One of the most important elements dissolved in water used to irrigate cannabis plants (and others as seen in the chart below) is Boron.
Too little or too much will cause severe withering and discoloration of a plant which severely reduces monetary value. The typically expressed range is from 0.05 to 0.5 mg/l in the irrigation water.
Boron in excessive amounts in well water sources is found in the US particularly in California, Russia, and Turkey. It is also found in membrane desalinated seawater throughout the world.
lf too low or absent, a simple, cost-effective “fix” is to mix a small amount of very soluble boric acid crystals (H3BO3) in water and feed it into the irrigation stream with an inexpensive chemical feed pump. Because of common consumer uses of boric acid such as a laundry aid or a foot bath, boric acid is available in drug stores, supermarkets, and, of course, online.
If too high (above 0.5 mg/l) it must be reduced. One would think that reverse osmosis would be the ideal solution. It is not. Reverse osmosis doesn’t reduce dissolved solids by forcing water through microscopic holes in the membrane, but instead by repelling charges between the membrane and the element (ion) in solution. Boron has no discernible charge so it passes through membranes. Even high-pressure seawater membranes allow too much through. Seawater can contain 5 PPM or more boron.
The removal is best accomplished using an extremely rare and expensive resin that is uniquely capable of being selective to boron and not blinded by other elements in the water. This feature permits minimal and relatively inexpensive pretreatment. The resin must be stripped of the boron with hydrochloric acid (HCl) and then returned to a boron removal state with sodium hydroxide(NaOH). The use of these chemicals in a sequential pattern necessitates special materials of construction and very unique controls for safety and efficacy.
Boron upper levels to minimize toxic sensitivity in mg/l*
<0.5 blackberry, cannabis
0.5-1 peach, cherry, plumb, grape, onion, garlic, sweet potato, wheat, barley, sunflower, strawberry
1-2 red pepper, pea, carrot, radish, potato, cucumber
2-4 lettuce, cabbage, celery, turnip, oat, corn, artichoke, tobacco, mustard squash
4-6 Tomato, alfalfa, parsley, sugar-beet
* Information provided by the Australian Department of Agriculture