ALFALFA SOIL SAMPLING

By Brad Schick

How did alfalfa fields fare this year? Drought, fall armyworms, and now fertilizer prices will affect what management decisions are made. This makes soil sampling this fall even more important.
 
The fertility of our soils is essential to creating good yields in alfalfa. All alfalfa fields should be sampled, especially for phosphorous, potassium, and pH while sulfur should be checked on sandy and weathered soils. Nitrogen is usually not needed because alfalfa fixes its own.
 
Soil cores and recommendations are often based on cores taken down to 8 inches. However, if historically samples have been at a different depth, such as 6 inches, stay with the historical and adjust accordingly if recommendations are based on an 8 inch depth. Due to mineralization, soils have more nutrients readily available nearer the soil surface, so deeper sampling depths can dilute the samples and increase nutrient supplement recommendations.
 
If sampling yourself, make sure the sampling is done in an accurate and precise manner. Soil type, grid, or a composite sample of 20 to 40 acres are several ways to take soil samples. With the latter, 10-15 cores will need to be composited to create a more accurate sample.
 
Precision and accuracy will change by sampling method and is affected by the element being analyzed. For example, phosphorous is accurate but not always precise with the variability across a field. Keep in mind that soil sampling may not reduce the overall cost of fertilizer needed, but will help ensure that it is applied to the places in the field more appropriately, which can result in a better yield.

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