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Agronomy Update

Soil Tests 101: How to Read Your Results

Published on Monday, January 02, 2017

One of the staples for growing healthy, high-yielding crops is to maintain good soil fertility. That’s why most agronomists will suggest soil sampling every two to three years to evaluate how your fields are holding up. These tests however are not always the easiest to read and many farmers often need help interpreting the results. Here are some key tips and areas to focus on when evaluating your soil test report. 

The first, and in my opinion, most important thing to look at is the soil pH. If your pH is off, it greatly limits the availability of all other nutrients. Figure 1 below shows the availability of nutrients at different soil pH levels. Let’s use phosphorous (P) as an example. As the pH drops below 6.0, the availability of P quickly diminishes. Phosphorous is an essential macronutirent that our crops need in high amounts to produce a favorable yield. It’s also a nutrient that most farmers have to apply each year to fulfill their crop needs. 

If the pH is too low, more P will need to be applied to produce the same result. Most of our row crops prefer a somewhat acidic pH (below 7), but just slightly. Ideally, we want our soil pH to range from 6.3 to 6.7. Keep in mind that the fertilizers we use, specifically ammonia based products like UAN or anhydrous ammonia, will quickly reduce soil pH because they flood the soil with hydrogen (H) ions, which in turn lower the pH. Buffer pH, which is a measure of a soils ability to resist change, is used to determine how much lime is necessary to achieve the desired pH level. 

 

Figure 1. pH impact on nutrient availability

The second thing I focus on is the percent base saturation levels and Cation Exchange Capacity (CEC). Base saturation is a measure of the basic cations: magnesium (Mg), calcium (Ca), and potassium (K) that are bonded to exchange sites on the soil. This number will always add up to 100 percent and will also show the measure of H and sodium (Na). The percent saturation of Mg, Ca, and K quantifies how many of those CEC sites are occupied by the basic cations while identifying if there is an imbalance. For example, in very tight soils you would want a Ca:Mg ratio to be closer to 7:1. In lighter soils however, you would want that number to be closer to 4:1.

Finally, I take notice of the percent organic matter (OM). This is the amount of the non-mineral content found in the soil. Your soils OM can influence the CEC, water holding capacity and nutrient cycling, and it can also contribute to the overall soil structure. The OM will mineralize throughout the year and release nutrients along the way. I suggest treating these mineralized nutrients as a bonus to your crop. Do not reduce your applied fertilizer rates. Typically OM content in many southern fields is between 0.5 to 2.5 percent. Soils with greater than three percent would be considered a high OM soil, which is typically beneficial for crop production. While OM can change over time depending on your farming practices, some ways to increase OM is using high biomass cover crops, applications of manure, reduced or no-tilling and by utilizing root bio-stimulants.

These are just a few things I focus on while looking at soil test reports, but are by no means all that should be evaluated. Please consult with your local Beck’s representative to help you dive into your soil test results further.

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Austin Scott
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