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Soil - Testing 


  Soil Test

One way to determine the soil type and fertilizer need is to have your soil tested. Soil sample kits and instructions are available in every county Extension office. The local Extension Educator can help with sampling approaches, testing needs, and provide you with the costs of the various soil testing services performed by the USU Analytical Laboratory. Knowledge of the current soil fertility can reduce fertilizer application rates and better match soil fertility level, past cropping history, and soil management practices to the crops grown. To minimize potential soil damage and water pollution, nutrient recommendations are based on the soil test results and past cropping and fertilization practices. For more information on soil testing and interpreting results, visit the Utah State University Analytical Laboratories website (www.usual.usu.edu).

  Soil Test Interpretation

A soil test evaluates the nutrient-supplying capabilities of a soil. A common misunderstanding is that the test provides you the total amount of nutrients that are available for plant growth. The soil test only provides a prediction of how much fertilizer is required for optimum plant growth. If fertility levels are below optimum, addition of the nutrient should enhance or increase plant growth and productivity (provided something else is not limited). If the soil test indicates that a nutrient is at adequate or excessive levels, no applications are needed.

The basic soil test determines the soil texture (sandsilt-clay), soil pH, salinity, and phosphorus (P) and potassium (K) levels. A “complete” analysis also tests for nitrates, micronutrients, sulfate, and organic matter. Soil test recommendations are commonly expressed in units of pounds of the particular nutrient per acre (Table 2.1). Reading and understanding the soil test depends on knowing what method was used in the test laboratory and what units are used to express the soil nutrient levels. If the soil test report does not state the method used, call the laboratory to find out. This information is needed before interpreting the soil test results.

  Nutrient Categories

       Soil test categories for nutrients.

Test Category Soil Test Value (mg/Kg)
Phosphorus (P)
Soil Test Value (mg/Kg)
Potassium (K)
Very Low 0-10 0-70
Low 11-20 70-125
Adequate 21-30 126-300
High 31-60 300+
Very High 60+  

  Soil Test

Always base nutrition applications on a current soil test. When soil test results are not available, use recommended amounts of P2O5 and K2O listed under adequate phosphorus and potassium soil test levels for the crop to be grown. This is not as accurate, but is a conservative approach that minimizes the chance of over-application. Refer to Table 2.1 to interpret the relative levels of phosphorus and potassium in the soil based on the soil test report from the laboratory. When a current soil test is available, use the crop specific recommendations or consult your local county Extension Educator.

EXAMPLE:
If the soil test recommends a 100 pounds of nitrogen (N), 100 pounds of phosphate (P2O5), and 100 pounds of potash (K2O) per acre, you would need a fertilizer with a 1:1:1 ratio, such as a 16-16-16. To determine the quantity of fertilizer to apply:

      • Divide the percentage of N, P2 O5, or K2O  in the fertilizer into the quantity of nutrient needed per acre.
      • Multiply that value by 100.
      • Total fertilizer required to provide 100 pounds of N per acre would be 625 pounds of the 16-16-16 (100/16=6.25 x 100 = 625).