What is pH?

pH, Excerpted from Chapter Three, Indoor Marijuana Horticulture, The Indoor Bible

pH

pH is a scale from 1 to 14 that measures acid-to-alkaline balance. One is the most acidic, 7 is neutral and 14 is most alkaline. Every full point change in pH signifies a 10-fold increase or decrease in acidity or alkalinity. For example, soil or water with a pH of 5 is 10 times more acid than water or soil with a pH of 6. Water with a pH of 5 is 100 times more acidic than water with a pH of 7. With a 10-fold difference between each point on the scale, accurate measurement and control is essential to a strong healthy garden. Cannabis grows best in soil with a pH from 6.5 – 7. Within this range, marijuana can properly absorb and process available nutrients most efficiently. If the pH is too low (acidic), acid salts bind nutrients chemically, and the roots are unable to absorb them. An alkaline soil with a high pH causes nutrients to become unavailable. Toxic salt build up that limits water intake by roots also becomes a problem. Hydroponic solutions perform best in a pH range a little lower than for soil. The ideal pH range for hydroponics is from 5.8 – 6.8. Some growers run the pH at lower levels and report no problems with nutrient uptake.

This pH chart shows the “Safe Zone” is between 5.8 and 6.8.

An inexpensive electronic pH tester is easy to use.

Measure 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 are a 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 a good set of directions that are easy to understand and ask the sales clerk for exact recommendations on using it.

Cannabis Clue: The pH level is much more important in organic soil gardens than in chemical hydroponic gardens. The pH dictates the environment of bacteria necessary to the uptake of organic nutrients.

If using litmus paper, collect soil 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 10 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. To learn the pH, 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 the substance to within a point. The pH readings will not be accurate if altered by water with a high or low pH, and litmus paper could give a false reading is 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. Some meters measure the pH perpetually.

Cannabis Clue: Always test the pH of raw water and drainage water with a pH meter.

For an accurate pH test with an electronic pH meter:

• Clean the probes of the meter after each test and wipe away any corrosion
• Pack the soil tightly around the probes
• Water soil with distilled or neutral pH water before testing.

Check the pH of irrigation water. In dry climates, such as the desert Southwest US, Spain, Australia, etc., irrigation water is often alkaline with a pH above 7. The water in rainy climates, such as the Pacific Northwest of North America, the UK, Netherlands and maritime Northern Europe, is often acidic with a pH below 6. 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. 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 pH and there are few water quality problems. None-the-less, be on the lookout for any major environmental changes that could affect water pH. Check pH at least once a week.

Cannabis Clue: Raw water pH above 6.0 helps keep fertilizer mixes from becoming too acidic.

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 of potting soil. Mix dolomite lime thoroughly into dry soil. Remix the soil in the container after it has been watered.

A few growers place charcoal in the bottom of containers to absorb excess salts and maintain sweet soil if plants are grown more than a few months.

*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 been a favorite pH stabilizer of gardeners for years.