Will bad tap water affect my crop?

Osmosis and Hard Water, Excerpted from Chapter Four, Indoor Marijuana Horticulture, The Indoor Bible

Osmosis

Roots draw nutrient solution up by the process of osmosis. Osmosis is the tendency of fluids to pass through a semi-permeable membrane and mix with each other until the fluids are equally concentrated on both sides of the membrane. Semi-permeable membranes located in root hairs allow specific nutrients dissolved in water to enter the plant while other nutrients and impurities are excluded. Since salts and sugars are concentrated in roots, the EC inside roots is (almost) always higher than that outside the roots. Nutrient transport 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 electrical conductivity (EC) of the solution. For nutrients to be drawn in by roots via osmosis, the strength of individual elements must be greater than that of the roots.

But, the transport of water (instead of nutrients) across the semi-permeable membrane depends on EC. For example, if EC is greater outside roots than inside, the plant dehydrates as water is drawn out of the roots. In other words, salty water with a high EC can dehydrate plants.

Reverse osmosis (RO) machines are used to separate dissolved solids from water. These machines move the solvent (water) through the semi-permeable membrane, but the process is reverse; 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 an EC of “0”, but most of the dissolved solids are removed. The efficiency of the reverse osmosis depends on the type of membrane, the pressure differential on both sides of the membrane and the chemical composition of the dissolved solids in the tainted water.

Unfortunately, common tap water often contains high levels of sodium, calcium, alkaline salts, sulfur and chlorine. 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 to 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 and salt-softened water is detrimental to cannabis. Chlorine also tends to acidify soil after repeated applications. The best way to get chlorine out of 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 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 dry residue per liter by pouring a liter of water on a tray and allow it to evaporate. The residue 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 packed with high of levels of dissolved solids (salts in solution) is possible to mange but require 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 the bottom of containers with each watering. If raw water contains more than 300 PPM of dissolved solids, use reverse osmosis device to purify the water.

Flushing plants with plain water will wash out most built up toxic salts. Flush again with a dilute nutrient solution.

Dissolved salts caused by saline water and fertilizer build up to toxic levels quickly 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 built up from growing mediums by applying 2 gallons of water per gallon of medium. Repeat leaching using a mild pH-corrected fertilizer solution. Leach growing medium every 2 – 4 weeks if using soft water or saline water. Hard water and well water in dry climates is often alkaline and usually contains notable amounts of calcium and magnesium. Cannabis uses large quantities of both nutrients, but too much calcium or magnesium can build-up in soil. In general, water that tastes good to people, also “tastes” good to cannabis.

Hard Water

The concentration of calcium and magnesium indicate how “hard” the water is. Water containing 100 to 150 milligrams of calcium (CaCO3) per liter is acceptable to grow marijuana. “Soft” water contains less than 50 milligrams of calcium per liter and should be supplemented with calcium and magnesium.

Sodium Chloride and Water Quality

Water with high levels of chloride frequently contains high levels of sodium, but the opposite is not true. Water with high levels of sodium does not necessarily contain excessive levels of chloride.

At low levels, sodium appears to bolster yields, possibly acting as a partial substitute for potassium deficiencies. But when excessive, sodium is toxic and induces deficiencies of other nutrients, primarily potassium, calcium and magnesium.

Chloride (chlorine) is essential to the use of oxygen during photosynthesis and is necessary for root and leaf cell division. Chloride is vital to increase cellular osmotic pressure, modify stomata regulation and augment plant tissue moisture content. A solution concentration of less than 140 PPM is usually safe for marijuana, but some varieties may show sensitivity when foliage turns pale green and wilts. Excessive chlorine causes leaf tips and margins to burn and leaves to turn a bronze color.

Super Size Secret: Run water with more than 300 PPM dissolved solids through a reverse osmosis machine.

Super Size Secret: Add nutrients to pure water and avoid many nutrient problems.

Simple water filters do not clean dissolved solids from the water. Such filters remove debris emulsified (suspended) in water. Releasing dissolved solids from their chemical bond is more complex. A reverse osmosis machine uses small polymer semi-permeable membranes that let pure water pass through and filters out dissolved solids from water. Reverse osmosis machines are the easiest most efficient means clean raw water.