Buffering Up: Adjusting the Cation Exchange Capacity in Coco Growing Media
The adjustment of the cation exchange capacity of grow media, referred to by many as the buffering of grow media, is a subject that most hydroponic growers could benefit from knowing a bit about. Mark Wittman provides growers with a technical look into this process and how buffered media has a big impact on what you feed your plants.
The widespread use of calcium and magnesium supplements in the indoor gardening industry is an indication that many growers have discovered the cation exchange capacity (CEC) phenomenon in coco coir media.
Growers are observing deficiencies and trying to correct those deficiencies with calcium and magnesium supplements. This article explores why these deficiencies happen and how adjusting the cation exchange capacity, or buffering, the media corrects this problem.
Coco growing media has changed a lot in the last decade or so. Previously, the majority of coco products were washed to an EC of 1 or 1.6, and few products on the market were buffered. Today, the higher-quality coco products are washed multiple times and will have an EC of 0.5 or lower. They will also be buffered in some way or another.
Coco Coir's Cation Exchange Capacity
Soil particles and organic matter have negative charges on their surfaces that attract cations. The total of these negative charges are collectively referred to as the CEC.
The CEC is significant in growing media because it is a measure of the quantity of nutrients the media is able to hold on to before nutrients start leaching out. Plants are able to access the cations attached to the cation exchange capacity.
Coco often has a CEC in the range of 90-100 meq/100 g of media. The CEC of coco is naturally loaded with potassium (K) and sodium (Na), with little or no calcium (Ca) or magnesium (Mg). These are the four cations that are most important when buffering coco.
The goal is to significantly lower the percentage of CEC sites that have K and Na on them and increase the percentage of CEC sites that have Ca and Mg attached. Potassium can be attached to up to around 40% of the sites and sodium can be attached to up to around 15% of the sites.
This is significant because if 40% of the exchange of un-buffered coco is holding potassium, then that equals 40 meq/100 g of media of the single-charge K molecule.
The 100 g of weight in the above equation is the dry weight of the coco, not the weight of the coco straight out of a bag when it is moist. Hydrated coco should make 12-15 L (3-4 gal.) of coco growing media per kilogram of dry coco and of course 100 g is one-tenth of a kilogram.
This does not sound like much, but would be as much as 1.56 g of potassium per 100 g of media. This is a lot of potassium, most of which will be slowly released into the aqueous solution around a plant’s roots. Compare this 1.56 g to 0.22 g of potassium per liter of nutrient solution (which is feeding potassium at 220 ppm, the amount that one would have in a well-balanced feed).
If you have a 4-gal. pot and give it 1 gal. of feed per day, you would be feeding about 0.9 g of potassium and the CEC may be holding 15.6 g of potassium.
Sodium may be present in up to 0.35 g per 100 g of media. With these numbers, an unbalanced nutrient solution will quickly result, as I explain in more detail below.
Read More: The Benefits of Using Coco Coir
What Is Coco Buffering?
Buffering coco media is accomplished by exposing the cation exchange to a solution of water with a high concentration of the cations that are desired on the exchange sites—in this case, calcium or calcium and magnesium.
Because the cations on the exchange sites are held reasonably tight, washing coco does little to change the makeup of the cations on the exchange sites. The washing will change the EC but not the CEC. CEC sites have a preference for some cations over others.
If the cations of Ca, Mg, Na and K are all present in the solution at the same concentration, they will be adsorbed at different levels, with calcium and magnesium being adsorbed at double the rate as they both have a double-positive charge, while potassium and sodium have a single-positive charge (Ca++, Mg++, K+, Na+).
For example, many coco product manufacturers buffer their coco with 8 kg of calcium nitrate per cubic meter of coco. Calcium nitrate has a value of 19% calcium, which equals 1,520 g of Ca with almost no Mg, K or Na if the water is clean.
As the process commences, a high concentration of Ca molecules attaches to the media—as each Ca++ molecule is adsorbed, two molecules of K+ or Na+ are released because the Ca has a double-plus charge and K and Na are single-plus charges.
In the beginning, the exchange goes very quickly, but as the exchange continues, the concentration of the K and Na molecules released into the solution slows the exchange down and it will eventually come into equilibrium. The buffering process can be done in 10-15 minutes—the point at which the exchange slows down enough that the greater exchange is not worth the wait.
Some coco products have been buffered with a higher treatment of Ca and Mg concentration. This creates a lower K and Na percentage on the exchange and adds the benefit of Mg to the CEC. These more advanced buffering processes involve a much greater amount of time, but result in much lower K and Na levels on the exchange.
This essentially creates a better coco product from day one, ensuring all of a nutrient mix goes straight to the plant versus amending the coco’s CEC.
Read More: The Chemistry of Coco Coir
What Does This All Mean for Growers?
As a grower, your goal is to create and use a well-balanced nutrient solution. If you are using an un-buffered coco product, a well-balanced nutrient solution goes into the coco and starts to buffer the coco as well as feed the plants, instead of all of the nutrients going directly to plants. So, the CEC in the coco is exchanging some of the K and Na for Ca and Mg.
This exchange is now unbalancing your nutrient solution, increasing the K and Na while decreasing the Ca and Mg. How much unbalancing, you ask?
Earlier, I mentioned the coco could have as much as 1.56 g of K and 0.35 g of Na per 100 g of coco. Your nutrient mix is not highly concentrated with Ca and Mg, but it is enough to get some of the K and Na released from the CEC.
About 15 years ago, I was growing roses in coco and we did a weekly chemical analysis of our feed and drain water. The first time we used coco, we noticed the Ca in our drain water was less than 40 ppm (we would have normally expected the Ca to read 100-150 ppm in the drain water), and we were feeding Ca at a rate of about 200 ppm.
For the next two weeks, we had the same result, so we doubled our Ca to 400 ppm. The analysis of our drain water went up to about 50 ppm of Ca. We watched that for about three weeks and then started feeding Ca at about 500 ppm and still had very little change in the Ca ppm in our drain water. It took about four months for our drain water Ca analysis to read about 100 ppm.
The loss of the Ca and Mg is one thing, but you also get an increase of K and Na. High levels of K will hinder the uptake of Mg by plants. Sodium can negatively impact plant health even at low levels and is toxic to some plants starting at 50 ppm.
The widespread use of calcium and magnesium supplements in the indoor gardening industry is an indication that many have experienced the CEC phenomenon in coco that I am talking about here.
The deficiencies are observed and can be corrected to a certain extent with calcium and magnesium supplements, but there are also coco products out there buffered to a higher level, which don’t need the calcium and magnesium supplements.
Read More: A Quick Guide to Growing Plants in Coco Coir
A Coco-Buffering Glossary
- Cation Exchange Capacity (CEC) – The degree to which a growing media can adsorb and exchange cations. The value of a cation exchange capacity is normally expressed as meq/100 g.
- Cation – A positively charged ion, such as Ca++, Mg++, K+, Na+, NH4+, H+, Al+++.
- Milli-equivalent (meq) – A measurement often used in quantifying a cation exchange capacity (meq/100 g = milli-equivalent per 100g of dry media). The milli-equivalent is based on the value of an equivalent. An equivalent is the weight in grams of a molecule divided by its molecular weight multiplied by its charge. For example, hydrogen (H+) has a molecular weight of 1 and a single positive charge, so 1 g of hydrogen is one equivalent. Calcium (Ca++) has a molecular weight of 40 g and a double-positive charge, so 40 g of calcium is two equivalents.