As fertilizer prices climb higher, the value of soil testing becomes more critical. Knowing what nutrients are available in the soil for crop use is important and an essential part of understanding the condition of the soil. Soil tests begin with the proper collection and processing of soil cores (Figure 1).
Depending on the lab completing the soil analysis, the results appear in similar styles (Figure 2). Each report can be used to establish a baseline for understanding what nutrients are present and help determine what nutrients and amounts are needed (if any) for optimum plant growth and yield potential.
Key components that should be reviewed in addition to nutrient information include pH, cation exchange capacity (CEC), and % base saturation.
Soil pH is very important when it comes to the availability of nutrients in the soil. Although soil test results may read high for some nutrients, those nutrients may not be available to plants if the soil pH is too high or too low. Figure 3 shows the availability of plant nutrients in relation to soil pH.
Most macro and micro soil nutrients are available when soil pH is between 6.5 to 7.5.1 A low or high pH can limit nutrients needed for plant development; therefore, it is critical to make necessary adjustments. Agricultural lime can be used to raise pH or a sulfur containing amendment can be used to lower pH. In some cases, pH amendments can help reduce the need for additional fertilizer.
Cation exchange capacity, or “CEC”, is provided on soil test results and is a measure of a soil’s ability to hold positively charged nutrients, such as Ca++ and Mg++. It is also directly related to the soil’s texture – that is, the makeup of sand, silt, and clay. Clay and organic matter are responsible for a soil’s negative charge; therefore, higher clay and/or organic matter composition would reflect a higher CEC. For example, CECs in the 20+ range suggest a “heavy” soil with high amounts of clay and a highwater holding capacity. These soils also tend to stay wetter longer under normal weather conditions. Conversely, the lower the CEC number, such as 8 to 10, the sandier the soil.
Percent (%) Base Saturation is a percentage of total CEC occupied by the basic cations: Potassium (K), Calcium (Ca), Magnesium (Mg), Sodium (Na), and Hydrogen (H), with the total of the five equaling 100%. Base saturation for each individual cation can be calculated; for example: Potassium saturation (0%) = (Potassium cations/CEC) x 100. Below are acceptable percentage guidelines for each nutrient:
Calcium2: 60 to 80%
Hydrogen2: 10 to 15%
Magnesium2: 10 to 20%
Potassium2: 2 to 6%
Sodium3: 0.5% to 1%
A nutrient to review closely for percent base saturation is K. Potassium is considered the “quality” nutrient and is critical to crop growth, development, and yield potential. Low levels of K can lead to weaker stalks and stems, lower drought tolerance, and less tolerance to plant diseases.
Making sure each of these nutrients are in an acceptable range can help ensure soils are well balanced and are able to provide the maximum allowable nutrients to the upcoming crop.
1Jensen, T.L. 2010. Soil pH and the availability of plant nutrients. Plant Nutrition TODAY. International Plant Nutrition Institute (IPNI).
2Understanding cation exchange capacity and % base saturation. Fact Sheet 552. A & L Canada Laboratories, Inc.
3Hefty, D. and Hefty, B. 2018. Base saturation sodium (video). Farms.com™.
ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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