Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

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.
Irrigation
    Large plants use more water than small plants, but there are many more variables than size that dictate a plant’s water consumption. The age of the plant, container size, soil texture, temperature, humidity, and ventilation all contribute to water needs. Change any one of these variables, and the water consumption will change. Good ventilation is essential to promote a free flow of fluids, transpiration, and rapid growth. The healthier a plant, the faster it grows and the more water it needs.

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

Plants in small containers require more frequent watering.

When they are small, plants in medium-sized containers can be watered every other day.

A water meter takes the guesswork out of watering. Remember to keep probes clean for an accurate reading.

Tip the containers to check if they are heavily laden with water. Irrigate the light containers.

Add a few drops of biodegradable liquid dish soap concentrate to the irrigation water. Detergent makes the water penetrate the soil more thoroughly.
    Small plants with a small root system in small containers must be watered often. Water frequently–as soon as the soil surface dries out. If exposed to wind, the small plants will dry out very quickly.
    Irrigate soil and soilless mixes when they are dry one-half inch below the surface. As long as the drainage is good, it is difficult to overwater fast-growing cannabis. Four-week-old clones flowering in 2- to 3-gallon (7.5 to 11 L) containers need to be irrigated once or twice daily. In fact, most growers prefer smaller containers because they are easier to control.
    Irrigate larger plants in the vegetative and flowering stages when soil is dry one-half inch below the surface. Flowering marijuana uses high levels of water to carry on rapid floral formation. Withholding the water stunts the flower formation.
    Plants that are exposed to wind dry out much faster. Outdoor, terrace, and patio plants will use up to three or four times more water on a hot, windy day. Keeping up with the watering is difficult and time-consuming. Use an automated watering system or break the wind to lessen its impact on the plants. Mulch will also lessen the evaporation from the soil.
    Use plenty of water, and allow up to 10 percent runoff during each watering. The runoff will prevent the fertilizer from building up in the soil. Water early in the day, so excess water will evaporate from the soil surface and the leaves. Leaving the foliage and the soil wet overnight invites a fungal attack.
    Moisture meters take most of the guesswork out of irrigating. They can be purchased for less than $30 and are well worth the money. The meter measures exactly how much water the soil contains at any level or point. Often the soil will not hold the water evenly, and it develops dry pockets. Checking the moisture with a finger provides an educated guess but disturbs the root system. A moisture meter will give an exact moisture reading without disturbing the roots.
    Cultivate the soil surface to allow the water to penetrate evenly and guard against dry soil pockets. It also keeps the water from running down the crack between the inside of the pot and the soil and out the drain holes. Gently break up and cultivate the top half inch of the soil with your fingers or a salad fork. Be careful not to disturb the tiny surface roots.
    After you develop some skill at knowing when the plants need water, you can check to see how heavy they are simply by tipping them. Once you get the hang of it, all you will have to do is tip each container.
    It is easier to keep pots in straight lines when growing and watering, and it is much easier to keep track of watered pots when they are in a straight line.
    Overwatering is a common problem, especially with small plants. Too much water drowns the roots by cutting off their supply of oxygen. If you have symptoms of overwatering, buy a moisture meter! It will let both you and your garden breatheeasier. Sometimes, parts of the soil are overwatered and other soil pockets remain bone-dry. Cultivating the soil surface, allowing even water penetration, and using a moisture meter will overcome this