Salt content
Soil media that is high on salts, will have an adverse effect on the growth of plants. Coconuts harvested from the trees that are very close to a sea coast are usually rich in salts. Naturally, the husks also contain a good amount of salts. Ageing (weathering) and repeated washing (rain or river water) removes a bulk of these salts. This is one of the reasons why we give a lot of importance to aged husks. In spite of all this, coconut coir still has considerable salts that need to be washed off before it can be used as a growing media. For a detailed explanation, please see below:
The surface of the coir is made of negatively charged particles like Chloride (Cl-), Bromide Br-), Bicarbonate (HCO3- ) etc. These are called anions. By nature, fresh unwashed coconut coir has excess amounts of Sodium (Na +) and Potassium (K+); a low level of Magnesium (Mg 2+), Calcium (Ca 2+) and Iron (Fe 6+). These are positively charged particles called cations.
Like opposite poles of a magnet, the cations and the anions always are attracted to each other. Electricity is nothing but the flow of charge. The flow of these charges in a solution is measured as electrical conductivity (eC), expressed usually in millisiemens per centimeter (mS/cm). So, if you can measure the eC of the coir-water extract, you can precisely estimate the salt content of the soil medium. Generally speaking, if the eC is > 1.0 mS/cm, then the salt content is going to be high in the coir. If left unchecked, this can affect the roots of plants resulting in stunted growth.
Here is a picture of the lab results of UNWASHED, UNBUFFERED coco peat (Pic-1):
And here is a picture of the lab results of WASHED, UNBUFFERED coconut coir (Pic-2):
From Pic-1, notice it is clear that fresh, unwashed, unbuffered coir has a very high salt content (4.4 mS/cm). Pic-2 shows the lab results of coir that has been washed multiple times with water. The soluble salts like Sodium and Potassium are greatly reduced in washing. Hence in Pic. 2, the salinity is reduced to 0.8 mS/cm.
From Pic.2 it is clear that the levels of other cations like Calcium, Magnesium, Manganese, etc. in washed coir ranges from low to medium. But even after several washes, the levels of potassium (K= 152 ppm) and sodium (Na = 69 ppm) are very high.
While this is a step in the right direction, this coir media is far from perfect. Fact: It doesn’t matter how much the coir is washed. It is impossible to eliminate all the salts. This is why coir needs to be pre-treated prior to use. This pretreatment process is called “ buffering”. While it doesn’t eliminate the excess salts, for sure, it will “mask” them and prevent them from leaching to the roots of the plants.
Science behind buffering coconut coir
Any nutrient (Miracle Gro, FloraBloom, Dakine, etc.) added, is purely to benefit the plant - not the growing media. If the coir media is not pretreated, then it will fully absorb these nutrients. Coir media will not release the nutrients to the plant, causing stress. The first two weeks of rooting are critical in a plant's growth. This stress will cause the leaves to wilt, curve upward or turn yellow. Often growers think this is because of a rapid change in the pH. It is not. The technical term for this culprit is “nutrient lockout”. With us so far? Please read below for an explanation.
Here is a picture of the lab results of WASHED & BUFFERED coconut coir (Pic-3):
When a nutrient containing a combination of calcium, magnesium, iron, zinc, etc. is added to a coir mix, the excess Na+ and K+ cations present in the surface of the coir get exchanged for the Ca 2+ & Mg 2+ ions present in the nutrient. The ability of a soil media to absorb these cations is called cation exchange capacity (CEC). The CEC value of 308 meq/Kg (see pic.3) is simply the ability of the soil to hold or store the cations (or nutrients). CEC measures the media’s ability to hold nutrients. It is a measure of soil fertility. Soils with high CEC have the ability to hold more cations, making them sufficient in calcium, magnesium and other cations.
These Na+ and K+ cations that are present in the coir in a salt form (eg. Sodium Chloride). So, when the nutrient is mixed with water and added to coir media, these salts will dissolve in the nutrient-water solution. These two undesirable ions get exchanged for the desirable ions (Ca2+ & Mg2+). For every one unit of calcium or magnesium in the nutrient, only one unit each of sodium and potassium in the coir gets exchanged. In other words, the sodium ion that was bound to the coir gets released into the solution. A vacant spot is created now. The calcium from the nutrient fills this vacant spot. This is called an “exchange”. To make things worse, when this exchange happens, the harmful salts get released into the solution, which then gets absorbed by the roots. The good stuff (Calcium) could never reach the roots because it is locked up by the coir. This is called “nutrient lockout”.
Think of it this way. You rent your home to a guy (or gal!) because you want to make a profit and grow financially. Let's say this person never pays a dime to you. Instead, he subleases your home, skims a profit, and also ransacks the place. Will you ever see growth? In a similar way, if left unchecked, coco coir will eat all the added nutrients.
How buffering helps?
“Buffer”: to lessen the shock of; a means or device used as a cushion against the shock of fluctuations - Merriam Webster dictionary.
Buffering is simply a pre-treatment of the coir before it can be used as a growing media. The idea behind buffering coir is to reduce the stress to the plant and ensure the nutrients are readily available. By soaking coconut coir in a solution of calcium nitrate and magnesium nitrate for 24-48 hours, we let the coir absorb as much calcium and magnesium as possible.
During this buffering process, the excess potassium and sodium content in the coir gets exchanged (washed out) for calcium and magnesium. Since the Na+ and K+ levels are reduced in the buffering process, the coir does not have to compete with Ca2+ & Mg2+cations. So, the added nutrients in the liquid form will be readily available for the roots of the plant.
Comparing Pic.2 and Pic.3, the following can be concluded:
1. Buffering reduces the potassium level (from 66 ppm to 16 ppm).
2. Buffering reduces the sodium level (from 69 ppm to 64 ppm).
3. Buffering increases the calcium level (from 8 ppm to 34 ppm).
4. Buffering increases the magnesium level (from 4 ppm to 12 ppm)
5. Buffering increases the CEC (from 225 meQ/kg to 308 meQ/kg).
Since there is a big reduction in the salt levels, the exchange (or leaching) of unwanted salts to the plant is blocked. Likewise, with increased levels of Calcium and Magnesium in the coir, all nutrients in the fertilizer added will readily be available to the plant. The buffered coir media does not have to compete with the plant for nutrients anymore.
Isn’t that what we want in the first place for a growing media?