Calcium and magnesium are both important nutrients for plants, and they have a special relationship with coco coir that requires some special attention. While coco coir is a proven growing medium, it should be of the buffered variety as unbuffered coco coir can be problematic with use.
Calcium is a secondary nutrient that is critical for proper plant growth. Carried from the root system to the developing cell walls via xylem sap, it is required for proper cell wall development, it interacts with growth enzymes and is also used for internal signaling.
Calcium deficiencies can be caused by either a lack of calcium available to the plant, or a lack of transpiration (the movement of the water that carries the calcium from the roots to the leaves) such as if the plant is exposed to cold, wet conditions.
Without sufficient calcium available, deficiencies show in newly formed cell walls, often first visible at the youngest leaves which will show necrosis (dead or dying) tissue along the tips and margins of the leaves, or as leaf curl. Blossom end rot in tomatoes is an example of a calcium deficiency. As it is immobile once integrated into the plant, the plant cannot move calcium from older growth to supply the needs of new growth, so it is important to correct any calcium issues promptly. An untreated deficiency will continue to cause general ill health to the plant as it becomes less able to create healthy new cell walls.
Magnesium is another secondary nutrient that is required by plants. It is mobile, and can move through the plant either through the xylem or the phloem. It is used to form chlorophyll, which is required for photosynthesis and proper enzyme function.
Magnesium deficiencies can be caused by either a lack of magnesium available to the plant, or as a result of an overabundance of potassium, ammonium, calcium, or manganese.
As magnesium is mobile, a deficiency will result in the plant moving it from the lower leaves to newer growth. This will cause a yellowing of the lowest leaves, with the leaves starting to yellow between the leaf veins giving them a mottled appearance.
One reason the coco coir is a popular growing medium is that it has a good cation exchange capacity (CEC) rating. Cations are positively charged ions such as calcium, magnesium, potassium, and sodium (among others), and the cation exchange capacity of a growing medium describes how well it can hold and exchange them. One way to think of a CEC is a rating of how well a medium stores nutrients.
Since coco is pretty good at holding nutrients, it should be of no surprise that it naturally has some attached to its cation sites. Specifically it starts with high levels of potassium and sodium.
Quality coco coir has been treated to replace some of these cation sites with a calcium (such as calcium nitrate). Calcium has a stronger bond to the cation sites than potassium or sodium because it has a double positive charge as opposed to the single positive charge of potassium or sodium.
This is done because untreated coco coir, being high in potassium to start with, will exchange nutrients in the nutrient solution and release potassium and sodium when used. Not only can this create an issue with denying the plant the intended solution, but it will release an overabundance of potassium. This can not only lead to a magnesium deficiency as described above, but the issues associated with potassium overdosing. By rinsing the coco coir with a solution of calcium before use, much of this issue is corrected. Coco coir that has already been treated is referred to as “buffered coco coir”.
By understanding the interaction between coco coir and plant nutrients, it becomes apparent that buffering is an important step in getting reliable and consistent results from coco coir use.