Understanding a soil test is an important element of farming and not as hard as you think.
Accurate soil sampling and soil test reports continue to be one of the most valuable pieces of information for evaluating nutrient needs and identifying possible yield-limiting fertility factors. Soil test data are critical for developing economic fertility plans while maximizing return on investment, crop productivity and environmental stewardship.
Applying fertilizer without accurate soil testing can’t be achieved with a “spread by crop removal” approach. This is because some nutrients, such as pH and boron (B), may be yield-limiting and remain unaddressed. After weather and adequate soil drainage, soil fertility and plant-available nutrition are the most critical factors for high yield.
As seen in Figure 1, a “complete” soil test report can be viewed as a complex set of numbers. I believe many people may be intimidated and not know what to do with all the data and, more importantly, what it means for management strategies on their farms. Without a good understanding of how this information relates to fertility recommendations, crop nutrition and potential yield limitations, knowing how to start addressing any fertility-related issues will be very difficult.
Most farmers recognize that there are optimum soil nutrient levels for crop production (Figure 2). The optimum level of nutrients like potassium (K) and calcium (Ca) vary with soil type and cation-exchange capacity (CEC). CEC is determined by the amount and type of clay and amount of organic matter (OM%). It is an estimate of the amount of negatively charged sites in the soil. This information can be of value in seed selection and placement and determining successful nitrogen management strategies in corn.
Soils with higher CEC can hold more cation (positively charged ions) like ammonium (NH4), Ca, magnesium (Mg), K and zinc (Zn) than soils with a lower CEC. While it would be nice for farms to have consistent optimum soil test values, it’s unrealistic as soil is highly variable. Soil nutrients are very dynamic, as are crop production levels and nutrient availability and removal across a field. When evaluating soil test reports, it is important to first identify and address the largest controllable yield limiting factor on that test. If pH levels are not in the optimum range, it does not make sense to invest money into other nutrients since soil pH largely affects nutrient availability.
While many farmers are very familiar with the basic soil test that provides pH, OM%, P and K, it is important to recognize the value and meaning of other soil test data, including CEC, micronutrients and base saturations (B.S.%). Although soil test procedures are not as accurate for sulfur or micronutrients as they are for pH, OM%, P and K, the data can be very valuable when compiled over many areas, multiple years and when used in combination with tissue testing. The B.S. values are valuable, especially when CEC is low (i.e., <12), since this can tell us if a soil is “holding” an optimum level of Ca and K, even if soil test values are below optimum.
Some nutrient levels can be too high in the soil. For example, if Mg levels get too high (B.S. >20%), K or Ca availability could become limited since Mg is occupying too many sites in the soil. In this case, I usually recommend a high quality ag lime if pH needs to be increased. If pH levels are optimum, gypsum would be an acceptable form of Ca to help displace Mg in the soil and not change pH. Dolomitic limestone would not be a good choice in this scenario because it contains Mg.
Soil tests are useful guides to not only make fertilizer recommendations but also to identify trends in soil nutrient levels and chemistry that are being influenced by your management practices. Once you start reading soil tests on a field, tracking them becomes easy and invaluable.
Chad Kalaher grew up on a grain and livestock farm near Litchfield, IL. He earned a B.S. in agronomy from the University of Illinois in 1995 and an M.S. in weed science from North Carolina State University in 1997. He is currently Beck’s Hybrids field agronomist for East-Central Illinois and farm management advisor for the Midwest. He has served other positions in the seed industry as research agronomist, district, and regional sales manager.