Interpreting Your Soil Test

You have received the results of your soil test. Now what?

Do you need lime?

Step 1:

The most important information the soil test provides is based on the pH of the soil. If your pH is too low for the crop you are growing, there will be a lime recommendation. If there is a lime recommendation, it should be applied about 2 months before any fertilizer is added to allow the liming material to raise the pH to an optimal level for your crop.

When applying lime, it is best to mix it in with the soil. Lime is only slightly soluble in water and does not move into the soil as effectively as soluble fertilizers. Lime must be in direct contact with soil acids for it to react effectively. This is best accomplished when it is mixed with the soil. Water is also required for the reaction to occur.

Be aware that not all lime is created equal. Lime is graded according to its relative neutralizing value (RNV). This number is required to be listed on the bag of lime. MSU lime recommendations assume 100% RNV. If the liming product used is less than 100%, you will need to add additional material to compensate for the difference. See Publication 3762 Agricultural Limestone’s Neutralizing Value for more information on how lime RNV is calculated.

Once the soil pH is addressed, then it’s time to move on to the next step.

Step 2:

Calculate the amount of fertilizer required.

Some find it easier to calculate fertilizer requirements using a fertilizer calculator. There are many available online. Our friends at the University of Kentucky developed a fertilizer calculator that is very easy to use.

To use this calculator, first set the recommended unit/rate.

Triple 13 is a common fertilizer blend and almost always available at big-box stores. However, it is a “one size fits all” fertilizer blend. It may not be the most optimal for your situation. Using it may provide an excess of one element (phosphorus in this example) and a deficit of another (potassium in this example). In addition, because each element is present in relatively low amounts (13%), soil tests that recommend a larger fertilizer rate will require a substantial amount of the product (as compared to using urea which contains 46% N). However, using other sources of fertilizer to achieve optimal results might be impractical or cost-prohibitive. When deciding which fertilizer to use, weigh advantages and disadvantages.  

The second page of the soil test report contains additional details about your sample. Units for elements tested are in pounds per acre (ppa). Of particular interest are phosphorous and potassium: green bars indicate additional fertilizer will probably not result in additional plant growth or yield; yellow bars indicate a plant response may or may not occur; and orange/red bars indicate additional fertilizer will likely result in increased plant growth or yield.

What about nitrogen measurements? Plants require specific forms of nitrogen that are tricky to measure in the lab. Additionally, nitrogen is so mobile in the soil, measurement of current values would not be very helpful for predicting a nitrogen recommendation. Therefore, MSU Extension recommendations are based on research.

For horticultural crops, we also test total soluble salts (TSS). As salt concentrations in the soil increase, plant growth decreases. Excess salt concentrations in the soil limit the availability of water to plant roots. While there are many reasons why a soil may contain excess salts, a common source among homeowners is excess fertilizer. Remember when applying fertilizer, more is not always better!

TSS measurements on the soil test are accompanied by an interpretation. TSS ratings range from “low” to “excessive.” Readings less than 0.3 mmhos/cm are considered “low” and impacts to plant growth are considered negligible. Values higher than 0.3 mmhos/cm may impact plant growth—especially for crops that are sensitive to salts. 

Soil CEC (cation exchange capacity) is reported as estimated CEC. It is a measurement of how well the soil holds onto cations (like calcium and potassium). Soils with high clay content and/or high organic matter content are better at holding onto cations. Soils with a low CEC are less able to hold onto cations and so are more susceptible to potassium deficiencies. Sandy soils tend to have low CEC values (about 1–10 cmol/kg), loamy soils tend to have medium CEC values (about 11–20 cmol/kg), and soils with high clay content can have high CEC values (>30 cmol/kg).

Soil organic matter is the fraction of the soil composed of anything that once lived. It includes plant and animal remains in various states of decomposition, cells and tissues of soil organisms, and substances from plant roots and soil microbes. Well-decomposed organic matter forms humus, a dark brown, porous, spongy material that has a pleasant, earthy smell.

Mississippi soils generally are less than 2% organic matter (according to USDA Web Soil Survey results). Organic matter (OM) percentages reported on the soil test are an indicator of overall soil health. While the climate in Mississippi favors rapid breakdown of organic matter in our soils, comparing values from soil tests from one year to the next is a helpful way of evaluating land management practices. If OM values trend higher from one year to the next, that is one indicator that cultivation practices are improving soil conditions.