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Soil sampling and analysis is used to assess what soil amendments are present or may be needed for optimum plant growth and yield potential. Soil test results provide the concentration for specific parameters along with an interpretation value (low, optimum, high) and a recommendation.
Managing soil fertility begins with soil sampling and testing. Soil test results help establish nutrient levels available in the soil and provide locally calibrated fertilizer recommendations. Ideally, soil sampling should be performed annually to help maximize yield potential. If not annually, it should occur at least once every four years. Sampling in the spring is best to estimate nutrient availability during the growing season. However, due to time constraints and soil conditions, it may be more practical to sample soil in the fall or winter. Soil tests can be used as a diagnostic tool or to identify trends through time. For test results to be meaningful, soil samples need to be taken correctly, at the same time and place each year, and good records need to be maintained.
Soil tests measure plant-available soil nutrients. However, they do not measure the total amounts of nutrients in the soil because plants can access only a small portion of the soil nutrients. Also, soil tests do not measure inputs of nitrogen and other nutrients from the mineralization (breakdown and release of nutrients) of organic matter. A soil test can provide a measure of nitrogen (N), phosphorus (P), potassium (K), as well as other secondary and micronutrients. It can also provide values for organic matter, soil pH, cation exchange capacity (CEC), soluble salts, soil salinity, and soil physical characteristics.
Soil test values should not deviate significantly from year to year. Drastic changes may indicate an unrepresentative soil sample or improper sampling techniques. This can result in fertilizer recommendations that are too high or too low. Be sure to follow procedures for soil sampling that can be specific to your region for each type of test (such as sampling depth and sampling time) and take soil samples that are representative of the field.
Soil tests generally indicate whether a nutrient level is low (a fertilizer addition will likely increase growth and yield), medium (a fertilizer addition may increase growth or yield), or high (a fertilizer addition is not likely to increase growth or yield). Some reports may break it down further to very low, low, medium, high, and very high. The ‘critical level’ is sometimes referred to as the cutoff between a medium and high level which provides a value that indicates when fertilizer should (below critical level) or should not (above critical level) be added. Soil test results in the western United States are generally based on the philosophy that nutrients should only be applied when an economic yield increase is likely.
Soil test results typically include a fertilizer recommendation that is based upon the previous crop, the crop to be grown, and the yield goals. If soil nutrients are below the critical levels, fertilizer inputs will need to be applied in order to bring the soil nutrient level to the desired values for the region (buildup). This rate will be indicated on the soil test results. Additionally, fertilizer inputs should replace what will be removed by the crop based on the yield goal (maintenance). This is referred to as the buildup plus maintenance approach. While fertilizer applications for the buildup of nutrients will likely result in a yield response if the soil test levels were low, maintenance applications serve to maintain soil test values within acceptable ranges for the long-term. Soil test results are also used to determine lime requirements for decreasing soil acidity.
Nutrient removal rates will vary by crop, geography, cropping practice (for example, harvesting grain versus silage), and other factors. Use local sources for nutrient removal rate estimations. Additionally, the fertilizer recommendation should take into account credits from other sources such as nutrients in irrigation water, manure applications, and mineralization from organic matter.
Dinkins, C.P. and Jones, C. 2013. Interpretation of soil test reports for agriculture. Montana State University Extension publication MT200702AG. Horneck, D.A., Sullivan, D.M., Owen, J.S., and Hart, J.M. 2011. Soil test interpretation guide. Oregon State University Extension publication EC1478. 161012164538