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

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    Electronic pH testers are economical and convenient. Less expensive pH meters are accurate enough for casual use. More expensive models are quite accurate. Pay special attention to the soil moisture when taking a pH test with an electronic meter. The meters measure the electrical current between two probes and are designed to work in moist soil. If the soil is dry, the probes do not give an accurate reading. I prefer electronic pH meters over the reagent test kits and litmus paper because they are convenient, economical, and accurate. Once purchased, you can measure pH thousands of times with an electronic meter, while the chemical test kits are good for about a dozen tests. Perpetual pH-metering devices are also available and most often used to monitor hydro-ponic nutrient solutions.
    Check the pH of irrigation water. In dry climates, such as the desert Southwest United States, Spain, Australia, etc., irrigation water is often alkaline with a pH above 6.0. The water in rainy climates, such as the Pacific Northwest of North America, the United Kingdom, Netherlands, and maritime Northern Europe, is often acidic with a pH below 6.0. The pH and electrical conductivity (EC) of water supplies in municipalities and cities can also change throughout the year in some countries. After repeated watering, water with a pH that is too high or low will change the pH of the growing medium, especially in organically amended soils. Raw-water pH above 6.0 helps keep fertilizer mixes from becoming too acidic. Climatic conditions can also affect irrigation water pH. For example, the pH can become more acidic in late autumn, when leaves fall and decompose. Large municipalities carefully monitor and correct the pH, and there are few water-quality problems. Check the pH at least once a week.
    Cannabis will grow in almost any soil * , but it flourishes when the pH is between 6.5 and 7. Commercial potting soil almost never has a pH above 7.5. A lower pH is more common, even as low as 5.5. Some potting soils purchased at a nursery are pH balanced and near a neutral 7. However, most potting soils have a tendency to be acidic. The easiest way to stabilize soil pH is to mix in one cup of fine dolomite lime per cubic foot (10 liters) of potting soil. Mix dolomite lime thoroughly into dry soil. Remix the soil in the container after it has been watered.
    * Cannabis is a well-known accumulator plant that takes in heavy metals and sequesters toxins in vacuoles, which are impermeable. The heavy metals remain toxic Cannabis was planted around Chernobyl, the toxic nuclear site in Russia, to absorb toxic heavy metals.
    Fine Dolomite Lime has long been a favorite pH stabilizer for gardeners. It is difficult to apply too much as long as it is thoroughly mixed into soil. Dolomite has a neutral pH of 7, and it can never raise the pH beyond 7.0. It stabilizes the pH safely. Compensate for acidic soil by mixing dolomite with soil before planting. It will helpkeep the pH stable, and maintain the correct pH when applying mild acidic fertilizers. Dolomite, a compound of magnesium (Mg) and calcium (Ca), is popular among indoor and outdoor growers in rainy climates with acidic soil. Dolomite does not prevent toxic-salt accumulation caused by impure water and fertilizer buildup. A proper fertilizer regimen and regular leaching helps flush away toxic salts. When purchasing, look for dolomite flour, the finest fast-acting dust-like grade available. Coarse dolomite could take a year or more before it becomes available for uptake by roots. Mix dolomite flour thoroughly with the growing medium before planting. Improperly mixed, dolomite will stratify, forming a cake or layer that burns roots and repels water.
    Hydrated Lime contains only calcium and no magnesium. As the name hydrated implies, it is water-soluble. Fast-acting hydrated lime alters the pH quickly. Mix it thoroughly with warm water and apply with each watering for fast results. Many growers use a mix of 0.25 cup (6 cl) hydrated lime and 0.75 cup (18 cl) dolomite lime. Hydrated lime is immediately available, whereas the slower acting dolomite buffers the pH over the long term. Do not use more than 0.5 cup (12 cl) of hydrated lime per cubic foot (30 L) of soil. The larger quantity is released so fast that it can toxify soil, and stunt or even kill plants. The beauty of hydrated lime is that it washes out of the soil in about two weeks. Leach it quicker by flushing pots with copious quantities of water.