Extension > Agriculture > Nutrient Management > Nutrient/Lime Guidelines > Fertilizer Recommendations for Agronomic Crops in Minnesota > Understanding the Soil Test Report
Understanding the Soil Test Report
The concept of soil sampling and analysis of samples collected has been the basis of fertilizer recommendations used in crop production for many years. Regardless of the procedure used for the collection of the samples, the results of the laboratory analysis that reach the crop producer are frequently confusing. The units used to report the analytical results are not familiar. There are several numbers on the analysis sheet. There is a relationship between the analytical results and fertilizer and lime recommendations. Some explanation of the information which appears on the analytical report would probably be helpful.
The reporting units
The numbers found on any soil test report are the result of some analytical measurement of the nutrients in the soil. Most soil testing laboratories report this measurement as parts per million abbreviated as ppm. This reporting unit is used for nutrients other than nitrogen. Some laboratories report measurements in terms of pounds per acre. There is a simple conversion factor for these two reporting systems. That conversion is: ppm x 2 = lb. per acre.
When soil pH is measured, there are no units associated with the number that is reported. The same is true for the buffer pH.
When soil samples are analyzed for nitrate-nitrogen (NO3--N), most laboratories will report the analytical results in two ways. The concentration is reported in terms of ppm. Then, depending on the depth from which the sample was collected, the concentration is converted into pounds of NO3--N per acre for each increment of depth that was sampled. For example, if soil was collected from depths of 0 to 8 and 8 to 24 inches, the amount of NO3--N at each depth is reported in terms of lb. per acre. The total for the 0 to 24 in. depth is calculated as the total that is found at 0 to 8 and 8 to 24 in. The sample calculations also apply to other depths that might be sampled.
There are a variety of reporting units for soil organic matter content. Some laboratories report the organic matter in relative terms for low, medium, and high. Others report the measured percentage.
The units chosen to report the analytical results do not have any effect on fertilizer guidelines. It is important, however, to be aware of the difference between ppm and lb. per acre when reading the units associated with the numbers on the soil test report.
The procedures used
There are several analytical procedures that can be used to extract plant nutrients from soils. The procedures used in testing soils are not designed to measure the total amount of any nutrient present in the soil. The analytical procedure used to measure a specific nutrient is selected because it extracts the portion of the total amount of that nutrient that is best related to plant growth. The selection of an analytical procedure is not arbitrary. The procedure selected has been developed from considerable research as the one which best predicts the amount of that nutrient in the soil that can be used by plants. The Bray and Kurtz #1 procedure (sometimes referred to as the weak Bray procedure) used for measuring phosphorus in acid soils is a good example. Results of considerable research have shown that the amount of phosphorus extracted by this method is the best predictor of the need for phosphate fertilizer for acid soils. Bray and Kurtz also developed an analytical procedure that uses a stronger acid. However, the amount of phosphorus extracted by the stronger acid was not related to crop growth. Therefore, the use of this strong Bray or Bray P-2 procedure has no value for making phosphate fertilizer guidelines in Minnesota.
Analytical procedures used in soil testing are usually standardized. Currently, most soil testing laboratories that operate in the North-Central states use the same analytical procedure when analyzing for a specific nutrient. These laboratories also participate in a quality control program that produces confidence in the analytical results coming from that laboratory.
The numbers listed on most soil test reports are usually followed by a letter such as VL, L, M, H, or VH. These letters are abbreviations for very low, low, medium, high, and very high, respectively. These letters designate the relative level of the nutrient measured and provide a good indication of the probability of measuring an economic increase in yield if fertilizer supplying the nutrient in question is applied. For example, if the relative level is classified as being very low, there is a high probability that crop yields will increase if fertilizer supplying the nutrient in question is applied. By contrast, no increase in yield from the application of the nutrient would be expected if the relative level in the soil is in the very high range. The relative proportion of nutrient needed from either soil or fertilizer at the various soil test levels is illustrated in figure 1.
The relative levels of the various immobile nutrients in soils have been defined in terms of concentration (ppm) measured by the appropriate extraction procedures. These definitions, used by the University of Minnesota, South Dakota State University, and North Dakota State University, are listed in the following table.
Table 1. Description of each relative level for the phosphorus (P), potassium (K), and zinc (Zn) extracted by the appropriate analytical procedure.
|Relative Level||Bray and Mehlich III||Olsen||Potassium||Zinc|
|— ppm —|
|Very low (VL)||0-5||0-3||0-40||0-0.25|
|Very high (VH)||21 +||16 +||161 +||1.01 +|
The range of values for each relative level shown in the above table is not used by all soil testing laboratories. A soil testing laboratory can use any range of values that it chooses. A difference in the range of values for each relative level is one source of confusion that adds to the difficulty of evaluating results from more than one soil testing laboratory. The ranges in the preceding table are the end result of a considerable amount of research conducted in the field.
It is important to understand that the number associated with any nutrient on a soil test report is an index value to be associated with one of the five relative levels. It is not the amount of a nutrient that is available for crop use. It is not the total amount of a nutrient present in the soil. The number listed is an index value only and when combined with an expected yield can be used to develop a fertilizer guideline.
Understanding the lime guidelines
The soil test report also shows the pH of the soil sample and, if acid, provides a guideline for the amount of liming material to be used. When a soil sample is started through the analysis process, it is first mixed with water and stirred; then a pH reading is taken. This reading is the soil pH value. If this value is less than 6.0, the soil is mixed with a special buffer solution and another pH reading is taken. This second reading is known as the buffer pH. This buffer pH value is used to determine the rate of lime that is needed.
Interesting, but not useful
The reports from some soil testing laboratories list the Cation Exchange Capacity (CEC) of a soil. This is a fixed soil property that varies with soil texture and organic matter content. This soil property, however, is not useful for making fertilizer guidelines.
The laboratories that measure CEC also usually report values of exchangeable potassium (K), calcium (Ca), and magnesium (Mg). This is interesting information. But, this information is of little value for making fertilizer guidelines in Minnesota.
Don't be confused
The soil test report contains a substantial amount of information. At first glance, this report can be confusing. Hopefully, the information presented in the previous paragraphs can help to eliminate some of the confusion.