Cations and CEC

SpartacusCations are the positively charged elements that are necessary for plant growth. The ones we are concerned with for plant growth are:
Calcium (Ca), Magnesium (Mg), Potassium (K), Sodium (Na). Each plays a role in plant nutrient uptake and plant health. For example, magnesium is one of the key constituents of chlorophyll. Calcium aids in the uptake of all nutrients and helps to build strong cell walls. Calcium and magnesium also play a significant role in the texture of soils. Calcium tends to loosen tight clays; magnesium tends to bind soil particles together.

Cations are held in the soil on cation exchange sites. The exchange sites are negatively charged locations on clay particles or humus that attract and loosely hold the positively charged cations. One can think of the cations as having the “hook” on velcro, while the exchange sites have the “loop”. The different elements have differing “hook” strength, meaning that some are more easily freed from the exchange site than others.

The number of exchange sites in a soil is quantified by the parameter Cation Exchange Capacity or CEC. Exchange sites occur on the edges of soil particles and organic matter. Clay, which has a particle size just one hundredth to one thousandth the size of a sand particle, usually has a fairly high CEC. Soils with higher organic matter content tend to have higher CEC than those that do not. Sandy soils tend to have lower CEC’s. Sometimes the term “TCEC” is used, or “Total CEC”. TCEC is a particular method of calculating CEC.

Some of the exchange sites may be filled with hydrogen (H). Below a neutral pH of 7.0, the lower the pH, the higher the percentage of hydrogen ions on the exchange sites. Hydrogen is the element with the lowest binding energy to the exchange site (the weakest “hook” in our velcro model). Since it can easily be removed, it means that other elements can be exchanged with it fairly readily. The presence of some hydrogen creates a more dynamic situation and allows elements to move more rapidly, e.g. into the plant. However, too many hydrogen ions means that the necessary plant nutrients are not available in the soil — the exchange sites are full of hydrogen instead of the other plant nutrients — and the pH is lower than optimum.

In order to grow nutrient dense food, our job is to move the ratio of the four major cation nutrients towards balance. The optimum balance has been found to be approximately this:

  • Calcium (Ca): 60-70% of CEC
  • Magnesium (Mg): 10-20% of CEC
  • Potassium (K): 3-5% of CEC
  • Sodium (Na): 1-2% of CEC

The percentages will not add up to 100% unless the pH of the soil is 7.0 or higher. The exact optimum percentages depend on the soil type, the soil pH and the soil’s parent material. Different labs calculate CEC differently. The percentages mentioned here refer to Logan Labs and their Mehlich 3 extraction test.

In OrganiCalc, we target an initial percentage of 68% Ca and 12% Mg. We want to keep the Mg low, because it is hard to get rid of Mg once it is in. Also, we put the Ca at the high end, so it will displace excess cations. These initial targets work very well in practice.

If your ground is sandy, you will want more Mg and less Ca. If you have clay soil, you will want less Mg and more Ca. If you have achieved equilibrium and your plants are growing very well, the Ca:Mg ratio you actually have is more important than a theoretical ratio. There are actually broad ranges of balanced Ca:Mg ratios. Ca% + Mg% should equal 78% to 82%.

When your soil is mineral-balanced, it’s time to really focus on the biology. Keep your organic matter below 30%; 5% is a nice number where summer is hot; 8 to 9 % is a good number where summers are coolish, such as in the Maritime northwest.

Finally, if your soil is otherwise balanced and you have excess Ca, don’t try to fix it; leave it alone.

A word about units

We usually use units of lbs/acre (pounds per acre) to describe mineral amounts. This unit refers to the weight of the mineral in the top six inches (15 cm) of soil. The same amount of mineral spread over one hectare would weight one kilogram, hence 1 lb/acre = 1 kg/hectare. A density of one part per million (ppm) of an element in the top six inches of soil would weigh 2 pounds if spread over an entire acre. To convert ppm to lbs/acre multiply by 2.

next… Nitrogen

2/13/14 Copyright Erica Reinheimer, Steve Solomon Rev 03a