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

mineral particles; when it gets wet, these minute particles pack tightly together, slowing or stopping root penetration and water drainage. Roots are unable to breathe because very little or no space is left for oxygen. Water has a very difficult time penetrating these tightly packed soils, and once it does penetrate, drainage is slow.
    Sandy soils have much larger particles. They permit good aeration (supply of air or oxygen) and drainage. Frequent watering is necessary because water retention is very low. The soil’s water- and air-holding ability and root penetration are a function of texture.
    Loam soil is ideal for growing cannabis. It contains a mix of clay, silt, and sand. The different sized particles allow a large combination of pore spaces, so it drains well and still retains nutrients and moisture.
    To check soil texture, pick up a handful of moist (not soggy) soil and gently squeeze it. The soil should barely stay together and have a kind of sponge effect when you slowly open your hand to release the pressure. Indoor soils that do not fulfill these requirements should be thrown out or amended. See “Soil Amendments.”

pH
    The pH scale, from one to 14, measures acid-to-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 a pH of five is ten times more acidic than water or soil 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 in 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 roots 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 for 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 good, easy-to-understand instructions and ask the sales clerk for exact recommendations on using it.

An inexpensive electronic pH tester is easy to use.

When planting, add one cup of fine dolomite lime to each cubic foot (one ounce per gallon [30 ml per 4 L]) of planting medium to stabilize the pH and provide calcium and magnesium.
     
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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 around the probes.
Water soil with distilled or neutral pH water before testing.
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    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 the 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