Osmosis
Marijuana Horticulture
by Jorge Cervantes
The roots draw the nutrient solution up the plant by the process of osmosis. osmosis is the tendency of fluids to pass through a semipermeable membrane and mix with each other until the fluids are equally concentrated on both sides of the membrane. Semipermeable membranes located in the root hairs allow specific nutrients that are dissolved in the water to enter the plant while the other nutrients and impurities are excluded.
Since salts and sugars are concentrated in the roots, the electrical conductivity (EC) inside the roots is (almost) always higher than outside the roots. Transporting the nutrients by osmosis works, because it depends on relative concentrations of each individual nutrient on each side of the membrane; it does not depend on the total dissolved solids (TDS) or EC of the solution. For nutrients to be drawn in by the roots via osmosis, the strength of the individual elements must be greater than than of the roots.
But, transport of water (instead of nutrients) across the semipermeable membrane depends on EC. For example, if the EC is greater outside the roots than inside, the plant dehydrates as the water is drawn out of the roots. In other words, salty water with a high EC can dehydrate the plants.
Reverse osmosis machines are used to separate the dissolved solids from the water. These machines move the solvent (water) through the semipermeable membrane, but the process is reversed. It moves from lower concentrations to higher. The process is accomplished by applying pressure to the tainted water to force only pure water through the membrane. The water is not totally “pure” with EC of “0”, but most of the dissolved solids are removed. The efficiency of reverse osmosis depends on the type of of membrane, and the chemical composition of the dissolved solids in the tainted water.
Unfortunately, common tap water often contains high levels of sodium (Na), calcium (Ca), alkaline salts, sulfur (S), and chlorine (Cl). The pH could also be out of the acceptable 6.5 to 7 range. Water containing sulfur is easily smelled and tasted. Saline water is a little more difficult to detect. Water in coastal areas is generally full of salt that washes inland from the ocean. Dry regions that have less than 20 inches annual rainfall also suffer from alkaline soil and water that is often packed with alkaline salts.
Table salt, sodium chloride (NaCl), is added t many household water systems. A small amount of chlorine, below 140 ppm, does not affect marijuana growth, but higher levels cause foliage chlorosis and stunt growth. Do not use salt softened water. Salty, brackish, salt-softened water is detrimental to cannabis. Chlorine also tends to acidify soil after repeated applications. The best way to get chlorine out of the water is to let it sit one or two days in an open container. the chlorine will evaporate (volatize) as a gas when it comes in contact with the air. If chlorine noticeably alters soil pH, adjust it with a commercial “pH UP” product or hydrated lime.
The metric system facilitates the measurement of “dry residue per liter”. Measure the dry residue per liter by pouring a liter of water on a tray and allowing it to evaporate. The residues of dissolved solids that remains after all the water evaporates is the “dry residue per liter”. The residue is measured in grams. Try this at home to find out the extent of impurities. Fertilizers have a difficult time penetrating root tissue when they must compete with resident dissolved solids.
Water that is loaded with high levels of dissolved solids (salts in solution) is possible to manage but requires different tactics. highly saline water that contains sodium will block the uptake of potassium, calcium, and magnesium. Salt-laden water will always cause problems. If water contains 300 ppm or less dissolved solids, allow at least 25 percent of the irrigation water to drain out of the bottom of containers with each watering. If raw water contains more than 200 ppm of dissolved solids, use a reverse osmosis device to purify the water. Add nutrients to pure water as a way to avoid many nutrient problems.
Dissolved salts, caused by saline water and fertilizer, quickly build up toxic levels in container gardens. Excessive salts inhibit seed germination, burn the root hairs and tips or edges of leaves, and stunt the plant. Flush excess salt buildup from growing mediums by applying two gallons of water per gallon of medium and repeat leaching using a mild pH-corrected fertilizer solution. Leach growing medium every two to four weeks, if using soft water r saline water. hard water and well water in dry climates are often alkaline, and usually contain contain notable amounts of calcium and magnesium. Cannabis uses large quantities of both nutrients, but too much calcium and magnesium can build up in soil. in general, water that tastes good to people also tastes good to cannabis.