pH
Marijuana Horticulture
by Jorge Cervantes
The pH scale, from 1 to 14, measures acid-t-alkaline balance. One is the most acidic, seven is neutral, and 14 most alkaline. Every full point change in pH signifies a ten-fold increase or decrease in acidity or alkalinity. For example, soil or water with pH of five is ten times more acidic than water with a pH of six. Water with a pH of five is one hundred times more acidic than water with a pH of seven. With a tenfold difference between each point on the scale, accurate measurement and control is essential to a strong, healthy garden.
Cannabis grows best is soil with a pH from 6.5 to 7.0. Within this range marijuana can properly absorb and process available nutrients most efficiently. If the pH is too low (acidic), acid salts chemically bind nutrients, and the rots are unable to absorb them. An alkaline soil with a high pH causes nutrients to become unavailable. Toxic salt buildup that limits water intake by roots also becomes a problem. Hydroponic solutions perform best in a pH range a little lower than soil. The ideal pH range for hydroponics is from 5.8 to 6.8. Some growers run the pH at lower levels and report no problems with nutrient uptake. The pH of organic soil mixes is very important because it dictates the ability of specific pH-sensitive bacteria.
Measure the Ph with a soil test kit, litmus paper, or electronic pH tester, all of which are available at most nurseries. When testing pH, take two or three samples and follow instructions supplied by the manufacturer “to the letter”. Soil test kits measure soil pH and primary nutrient content by mixing soil with a chemical solution and comparing the color of the solution to a chart. Every one of these kits I have seen or used is difficult for novice gardeners to achieve accurate measurements. Comparing the color of the soil/ chemical mix to the color of the chart is often confusing. If you use one of these kits, make sure to buy one with with good, easy to understand instructions and ask the sales clerk for exact recommendations on using it.
If using litmus paper, collect samples that demonstrate an average of the soil. Place the samples in a clean jar, and moisten the soil samples with distilled water. Place two pieces of the litmus paper in the muddy water. After ten seconds, remove one of the strips of litmus paper. Wait a minute before removing the other one. Both pieces of litmus paper should register the same color. The litmus paper container should have a pH-color chart on the side. Match the color of the litmus paper with the colors on the chart to get a pH reading. Litmus paper will accurately measure the acidity of a substance to within a point. The pH readings will not be accurate if altered by water with a high or low pH, and the litmus paper could give a false reading if the fertilizer contains a color-tracing agent.
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 hydroponic 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 an 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 neutral7. However, most soils have a tendency to be acidic. The easiest way t stabilize soil pH is to mix in one cup of fine dolomite lime per cubic foot of potting soil. Mix dolomite lime thoroughly int dry soil. Remix the soil in the container after it has been watered.
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 stabilized the pH safely. Compensate for acidic soil by mixing dolomite with soil while planting. It will help keep 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 rots. 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 hydrates lime and 0.75 cup dolomite lime. Hydrate lime is immediately available, whereas the slower acting dolomite buffers the pH over the long term. Do not use more than 0.5 cup f hydrated lime per cubic foot of soil. The larger quantity is release so fast, that it can toxify the soil, and stunt or even kill plants. The beauty of hydrate lime is that it washes out of the soil in about two weeks. Leach it quicker by flushing pots with copious amounts of water. Hydrated lime is also used as a grow room fungicide. Sprinkle it on the floor and around the room. It kill fungus on contact.
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 of fine dolomite lime to one cubic foot of soil. An alternate fast acting mix would be to add 2.5 cups of dolomite and 0.5 cup of hydrate lime to one cubic foot of soil.
Pulverized eggshells, clam r 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 are very 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 indoor gardens unless you know their origin, pH and nutrient content. You can add cottonseed meal, lemon peels, coffee ground, r a high-acidity fertilizer to lower pH in soil to below 7.0.
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 the soil rather than experiment with changing the pH. Fertilizers are naturally acidic and lower the pH of a growing medium. Sulfur will lower 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 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 the pH. Calcium nitrate can also be used, but is less common. Keep a close eye on the pH, check it, and then check it again daily to make sure it remains stable.
Aspirin also lowers the pH. However, hormonal reactions appear to be triggered by aspirin. Some growers reported more hermaphrodites when using aspirin to alter the pH.