Soil Testing

Kevin Heaton

Utah State University Extension

 

            In cooperation with local farmers, USU Extension sampled the soil and prepared it for analysis in forty-six fields in Kane and Garfield Counties. Soil samples were submitted to Utah State University’s Soil Testing Laboratory and analyzed for texture, pH, salinity, phosphorous, and potassium. The average test results were pH 7.5 (normal), salinity 1.0 (normal), phosphorous 11.4 ppm (marginal), and potassium 202 ppm (adequate). If farmers base their fertilizer application rate on the average soil test, they could save a significant amount of money on fertilizer, but less than 15% of the farmers would be applying the appropriate amount of fertilizer and crop production would suffer. In fact, 30% of the field’s phosphorous test result indicated “very low” which would require 200-250 lbs of phosphorous (P2O5)/acre. Whereas, the average field (marginally deficient phosphorous field) would only require 50-100 lbs of phosphorous (P2O5)/acre. Approximately 30% of the phosphorous test results indicated that no fertilizer was needed which is a significant cost savings to farmers, particularly as fertilizer costs continue to skyrocket.  

 

When comparing individual soil analysis, the differences become more apparent.   For instance, phosphorous levels ranged from very low 2.4 ppm to high 36.0 ppm. Twenty-eight fields (62%) had “very low” or “low” phosphorous test results. Potassium levels ranged from “low” 52 ppm to “very high” 814 ppm. Soil texture, pH or salinity showed no influence to phosphorous or potassium test levels. All test results for soil pH were “normal” for local growing conditions.  Two of the forty-six fields, one field in Garfield and one field in Kane, had salinity values above 2 (EC = dS/m) which is the threshold for alfalfa. However, soil slightly above the threshold for salinity has minimal influence on production capability of the crop, usually resulting in less than a 10% reduction. If salinity issues continue to increase or is an agronomic problem for these fields, then the operator should apply excessive irrigation water (if there is adequate drainage) which will flush the salts below the root zone. 

The below graph highlights research conducted on alfalfa in Utah and shows yield response in relation to phosphorous fertilization (Koenig et al., 1999). Alfalfa fields testing less than 10 ppm phosphorous will yield between 60-90% of the maximum. Most definitely, farmers should evaluate the costs and net benefits derived from fertilizer applications.   

The below soil test data indicates that soil testing is essential to ensure proper fertilizer management. Farmers should not be fertilizing based off text book tables, salesman’s opinion or the Extension agents recommendation unless fertilizer applications are based on a soil analysis. A simple $12.00 test could save hundreds of dollars in fertilizer expenses or significantly improve crop production by application of the appropriate amount of fertilizer. Another good recommendation is to test agriculture fields every 3-5 years thus monitoring nutrients for optimum crop production.

 

 

           

 

 

 

 

References:

 

Koenig, R., B. Kitchen, C. Hurst and J. Barnhill. 1999. Phosphorus and potassium fertilization of

irrigated alfalfa in Utah. Pp 57-62 in T.L. Tindall (Ed) Proceedings of the Western Nutrient

Management Conference, March 4-5, Salt Lake City, UT.