By Grant McCarty
Local Foods and Small Farms Educator, University of Illinois Extension
So you’ve gotten your soil test results. How do you interpret them? Over the last years, soil test results have become easier to read. Most will tell you how much fertilizer to add based on what plants you specified you were growing. Soil test results will also show information on pH, phosphorus, potassium, soil organic matter (SOM), soluble salts, among others.
Your soil tests will also give rankings as to whether these are low or high. Additional information on other nutrients and soil qualities will be given but in most cases our main concerns are nitrogen, SOM, phosphorus, potassium, and soil pH.
When you examine the soil results, you’ll notice there is no mention of nitrogen levels. Nitrogen fluctuates in the soil so a test result for nitrogen is not accurate. The nitrogen recommendation from the results page comes from the SOM, plants you are growing, and other soil qualities.
Your SOM will be between 1-6% in the soil. If SOM is towards the low end, you may have limitations on the availability of nitrogen in the soil. As mentioned previously in my column, you should always be adding organic matter to the soil.
Both phosphorus and potassium are the macronutrients of Nitrogen-Phosphorus-Potassium (NPK) that we are concerned about. Optimum levels of P and K can vary among soil testing labs. In general, vegetables need soil with 18-50 ppm for phosphorus and 111-150 ppm for potassium.
Soil pH is a measurement of the acidity of your soil. Acidic soil is less than pH 7 while above 7 is considered alkaline. In general, vegetables and fruit grow well in pH between 6.0-7.5. In this range, your plants will acquire all of the soil nutrients they need. If the soil pH is too low or high, recommendations will be given to change these.
The last part of the results page will be your fertilizer recommendation. You want to match the NPK recommendation with a fertilizer. For instance, a recommendation of 0.3-0.6-0.3 would be a fertilizer that is a 1-2-1 ratio. It’s important to get as close to this ratio as you can. Information for the lbs of fertilizer to apply will be provided.
With these results, you are ready for a productive season.