Spectrum Agronomic Library

Soil analysis is used to express the approximate availability of nutrient to plants. There is no soil extractant used that will indicate the exact amount of that nutrient which will be available during the growing season. Soil chemistry is too dynamic and complex to interpret such results. That is why universities, consultants and laboratories have done research on the addition of fertilizer based on soil analysis results. At Spectrum Analytic, Mehlich 3 is the primary extractant in use. There are other extractants that we can perform upon request for additional fees.

Major nutrients have been researched for many years and since they are required in large amounts by plants for optimal growth there is a lot of data available. Interactions have been studied, but for the most part the major and secondary nutrients are pretty straight forward when it comes to soil test results and recommendations. Nitrogen is an exception to this rule; we will not discuss it within this paper. We know that as soil pH drops and becomes more acidic the phosphorous in the soil becomes less available due to tie up. In our recommendation system we compensate for this by recommending a higher level of phosphorous in the soil when compared to a phosphorus level in a more neutral soil pH. This additional phosphorus helps overcome the tie up making more available for the plant. This additional phosphorus has been demonstrated with higher P levels in the plant analysis.

Early agronomists at our lab looked at thousands of plant analysis along with soil analyses in the early years and noticed patterns with copper and manganese. They saw how organic matter, soil pH, and soil test levels of certain nutrients influenced the availability of these elements to the plants. Equations were developed to take Mehlich 3 soil test results and provide our clientele with a better recommendation for the addition of these micronutrients. In the early days these numbers were expressed on the soil test report as an “index”. This worked well for many years, however with the nutrient management plans that are developed the use of these index number had to be abandoned. More standardization is being required for these plans, to make them easier to write and less confusing to the reviewers. To compensate for this and try to make it easier for people we tried to unindex our equations to the different status levels on our reports, in doing this some of the ranges would start at 1 ppm as being in the good range. And we know that this is not a true representation. We therefore decided to abandon the numeric optimal range on the soil test report in the status range. The graph on your report as well as the letter designation (L, M, G, H, VH) are put through the index equation in order to make a better approximate copper and manganese availability number.

The equation for copper contains soil pH and soil phosphorus level. The manganese equation contains organic matter and soil pH as part of the components. If you would like to discuss the full equations please contact one of the agronomists at the lab and we will be happy to talk with you on a one to one basis.