Soil Testing and Testing Results

Image Credit: 
Sara BhaduriHauck

Soil testing is the basis of developing nutrient recommendations for maximum production of agronomic crops. Therefore, dependable recommendations demand good soil sampling techniques. Read more about soil sampling procedure here.

Soil sampling for nutrient management plans only require the basic soil test provided by the soil testing labs. Soil test results must also be less than three years old. In general the basic soil test includes testing for  nutrients including phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg); pH; organic matter (OM);  and cation exchange capacity (CEC).

The nutrient results of the soil test will not measure all of the nutrients present in the soil. They will measure the amount of nutrients in the soil that are expected to be plant available during the current growing season. The levels of the different nutrients can be grouped into nutrient classes: low, medium, optimum and excessive. By knowing the nutrient class, a prediction of the crops response to additions of a particular nutrient can be determined. The chart below is applicable for the University of Maryland nutrient FIV levels. (As an example, a phosphorus level of FIV 30 would indicate that a phosphorus application following the recommendation would give a possible increase in crop yield, whereas a phosphorus level of FIV 110 would indicate that a yield increase would be unlikely if more phosphorus was applied.)

Soil Test Fertility Index Value

Soil Test Category

Likelihood of Yield Response to an additional application of nutrient

0-25

Low

Likely

26-50

Medium

Possible

51-100

Optimum

Unlikely

<100

Excessive

Unlikely

Phosphorus and potassium levels in the soil determine if additional nutrients are needed for maximum crop production. If additional nutrients are needed beyond soil level, they will be reflected in the recommendations. The recommendations are crop dependent.

Excessive phosphorus levels can result in a need for a Phosphorus Management Tool (PMT) analyses to be completed prior to additional applications of phosphorus. The PMT will require that the soil analysis include a Phosphorus Saturation Ratio (PSR). This is a new test requirement that is not included in all soil analyses.

The test for pH measures the acidity level of the soil on a scale of 0 to 14. A pH of 7 is neutral where below 7 is acidic and greater than 7 is alkaline or basic. When pH indicates the soils is acidic, liming materials are used to neutralize the soil or bring the soil closer to a basic level. The lime recommendation takes into consideration the pH level of the soil as well as the crop to be grown.

Crushed limestone is the most widely used liming material. Limestone is composed mainly of calcium carbonate. Some limestone, particularly dolomitic limestone, also contains magnesium. The type of limestone used will depend on the Ca or Mg levels in the soil and the crop to be grown. For more information on pH and lime, see Nutrient Manager Vol. 3, Issue 2

The organic matter (OM) levels give an idea as to amounts of humus, bio-mass, residuals, and by-products present in the soil. Organic matter works to increase earth worm and microbial activity, loosens clay soils which improves drainage, slowly releases nutrients to plants, and holds water  for plant uptake. Cover crops can improve organic matter levels. For more information, see Nutrient Manager Vol. 2 Issue 2

CEC measures the cation exchange or ability of the soil’s clay sized particles to hold nutrients like calcium, magnesium, and potassium in reserve. A soil with a high CEC will be able to release these nutrients into the soil solution, making them available to plants, when levels of these nutrients become low in the soil.

Although most soil labs offer a basic soil testing package that will fulfill nutrient management plans requirements, remember that labs report these results in different ways; results from different labs are not necessarily comparable. A variety of chemical extracts, extraction procedures, and reporting forms make it necessary to convert results to a standard form to compare results.

In developing nutrient management plans, soil test results are all converted to the Fertility Index Value (FIV). The University of Maryland FIV makes it easier to compare nutrient levels and make recommendations for various crops using one scale. Furthermore, recommendations are based on research by our University of Maryland scientist on our own Maryland soils. When this information is combined with the proven yield generation for a given agriculture operation and a management history, a true customized recommendation is generated.

Keep in mind that it is important that the soil analyses and nutrient management plans reflect current crops and management of your operation. That is why nutrient management plans are only good for a period of time. Expiration dates are clearly marked on the nutrient management plans. Check the date on your plan today.

For more information, visit the Agricultural Nutrient Management Program website.

 

Maintained by the IET Department of the College of Agriculture and Natural Resources. © 2017.