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SOIL


The best soils for growing vegetables are well-drained, deep, fertile soils, with adequate levels of organic matter. Soil textures like sandy loam or loamy sand are suitable for early market crops since they are accessible to machinery and workers even when wet. Loam and silt loam soils are better suited for growing crops for later fresh-market use. Regardless of the soil type, develop a best management practices (BMP) plan for the farm, which includes a good soil management program, proper fertilization, good tillage practices, suitable crop rotations, strategies to increase organic matter, and managed irrigation. Consider integrating cover crops between vegetable plantings to maintain or improve soil structure and retain topsoil.

Many factors influence the nutrient requirements of a given vegetable. Soil textural classification, cation exchange capacity, organic matter content, and drainage are important properties that influence the nutrient needs of vegetables. Rainfall, irrigation methods and management, and environmental conditions during the growing season can alter the retention, availability, and uptake of nutrients.

Soil Tests

One way to determine the soil type and fertilizer hneed 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.

Table 2.1. 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+ -

Nutrient Recommendations

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).