So I’ve been tasked with making a coconut coir-based organic “hot” soil (with pre-amended dry fertilizers) and I was thinking that I could buffer the coco during the “cooking” process with organic Calcium and Magnesium fertilizers.
According to most sources, unbuffered coco has up to 1.5g of potash per 100 grams (1250 cm^3) of substrate. So I’m thinking I would want to replace around 2/3 of this potash with calcium and magnesium during the 2-3 week “cooking” process between mixing and using the media.
So for every 1 gram K (=0.025 meq) I want to buffer, I calculate I would need 0.4 gram Ca (=0.02 meq) and 0.1 gram magnesium (=0.005 meq). Given that my base mix is 50/50 coco/perlite I would have ~30.56 kg (dry weight) coco coir per cubic yard of mix. Buffering 2/3 of the K in the coir would thus require displacing 305 g K/cy = 7.8 meq K.
Therefore, if I wanted to replace 3/4 of the K with Ca and 1/4 with Mg I would need to add 521g Gypsum and 240g Epsom Salts per cubic yard of mix on top of any calcium and magnesium I would be adding for the crop.
This would release 305g of K (equal to 366g K2O) from the coir, or about 281 cubic cm (1.2 cups) of Potassium sulfate per cubic yard, which is about a third of what I want to use in the mix, so the way I see it, I would just need to reduce potash in my recipe by 1/3 and I’d definitely use a granular potash so it releases more slowly since I’ll have a considerable amount in the soil solution to start with.
Making these adjustments I assume that the coir would get buffered while it is “cooking”, leaving a perfectly balanced organic mix ready to plant. Has anyone done this before? Do my numbers look right?