Environmental Implications Of Soil Testing For Phosphorus
New tests are available for predicting phosphorus mobility.
Historically, soil testing has been used to evaluate P availability to crops and to make P fertilizer recommendations. Sometimes people use these soil tests to estimate the potential for P runoff. For example, both the new hog regulations and the biosolids regulations use a cutoff of 100 ppm Olsen P (50 ppm AB-DTPA). If a field has an Olsen P level above 100 ppm, it is illegal to apply hog manure or biosolids to that field unless proof is given that P runoff is not occurring.
Research has shown that traditional agronomic soil tests for P are often correlated with runoff P levels. However, these relationships are different for different soil types. Some soils bind more P, and some soils are more susceptible to runoff than others.
In
the past few years, numerous soil tests have been developed for the purpose
of predicting P mobility. One approach is to use water soluble P to simulate
the release of P to rainfall and runoff water. Another promising method
is called the iron oxide strip method. In this procedure, the potential
of a soil to release or “desorb” P is measured. This method is a good
indicator of the biological availability of P to algae in surface waters,
but it is a much more tedious method than either the water soluble P test
or typical agronomic tests, such as Olsen or AB-DTPA.
Dutch scientists have developed another analytical tool called the degree of P saturation. In this method, the P sorption capacity of a soil is determined, and then the current P status of the soil is presented as a percentage of the total sorption capacity. Unfortunately, this method was developed for acid soils in which the P sorption capacity is determined by the amount of iron and aluminum oxides and hydroxides. In calcareous and high pH soils, on the other hand, the amount of soluble calcium and the amount of lime (calcium carbonate) are important factors in P adsorption. Scientists in the U.S. are currently working on modifying this method so that it can become useful for high pH soils, which are typical in Colorado.
Be cautious about using agronomic soil P tests for predicting P runoff potential. A high level of plant available P is not necessarily equivalent to a high level of environmentally mobile P.
