Step by Step Soil Test Interpretation

Step by Step Soil Test Interpretation

IMPORTANT NOTE: all suggested application rates from the linked forms in this guide are for correcting imbalances only and do not include fertilization for crop removal. Please refer to our Application Guide which provides fertility programs for various crops.

Some labs give recommendations and some don’t. Use the steps below if you need to calculate your own recommendations or want a second opinion about what your lab is recommending.

1. The first thing to look at is the pH on your soil test report. Use our Calculate your lime application form. to determine if you need lime, how much, and what kind. If the pH is greater than 7.0, you may need to acidify the soil (see step 2).

2. If the pH indicates an alkaline condition (7.5 or higher), you may want to consider applying elemental sulfur to your soil. Use the Calculate your sulfur application form to determine the amount of sulfur to apply. Sulfur bacteria use sulfur as energy and create sulfate ions (SO4). Sulfate mixes with water to create sulfuric acid, which reacts with carbonates in the soil to make carbon dioxide and water. Removal of carbonates lowers pH. Soil temperature has to be above 60o F for these reactions to occur because sulfur bacteria are inactive below that temperature.

3. Look at the values for potassium (K), magnesium (Mg), and calcium (Ca) and see if any of the values are labeled VL (very low) or L (low), or are below the lab’s recommended range. If any appear to be low, look at the percent base saturation and compare those values with the following ranges or the ranges offered by your lab.

Cation % Range
K 2-5%
Mg 10-15%
Ca 65-75%
If your values fall well within those ranges, then the low rating may not be a major concern. Also, don’t worry if your values are above those ranges. If the values are labeled VL, are well below the recommend range, or fall below the base saturation ranges above, K, Ca, or Mg may need to be applied. Cation exchange capacity (CEC) is important to determine the amount of these elements needed. If your lab doesn’t report CEC or reports ECEC instead, you may be able to discover CEC using this form. When you’ve found the CEC, use the calculation forms for potassium, calcium, or magnesium to determine what you need. If you’re not sure what material to use to correct the deficiency, consult our Material Nutrients List.

IMPORTANT NOTE: Calcium and magnesium deficiencies can be corrected with calcitic or dolomitic lime (respectively) but these materials will also raise the soil’s pH. If changing the pH is undesirable, gypsum and Epsom salt will add calcium and magnesium (respectively) without raising the pH.

4. Potassium (K) is usually expressed in parts per million (ppm) of the actual element on the analysis report but fertilizers measure potassium as potash (K2O) and application rates are usually expressed in pounds per acre (ppa). These forms will convert the application rates to ppa for you. But if the lab reports K as ppa, you’ll have to convert to ppm before entering values into our form. The conversions are available on this form.

5. Magnesium (Mg) is also usually express in ppm on the analysis report. The conversions are available on this form.

6. Like Mg and K, Calcium (Ca) is also typically expressed in ppm on the analysis report. Conversion is the same as it is with Mg (see conversion form).

7. Phosphorus is calculated differently than K, Mg, and Ca because it has opposite magnetic properties and is not related to the soil’s CEC.  There are several accepted methodologies used to test for phosphorus but they give different results so it’s necessary to know the methodology used. Most land grant university labs use a Morgan extraction while many others use Mehlich 3. Open the Calculate Phosphorus Application form and follow the instructions to determine your application rate.

8. If your lab report indicates deficiencies in one or more trace elements, use this form to determine the amount you’ll need for correction.

9. Now look at the percentage organic matter (OM). Soil OM can contain  nitrogen (N) and other nutrients and it’s valuable to be able to estimate the amount of N that can possibly be released. If the lab you’ve used offers an OM value, take a look at our Estimated Nitrogen Release table to see the reserve in your soil. It is important to understand that the amount of nitrogen released from you soil’s organic matter is very dependent of environmental factors such as soil temperature, moisture, and the character of the organic matter.

If the application rates given in pounds per acre (ppa) need to be converted to smaller areas, use this form.