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The value of a soil sample

04/28/2021

The value of a soil sample

By Graham McLean, CCA-ON, 4R NMS
Crop Sales Specialist
AGRIS Co-operative - Glencoe

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The results from a properly taken soil sample are invaluable in managing essential crop nutrient levels. It also contains other important information for modifying other crop inputs.

The first test result to check is soil pH and then Buffer pH (BpH). Most broadacre crops grow best in pH levels ranging from 6.5 to 7.5. The pH measures the soil reaction; a pH below seven indicates that the soil is becoming acidic. When the soil pH is below 6.5, we have the lab conduct a BpH test. A low pH means the soil is acidic, and the BpH is used to determine how much limestone we need to apply to increase the soil pH to the proper range. The reason that is so important is the pH determines the availability of essential nutrients.

The soil pH has another useful utility; it can be used to adjust the choice of herbicides used and the rate adjustments that may be necessary to enhance weed control or reduce crop damage. The Cation Exchange Capacity or CEC is also useful in adjusting herbicide rates as it is used as an indirect measure of relative soil textures with lower readings <10 indicating a sandy soil and a CEC above 20 indicating a higher clay content. Both pH and CEC can affect the choice of herbicide and the rate of application.

Most often, we are looking at the phosphorus (P) or potassium (K) levels to determine if we need to add any more nutrients to obtain the desired yield goals or not. In addition, the essential secondary elements and micronutrients are also measured and are of equal importance. The soil test values are a measure of the soils ability to supply nutrients at sufficient enough amounts to meet stated yield goals. A high soil test rating for P and or K is indicating your soil has sufficient nutrient levels to meet stated yield goals, and the need for amendments of additional nutrients from any source, including manures and other organics, is low or a low yield response is expected from applying high rates of these nutrients.

Conversely, a low soil test for P and K is indicating a low ability to supply the needs of the crop and higher rates of application are required to meet the yield objectives. Stable values over time in the appropriate range are maintained by matching applications to crop removal levels.

Crop rotation has a profound influence on soil nutrient removal, with forages and corn silage removing large amounts of P and especially K. 

Regardless of the levels found, there are few mysteries as to why that might be. If the tests are high, then likely, you have been applying nutrients at rates that exceeded crop removal. In contrast, if the values are low, then crop removal from the field is exceeding application rates. The test is a direct measure of how you have chosen to manage nutrient applications. 

Either scenario has economic consequences. Over-applying is not optimizing the dollars spent, and under-applying is likely lowering potential income by limiting yields—this is the very principle behind the 4R Nutrient Stewardship Program. Applying the right source at the right rate at the right time and in the right place. It all begins with a current soil test no older than four years upon which to base a recommendation.

Ask your local AGRIS Co-operative crop specialist for a deeper dive into discussing your nutrient management plan.

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