5;can vary based on conditions from 10-35 lbs. (Iowa State University Extension, Bulletin PM 1688). If soil P concentrations are excessively low, extra fertilizer may be needed to build up soil P concentrations in addition to the amount needed for maintenance. Soils that test very low to low in P will generally see yield increases when additional P fertilizer is applied. Maintenance fertilizer may be needed if soil P values are between 40 to 70 lbs. per acre. Fertilizer may not be needed if soil P values are greater than 60 to 70 lbs. per acre. Research is currently ongoing to update these recommendations in Illinois.

Phosphorus Application Recommendations