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

Hydrated lime is also used as a grow room fungicide. Sprinkle it on the floor and around the room. It kills fungus on contact.

The pH of these large sativa plants growing on a terrace is kept at 6.5 to 6.8.
    Do not use quicklime; it is toxic to plants. Calcic lime (quicklime) contains only calcium and is not a good choice. It does not have the buffering qualities of dolomite nor does it contain any magnesium.
    Raise the pH of a growing medium or irrigation water by adding some form of alkali, such as calcium carbonate, potassium hydroxide, or sodium hydroxide. Both hydroxides are caustic and require special care when handling. These compounds are normally used to raise the pH of hydroponic nutrient solutions but can be used to treat acidic nutrient solutions when applied to soil. The easiest and most convenient way to raise and stabilize soil pH is to add fine dolomite lime and hydrated lime before planting. To raise the pH one point add 3 cups (65 cl) of fine dolomite lime to one cubic foot (30 L) of soil. An alternate fast- acting mix would be to add 2.5 cups (590 cl) of dolomite and 0.5 cup (12 cl) of hydrated lime to one cubic foot of soil.
    Pulverized eggshells, clam or oyster shells, and wood ashes have a high pH and help raise soil pH. Eggshells and oyster shells take a long time to decompose enough to affect the pH; wood ashes have a pH from 9.0-11.0 and and are easy to overapply. Ashes are often collected from fireplaces or wood stoves that have been burning all kinds of trash and are, therefore, unsafe. Do not use wood ashes on indoorgardens unless you know their origin, pH, and nutrient content. You can add cottonseed meal, lemon peels, coffee grounds, or a high-acidity fertilizer to lower pH in soil to below 7.0.

Outdoors, the soil temperature can climb quickly when sunshine warms the containers.
    Commercial potting soils and soilless mixes are often acidic and the pH seldom needs to be lowered. If new soil pH is under 6 or above 8, it is easier and less expensive in the long run to change soil rather than experiment with changing the pH. Fertilizers are naturally acidic and lower the pH of the growing medium. Sulfur will lower the pH, if necessary, but it is tricky to use. I advise using an acid to alter the pH. Add distilled white vinegar at the rate of one teaspoon per gallon (1.2 ml per L) of irrigation water, allow the water to sit for a few minutes, and then recheck it. The pH should drop by a full point. If it does not, add more vinegar in small increments. Often when using vinegar, the pH drifts up overnight. Check the pH the next day. Hydroponic growers use phosphoric and nitric acid to lower pH. Calcium nitrate can also be used, but is less common. Keep a close eye on the pH and control it accordingly. After altering the pH, check it, and then check it again daily to make sure it remains stable.

Soil temperature should stay between 65-70°F (18-24X) for best results.
    Aspirin also lowers the pH. However, hormonal reactions appear to be triggered by aspirin. Some growers have reported more hermaphrodites when using aspirin to alter the pH.
Humates Chelate
    Humic and fulvic acids chelate metallic ions making them readily transportable by water. This ability is dependent upon the pH level. Copper, iron, manganese, and zinc are difficult to dissolve. When mixed in a chelated form, they become readily available for absorption.

Soil Temperature
    Raising the soil temperature speeds the chemical process and can hasten nutrient uptake. Ideally, the soil temperature should range from 65-70°F (18-24°C) for the most chemical activity. Warm the soil with soil-heating cables or a heating pad. Fasten heating cables to a board or table and set a heat-conducting pad on top of the cables to distribute heat evenly. Set cuttings and seedlings in shallow flats or growing trays on top of the heat-conducting pad. The added heat speeds root growth when soil temperature is below 65°F (18°C).
    Soil heating cables cost much less than soil heating pads but must be installed, whereas the pads are ready to use. Most commercial nurseries carry cables, and hydroponic stores carry heating pads. When rooting clones, a heating pad or cables virtually ensure success and expedite root growth.
    Cold soil slows water and nutrient uptake and stifles growth. Growers often overwater when the soil is too cold or the room cools unexpectedly, which further slows growth. Pots on coldconcrete floors stay as cold as the concrete,