High Tunnel Soil Test Report: Organic Matter and Cation Exchange Capacity

 

Cation Exchange Capacity or CEC is a measure of the ability of a soil to hold cations or elements with positive charges.

 

Examples of cations are potassium, calcium, magnesium, ammonium, hydrogen, and sodium. A higher CEC indicates the soil can hold more cations. One way to visualize this by thinking of a magnet and paperclips. Imagine a magnet as the soil and paperclips as cations. If you pour the paperclips over the magnet, the amount of paperclips the magnet can hold is analogous to CEC.

 

It is a little more complicated though because not all cations are created equally. Some have higher charges, for example, calcium (Ca+2) has a higher charge than potassium (K+1). This means that calcium has a higher tendency to be held to the soil than potassium. 

Back to the magnet/paperclips analogy. Think of calcium as being a regular metal paperclip and potassium as being a plastic-coated metal paperclip. Now when you pour the paperclips over the magnet, more regular paperclips will be held than plastic-coated ones.

 

Cation Exchange Capacity is related to several factors including the amount and type of clay and organic matter in soils. Clay and organic matter have negative charges and hold cations. In general, soils with larger amounts of clay or organic matter have more negative charges and therefore a higher CEC than ones with lower amounts. You can think of this as a stronger magnet holding more paperclips.

 

CEC at pH 7.0 for various soil textures. Table adapted from Cation Exchange Capacity and Base Saturation

 

This bar is on the bottom of soil test reports from the Agricultural Analytical Laboratory at Pennsylvania State University. Orange boxes highlight the Cation Exchange Capacity (CEC) and organic matter content.

 

 

Cation Exchange Capacity for the 27 high tunnels ranged between 10.3 to 36.3 meq/100 g. The average was 10.9 meq/100 g. We did not record soil types or textures for the tunnels, so it is difficult to draw any conclusions from these values. It does appear that in general, the soils had heavy texture based on the range. The Lab tracks CEC for commercial vegetable soil samples analyzed. These would be a mixture of samples from fields and high tunnels. The average CEC for 1359 samples analyzed in 2016-17 was 13.9 meq/100 g.

 

Soil organic matter (SOM) is the organic component of soil, consisting of three primary parts including small (fresh) plant residues and small living soil organisms, decomposing (active) organic matter, and stable organic matter (humus). Organic matter content for the 27 high tunnels ranged from 2.4 to 15.5 %. The average was 6.3 %. In general for the field, we consider a range between 2 and 5 % to be a good target in Pennsylvania for our soils high in clay. Organic matter accumulates in high tunnels much quicker than in the field. At too high levels, problems from a nutrition imbalance can occur. When values reach about 15 % or more, the soil acts more like soilless media or potting soil.

 

It is estimated that you can get about 20 lb/acre of nitrogen from each percent of organic matter above 2 in the field. For example, a value of 6.3 % could result in 86 lbs/acre of nitrogen (6.3 - 2 = 4.3; 4.3 x 20 = 86). This is for the field though, and we are not sure how this translates to high tunnels. However, you should be getting some nitrogen as the organic matter decomposes.

 

As indicated, organic matter content influences CEC. However, CEC is also related to other factors including soil type and texture. As an example, two samples from our project had identical CEC values of 22.9 meq/100 g. The organic matter contents of the samples were 7.9 and 15.5 %. As another example, two samples had identical organic matter contents of 4.1 % and CEC values of 17.8 and 24.2 meq/100 g.

 

We are continuing our series examining soil test reports for high tunnels. This is based on results of soil tests from 27 Pennsylvania farms. Farmers sent soil samples from their high tunnels to Penn State’s Agricultural Analytical Services Laboratory (“the Lab”) for analysis. In this article, we looked at organic matter and cation exchange capacity. In past articles, we have examined soil pH, nutrient levels, and soluble salts levels: