Using Laboratory Analysis of Soil as a “Guide” for Fertilizer Recommendations

Posted on: Aug 14, 2015

When developing recommendations for our seasonal fertilizer programs, soil samples are pulled and sent to the lab, which issues a report. It is important to note that while values are given for every nutrient tested, don’t assume that the numbers are absolute values of parts per million (ppm) of nutrients per acre. Instead, the values listed should be seen only as a guide for making accurate fertilizer applications.

Potassium, for example, is analyzed with an ammonium acetate extractant that gives an indication of soluble and extractable K. Most soils actually contain thousands of pounds of potassium-bearing feldspars and micas per acre, minerals that contains slowly available potassium. The K locked in the mineral fraction is not accounted for in the lab test.

With every lab report should be an accompanying interpretive guide to give direction for the fertilizer recommendation process. A rating or index, is an indicator of expected plant response to fertilizer. A soil having a “LOW” rating has a very high probability that the crop will respond to a fertilizer application, whereas a soil with a “HIGH” rating indicates that little or no response would be expected. Below is a general guide for potassium prepared by Dr. Subarrao Yarlagadda, Helena’s National Agronomist.

  • Potassium (AA)
    • Less than 2% or 150ppm is Low
    • Between 2-5% or 150-300ppm is Medium
    • More than 5% or 300ppm is High

Each of the sixteen nutrients are analyzed and ratings are developed over multiple years by the lab, or research affiliates, and are based on hundreds of soil samples that have been calibrated with fertilizer applications. They are often specific and exclusive to the lab or researcher. And even though there are standardized methods of nutrient extraction among labs, ratings still vary from lab to lab.

Rather than using one lab’s interpretation for another lab’s results, it’s important to understand the method of analysis, ratings and interpretation for each agricultural lab that we use. Helena Agronomists have a close working relationship with a few select labs and have a good grasp on their analytical methods and ratings process. This is what allows us to use the lab report to make fertilizer decisions on the right rate and application method - with confidence.

- Don Wolf, Western Division Agronomist