Soil Amendments

Marijuana Horticulture

by Jorge Cervantes

Soil amendments increase the soil’s air, water, and nutrient retaining abilities. Soil amendments fall into two categories: mineral and organic.

mineral amendments are near neutral on the pH scale and contain few, if any, available nutrients. Mineral amendments decompose through weathering and erosion. Add mineral amendments to augment air and increase drainage. They have the advantage of creating no bacterial activity to alter nutrient content and pH of the growing medium. Dry mineral amendments are also very lightweight and much easier to move in and out of awkward spaces.

Perlite is sand or volcanic glass expanded by heat. It holds water and nutrients on its many irregular surfaces, and it works especially well for aerating soil. This is a good medium to increase drainage during vegetative and flowering grwth, and it does not promote fertilizer-salt buildup. Versatile perlite is available in three main grades: fine, medium and coarse. Most growers prefer the coarse grade as a soil amendment. Perlite should make up one third or less of any mix to keep it from floating and stratifying the mix.

Pumice, volcanic rock, is very light and holds water, nutrients, and air, in its many catacomb-like holes. it is a good amendment for aerating the soil and retaining moisture evenly. But like perlite, pumice floats and should constitute less than a third f any mix to avoid problems.

Hydroclay is used more and more as a soil amendment in containers. The large expanded clay pellets expedite drainage and hold air within the growing medium.

Vermiculite is mica processed and expanded by heat. It holds water, nutrients, and air within its fiber and gives body to fast-draining soils. Fine vermiculite holds too much water for cuttings, but does well when mixed with a fast-draining medium. This amendment holds more water than perlite or pumice. Used in hydroponic wick systems, vermiculite holds and wick much moisture. Vermiculite comes in three grades: fine, medium, and coarse. Use fine vermiculite as an ingredient in cloning mixes. If fine is not available, crush coarse or medium vermiculite between your hands, rubbing palms back and forth. Coarse is the best choice as a soil amendment.

Organic soil amendments contain carbon and break down through bacterial activity, slowly yielding humus as an end product. Humus is a soft, spongy material that binds minute soil particles together, improving the soil texture. New, actively composting organic soil amendments require nitrogen to carry on bacterial decomposition. If they do not contain at least 1.5 percent nitrogen, the organic amendment will get it from the soil, robbing roots of valuable nitrogen. When using organic amendments, make sure they are through composted (at least one year) and releasing nitrogen rather than stealing it from the soil. A dark, rich color is a god sign of fertility.

Rich, thoroughly composted organic matter amends texture and supplies nutrients. Leaf mold, garden compost, and many types of thoroughly composted manure usually contain enough nitrogen for their decomposition needs and release nitrogen rather than using it. Purchase quality organic amendments at a reputable nursery. Look carefully at the descriptive text on the bag to see if it is sterilized and is guaranteed to contain no harmful insects: larvae, eggs, and fungi or bad microorganisms. Contaminated soil causes many problems that are easily averted by using a clean mix.

Garden compost and leaf mold are usually rich in organic nutrients and beneficial organisms that speed nutrient uptake, but they can be full of harmful pests and diseases, too. For example, compost piles are a favorite breeding ground for cutworms and beetle larvae. Just one cutworm in a container means certain death for the defenseless marijuana plant. Garden compost is best used in outdoor gardens and not indoors.

Manure – barnyard manure, a great fertilizer for outdoor gardens, often contains toxic levels of salt and copious quantities of weed seeds and fungus spores that disrupt and indoor garden. If using manure, purchase it in bags that guarantee its contents. There are many kinds of manure: cow, horse, rabbit, chicken, etc. When mixing manures as amendments, do not add add more than 10-15 percent, to avoid salt buildup and overfertilization. The nutrient content of manure varies, depending upon the animal’s diet and the decomposition factors.

Peat is the term used to describe partially decomposed vegetation. The decay has been slowed by the wet and col conditions of the northern United States and Canada where it is found in vast bogs. The most common types of peat are formed from sphagnum and hypnum mosses. These peats are harvested and used to amend soil and can be used as a growing medium. Peat moss is very dry, and difficult t wet the first time, unless you bought it wet. Wet peat is heavy and awkward to transport. When adding peat moss as a soil amendment, cut your workload by dry-mixing all of the components before wetting. Use a wetting agent. Another trick to mixing peat moss is to kick the sack a few times to break up the bale before opening.Peat tends to break down and should be used for only one crop.

Sphagnum peat moss is light brown and the most common peat found at commercial nurseries. This bulky peat gives soil body and retains water well, absorbing 15 to 30 times its own weight. It contain essentially no nutrients of its own, and the pH ranges from 3-5. After decomposing for several months, the pH could continue to drop and become very acidic. Counter this propensity for acidity and stabilize the pH by adding fine dolomite lime to the mix.

Hypnum peat moss is more decomposed and darker in color with pH from 5.0 to 7.0. This peat moss is less common and contains some nutrients. Hyphum peat is a good soil amendment, even though it cannot hold as much water as sphagnum moss.

Coconut fiber is also called palm peat, coco peat, cocos, kokos, and coir. Coir is coconut pith, the fibery part just under the heavy husk. Pith is soaked in water up to nine months to remove salts, natural resins, and gums in a process called retting. Next, they beat the straw-brown cir to extract the husk.

Coir is biodegradable and a good medium for propagation through flowering and fruit growth. Coir holds lots of water while maintaining structure. It is durable, rot-resistant, and a good insulator, too. It is inexpensive, easy to control, and holds lots of air.

Washed, pressed blocks or bricks of coir are virtually inert. Bricks weigh about 1.3-2.2 pounds. The pH is between 5.5 and 6.8. Some f the best coconut coir is from the interior of the Philippine Islands, where the environment is not packed with coastal salts. Quality coconut coir is guaranteed to have sodium content of less than 50 ppm.

Growers use coir by itself or mixed 50/50 with perlite or expanded clay to add extra drainage to the mix. Some growers sprinkle coconut coir on top of rockwool blocks to keep the top from drying out.

Flake dry bricks of coconut coir apart by hand or soak the bricks in a bucket of water for 15 minutes to expand and wet. One brick will expand to about 9 times its original size. Growing in coconut coir is similar to growing in any other soilless medium. Coconut coir may stay a little too wet and require more ventilation and air circulation.