Soil Testing/Fertilizer Recommendation

DR. A.J. FOSTER

BLOOMFIELD, MO.
   How often do you find yourself in a friendly argument with someone on an issue, to later find out the reason for the disagreement is a simple difference in definition? Recently, I found myself in such a situation on the issue of soil testing and fertilizer recommendation. I learned from the experience that providing a simple explanation of my personal definition could have resolved the issue. So in this article I hope to define soil testing and fertilizer recommendation and to explain the difference between them.
   The simplest way for me to define soil testing, is that it is an extraction method (chemical mixtures) that is used to determine the amount of nutrients present within a soil. Different laboratories use different extraction methods, for example University of Missouri uses Bray 1 extraction for P and Ammonium acetate for K, Ca and Mg. A common extraction method used is Mehlich III which is widely accepted and most often referred to as the universal extraction method.
   A fertilizer recommendation is a way or a philosophy for interpreting and drawing conclusions based on the soil test results. In general, there are two main philosophies behind fertilizer recommendations (mainly for P and K): 1) Buildup and maintenance and 2) Sufficiency Basic cation saturation ratio is also a minor philosophy that is gaining some interest by producers and laboratories.
   Buildup and maintenance
   This approach I liken to a savings account. The goal here is to maintain soil nutrient levels at certain levels for optimum yield for future years in other words fertilizing the soil. So to me it is like saving, you try to save and put away money for future yields. Fertilizer recommendation is made to apply enough nutrients to meet both the buildup (critical level for optimum yield) and to maintain at or above the critical level by replacing nutrients removed by the crop. Advantage of this approach is that the risk of nutrient deficiencies is decreased. Disadvantage is the risk of over-fertilizing.
   Sufficiency
   This approach I liken to a checking account. The goal here is to apply only to meet the nutrient requirements of the crop in other words fertilizing the crop. So to me it is like a checking account, only put in the required amount to take care of the bills. Fertilizer recommendation is made to meet the crop requirement that is most likely to result in a yield response. The advantage of this approach is that it tries to maximize yield while minimizing annual inputs and is well suited for short-term leases. The disadvantage to this approach is that it is short-term and provides no protection against times of high fertilizer prices.
   Basic Cation Saturation
   The goal of this approach is concerned with maintaining a specific ratio of cations (positively charged ion) mainly calcium, magnesium and potassium within the soil. A desired proportion of 65-85 percent Ca, 6-12 percent Mg and 2-5 percent K is the goal of this approach. The use of this method is not well substantiated by research. See link for further reading http://www.extension.umn.edu/agriculture/nutrient-management/soil-and-plant-sampling/soil-cation-ratios/; http://www.ianrpubs.unl.edu/epublic/live/ec155/build/ec155-4.pdf;
   Regardless of the approach, producer should take soil samples regularly to ensure the amount of nutrients in the soil is known. For more information on soil testing and fertilizer recommendations contact your local Extension office. Remember if you don’t soil test you will be forced to guess!!! Ô
   DR. A.J. FOSTER: Agronomy Specialist, University of Missouri