By George Rehm on 16 Jan 2009

For many years, there have been differences in fertilizer guidelines provided by private and public (University) soil testing labatories. Some have been minor. Others have been substantially different. Some guidelines were evaluated in a long term study (14 years+) conducted at five locations in Nebraska. To make a long story short, the fertiilizer programs from the various laboratories produced fertilizer bills with a substantial difference in cost while corn yields were not different.

Many crop producers have reached a similar conclusion without conducting detailed comparisons. So, it’s only logical for them to ask:” What is the basis for the U of M guidelines?”

Arriving at fertilizer guidelines is not an easy process. The guidelines are the end result of a considerable effort involving the efforts of several faculty and numerous support personnel. For example, the recent Corn Belt nitrogen fertilizer guidelines were the end result of a critical evaluation of data collected from over 500 sites across the Corn Belt. The information was collected over a period of several years. The new nitrogen guidelines are built on a broad and detailed research base.

Development of “University” guidelines for phosphate and potash can be thought of as a two step process. The first step is to relate crop yield of various crops to some measure of a nutrient in the soil.. With phosphorus, for example, soil samples are collected from numerous locations and depending on soil pH, are analyzed for phosphorus by either the Bray or Olsen analytical procedures. The yield at each of these locations is measured and the yield is then related to the measurement of phosphorus. Through this data collection process the yield is CORRELATED to soil test phosphorus. The relative soil test levels for phosphorus (very low, low, medium, high, very high) are then defined. For example, a range of 5 to 10 ppm as measured by the Bray procedure is defined as “low”.

The next step is to select sites with a specific soil test value or range of soil test values, apply severl rates of phosphate fertilizer, and measure yield. The rate that produces the most economic yield, then, becomes the suggested rate for the narrow range of soil test values studied. In this step, the rate of phosphate fertilizer producing optimum yield is CALIBRATED to the soil test level.

To develop effective guidelines, it’s necessary to conduct the correlation and calibration trials over a relatively large number of sites. This is not a process that can be completed in one year. Nor can it be completed in a limited geographical area. In Minnesota, for example, production environments are substantially different across the state and the research base reflects these differences. For example, phosphate fertilizer guidelines for hard red spring wheat should be based on data collected from both northwest and west-central MN. There are many other examples that could be cited. It would not be accurate to develop guidelines for the entire state using data fron a limited geographical area.

Fertilizer guidelines must be based oa a solid research base. For example, guidelines based on a ratio of calcium to magnesium in soils are not the end result of a solid research base because there is no relationship between this ratio and crop yield.

Because the data used as the basis for the fertilizer guidelines were collected by faculty and staff of a Land Grant Universsity (University of Minnesota), the guidelines are public information. This does not mean that an organization or other agency can stamp them with their logo and claim them as their own. This action was taken by NRCS for the Nutrient Management Initiative. It was not appropriate and was wrong.

In summary, University of Minnesota fertilizer guidelines are built on a solid data base. Much of this important information has been collected from fields of cooperating farmers. Development of fertilizer guidelines is a continuous process and will be an important component of the mission of the University of Minnesota in the future.