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Issue No. 22, Article 3/September 5, 2008

Prioritizing Fertilizer Applications

The recent increase in cost of fertilizer and other agricultural commodities calls for conscientious planning and sound strategies to continue to produce high yields while maintaining profitability. At times of high input costs, more often than not there are not enough resources to cover all the aspects of crop production and the allocation of such resources must be done by priority. Soil fertility decisions such as fertilizer and lime applications must be weighed against other crop production needs and farmers' goals. Since final crop yield is affected by a host of management decisions and weather conditions that are often interrelated, under limited financial resources, ranking cropping system needs can be very tough.

When considering the nutritional needs of a crop, the goal should be to obtain the greatest return on investment by focusing on inputs that will result in the greatest profit. This concept is known as the Liebig law of the minimum. This law likens plant growth or yield to a barrel with staves of unequal lengths, in which the shortest stave determines the capacity of the barrel. In terms of soil fertility, the staves could represent soil pH, nitrogen (N), phosphorus (P), potassium (K), or other nutrients needed by the crop. Thus, the first step in assigning priorities should be to determine what is limiting the crop. While high fertilizer prices do not allow for nutrient rate applications beyond those strictly needed by the crop, high crop prices call for adequate soil fertility to maximize yield. In light of current commodity and crop prices and the relatively inexpensive cost of soil analysis, soil testing for pH, P, and K is probably the single most important step toward increasing profit margins. Unfortunately, the many factors influencing N availability make it very difficult to predict it for a crop to be grown the following spring. However, a range of optimum N fertilization rates can be obtained using a corn N rate calculator that takes into account yield and the price of corn and N fertilizer costs. This calculator was developed using data from many years and sites across Illinois; it can be accessed at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

Once factors that likely would limit the crop have been determined, the obvious next step is to correct the potential limitation by developing a strategy to meet the needs of the crop within the constraints of the budget. For instance, if K levels are low and all other nutrients are in adequate supply, adding more P will not help increase yield since the crop is being limited by K. This sounds very simple, but often this concept is overlooked when people go ahead and apply "a typical rate" without first determining what is actually needed. Supplying the nutrient in shortest supply not only helps to maximize yield, but it also makes it possible to optimize the fertility already present in the soil. Often more than one nutrient is limiting the crop. In those situations applying at least a portion of every limiting nutrient will be a better strategy than focusing on only one of them.

Soil pH has an important impact on the availability of nutrients, so checking and adjusting soil pH should be one of the first steps to consider. It is recommended that pH be maintained at least at 6.0 for corn and soybean. If resources allow it, it is always better to bring pH to 6.5. However, adding lime to bring the pH to a level above 6.5 is not economical since there is very little chance for a crop response once pH is above 6.5. In a practical sense, adjusting soil pH means that if the same amount of fertilizer were to be applied at a pH below 6.0, the availability of that application would be lower than if the amount were to be applied at a pH slightly above 6.0.

Additional strategies that can help increase the efficiency of the fertilizer application include these:

  • Applying nutrients to the portions of the field with highest likelihood of response, instead of doing an average application across an entire area
  • Maximizing the efficiency of the nutrient by minimizing losses and by increasing its availability
  • Accounting for all nutrient sources and nutrient pools already present in the cropping system before making an application
  • Determining which crop will perform better under the given conditions. For instance, if limited resources do not allow for a full rate of lime application to bring pH to the desired level, corn may be a better planting option than soybean because corn is less sensitive to lower pHs. If you plant wheat this fall, it is critical to have adequate P available. If a full P rate cannot be afforded, supply as much as possible at planting and possibly do so by seed-banding the fertilizer so the young wheat plant has access to large amounts of P during establishment.
  • Not being afraid to cut or eliminate an application if soil test levels are well above the point where a response is expected. Doing this will allow resources to be used where they are truly needed.

--Fabián G. Fernández

Fabián Fernández

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