There is a lot of information these days about soil testing.  In some cases, almost too much information for anyone to interpret.  Here are some things to consider when looking at soil tests.  The soil comprises three components:

  • Physical
  • Mineral
  • Biological, which is the component most often forgotten

Without all three of these components working in harmony, yields can drag, and, the crop will struggle not only throughout the growing season but in subsequent years, as well.

The Mineral Component:

Most farmers send in samples for a standard soil test.  This test identifies the minerals in the soil and also the quantities of each mineral in the soil being tested.  Which results should we be looking at?  My suggestion is to start with the soil pH.  pH or “percent hydrogen” reveals a lot about the soil.

  • If a soil has a high pH, then the number of cations present, like Calcium (Ca), Magnesium (Mg), etc. are present in large amounts.
  • If it has a low pH, then the cations, like Calcium, Magnesium, etc. are there in small amounts and there is a larger amount of Hydrogen (H) present in the soil.

Why is this so important to the grower?

Remember the following chart (Figure 1.):

 

Figure 1.

http://articles.extension.org/sites/default/files/w/c/c6/PHandavailability.jpg

You can see that nutrient availability is at its greatest across the board when the pH of the soil is slightly acidic and in the 6.3-6.5 range.

By looking at the chart, we can see that if the pH is too low, the macro elements – Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S) – will be very difficult for the plant to access.  Therefore, in certain circumstances, it would make more sense for the grower to apply limestone then to do anything else to that field in the off season.

While pH is significant, analyzing the rest of the test results is also important. Be sure to look at the Ca and Mg levels in the soil.  Typically, the base saturation section of the soil test can help determine if the Ca:Mg ratio is too tight, which can negatively impact the soil.  A tight soil struggles to drain or remain “open” for roots to move down.   Air and gas exchange can be limited, leading to plant health issues.

One rule of thumb that I use is:

(Ca base saturation) + (Mg base saturation) = 80%

  • If the soil that is tested is made up of clay; then we would like as much of the 80% to be Ca. This will allow for the clay particles to spread apart from one another allowing water, air and other gases to move through the profile.  This will also ensure that the roots of the plant will dive deeper into the soil profile.
  • If the soil that is tested is made up of sand, then we would like to see 20% of the 80% made up of Mg. This will keep the soil/sand particles a little closer together.  This is important in this type of soil system because leaching will take place much quicker in a sand soil than a clay soil.

 

We can’t physically change the soil make-up of the fields that we are farming, but we can try to manipulate the chemistry of the fields to help with mineral retention, plant uptake and water movement.

Other Indicators:

I typically look at the Cation Exchange Capacity (CEC) (or Total Exchange Capacity (TEC)) of a soil along with the Organic Matter percentage (OM).  These two findings give you a very good idea of what the soil can hold and what can be given back up to the plant.  Recognize that the higher the CEC or TEC, the more minerals the soil can hold.  Be mindful, this does not guarantee that they will become available but, if they are present in the soil, there is a much better chance they will be plant-available than if they are not present at all.  Organic Matter can release nitrogen throughout the season through mineralization from the microbial community in the soil.

There is a lot of information available to help with the decisions that need to be made on the farm.  I would highly recommend that you look at your soil tests with your yield maps.  In most cases, if we overlay information we can get the full story of what is happening out in the field.  Utilizing the information, I touched on earlier and combining it with your soils map and yield data may help you make decisions that are both agronomical and economical.

Blog Post by Jake Straub, Performance Nutrition A division of LidoChem, Inc.