Chapter 10: Soil & Containers

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

by Jorge Cervantes

Soil is made up of many mineral particles mixed together with living and dead organic matter that incorporates air and water. Three basic factors contribute to the cannabis root’s ability to grow in soil: texture, pH, and nutrient content.

Soil texture is governed by the size and physical makeup of the mineral particles. Proper soil texture is required for adequate root penetration, water, and oxygen retention and drainage as well as many other complex chemical processes.

Clay or adobe soil is made up of very small, flat 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 god aeration (supply of air and 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 f 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 moisture, 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.


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 r 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.

Humates Chelate

Humic and fulvic acids chelate metallic ions making them readily transportable by water. This ability is dependent upon the pH level. Copper, iron, manganese, and zinc are difficult to dissolve. When mixed in a chelated form, they become readily available for absorption.

Soil Temperature

Raising the soil temperature speeds the chemical process and can hasten nutrient uptake. Ideally, the soil temperature should range from 65-70F for the most chemical activity. Warm the soil with soil heating cables or a heating pad. Fasten heating cables to a board or table and set a heat conducting pad on top of the cables to distribute heat evenly. Set cutting and seedlings in shallow flats or growing trays on top of the heat conducting pad. The added heat speeds root growth when soil temperature is below 65F.

Soil heating cables cost much less than soil heating pads but must be installed, whereas the pads are ready to use. Most commercial nurseries carry cables, and hydroponic stores carry heating pads. When rooting clones, a heating pad or cables virtually ensure success and expedite root growth.

Cold soil slows water and nutrient uptake and stifles growth. Growers often overwater when the soil is too cold or the room cools unexpectedly, which further slows growth. Pots on cold concrete floors stay as cold as the concrete, which is always colder than the ambient temperature. Increase soil temperature by moving pots up off the floor a few inches. Set them on an insulating bard or piece of Styrofoam.

Soil temperatures that climb above 75F dehydrate rots, and at higher temperatures the roots actually cook! It is relatively easy to heat the soil in a pot. If the light or any heat source is too close to small pots, it can easily heat up the outside layer of soil where the majority of the feeder roots are located. Once destroyed, roots take one or two weeks to grow back. Two weeks accounts for one quarter of the flowering cycle!

The more feeder root hairs there are to absorb water and nutrients, the faster and stronger plants will grow. Once roots go beyond their comfort zone, they send stress signals to foliage and stomata via hormones to close and conserve moisture.

Oxygen is essential for clones that are growing roots. Water holds under one percent dissolved oxygen at 70F. Bump the temperature up to 85F and it holds less than 0.5 percent oxygen.

Root temperatures below 40F make water expand, which causes cell damage. Temperatures above 92F cause excessive vapor pressure within the roots, which can cause damage. At high temperatures roots send stress signals t shut the leaves down before damage can occur.

Potting Soil

Potting soil fresh out of the bag often fulfills all the requirements fro a growing medium: good texture that allows good root penetration, water retention, and good drainage, a stable pH between 6 and 7, and a minimum supply of nutrients.

premium fast-draining soils with good texture that will not break down quickly are the best choice. Potting soils found at nurseries often formulated with a wetting agent and retain water and air evenly, drain well, and allow easy rot penetration. Organic potting soils are very popular. These soils are often fortified with organic nutrients including readily available high-nitrogen worm castings. Potting soils are very heavy, and transportation costs tends to keep them somewhat localized. There are many good brands of high quality potting soil. Ask your nursery person to help in selecting one for fast growing vegetables.

Stay away from discount brands of low-quality potting soil. These soils can be full of weed seed and diseases, hold water unevenly, and drain poorly. Ultimately, saving a few pennies on soil will cost many headaches and a low yield later.

Many potting soils supply seedling transplants and clones with enough food (fertilizer) for the first two to four weeks of growth. After that, supplemental fertilization is necessary to retain rapid, robust growth. Add fine=-grade dolomite lime to buffer and stabilize the pH. Trace elements in fortified soil and soilless mixes can leach out and should be replenished with chelated nutrients, if deficiency signs occur. Organic growers often add their own blends of trace elements in mixes that contain seaweed, guanos, and manures.

Although some growers reuse their potting soil, I do not recommend it. If used for more than one crop, undesirable microorganisms, insects and fungi start growing; nutrients are depleted; water and air retention are poor, causing compaction and poor drainage. Some growers mix their old potting soil with new potting soil to stretch their mix. Cutting corners this way most often costs more in production than is saved in soil.

Potting soil or soilless mix that contains more than 30 percent lightweight pumice or perlite may float and stratify when saturated with water before planting. Mix water-saturated soil thoroughly with your hands until it is evenly mixed before planting and transplanting, if necessary.

Mushroom Compost

Mushroom compost is an inexpensive potting soil amendment that is packed with organic goodies. Mushroom compost is sterilized chemically to provide a clean medium for mushroom growth. After serving its purpose as a mushroom growing medium, it is discarded. Laws usually require that it sit fallow for two years or more to allow all the harmful sterilants to leach out. After lying fallow for several years, mushroom compost is very fertile and packed with beneficial microorganisms. The high-power compost could also foster antifungal and antibacterial properties in foliage and below the soil line, which helps guard against disease. Mushroom compost is loaded with beneficial bacteria that hasten nutrient uptake. The texture, water holding ability, and drainage in some mushroom compost should be amended with perlite to promote better drainage. Check your local nursery or extension service for a good source of mushroom compost. Some of the most abundant harvests I have seen were grown in mushroom compost.

Soilless Mix

Soilless mixes are very popular, inexpensive, lightweight, and sterile growing mediums. Commercial greenhouse growers have been using them for decades. The mixes contain some or all of the following: pumice, vermiculite, perlite, sand, peat moss, and coconut coir. Premixed commercial soilless mixes are favorites for countless growers. These mixes retain moisture and air while allowing strong rot penetration and even growth. The fertilizer concentration, moisture level, and pH are very easy to control with precision in soilless mix.

Soilless mixes are the preferred substrate for many bedding plant and vegetable seedling commercial growers. Soilless mixes have good texture, hold water, and drain well. Unless fortified with nutrient, soilless mixes contain no nutrients and are pH balanced near 6.0 to 7.0. Coarse soilless mixes drain well and are easy to push plants into growing faster with heavy fertilization. The fast0draining mixes can be leached efficiently so nutrients have little chance of building up to toxic levels. Look for ready mixed bags of fortified soilless mixes such as Jiffy Mix, Ortho Mix, Sunshine Mix, TerraLite, and ProMix, etc. To improve drainage, mix 10-30 percent coarse perlite before planting. Fortified elements supply nutrients up to a month, but follow directions on the package.

Soilless components can be purchased separately and mixed to the desired consistency. Ingredients always blend together best when mixed dry and wetted afterwards using a wetting agent to make water more adhesive. Mix small amounts right in the bag. Larger batches should be mixed in a wheelbarrow, concrete slab, or in a cement mixer. Blending your own soil or soilless mix is a dusty, messy job that takes little space. To cut down on dust, lightly mist the pile with water several times when mixing. Always wear a respirator to avoid inhaling dust.

The texture of soilless mixes – for rapid growing cannabis – should be coarse, light , and spongy. Such texture allows drainage with sufficient water and air retention, as well as providing good root penetration qualities. Fine soilless mix holds more moisture and works best in smaller containers. Soilless mixes that contain more perlite and sand drain faster, making them easier to fertilize heavily without excessive fertilizer salt buildup. Vermiculite and peat hold water longer and are best used in small pots that require more water retention.

The pH is generally near neutral, 7.0. If using more than 15 percent peat, which is acidic, add appropriate dolomite or hydrated lime to correct and stabilize the pH. Check the pH regularly every week. Soilless mixes are composed mainly of mineral particles that are not affected by organic decomposition, which could change the pH. The pH is affected by acidic fertilizers or by water with high or low pH. Check the pH of the runoff water to ensure the pH in the medium id not too acidic.

Propagation Cubes & Mixes

Rockwool root cubes, peat pellets, and Oasis blocks are pre-formed containers that make rooting cuttings, starting seedlings, and transplanting them easy. Rot cubes and peat pots also help encourage strong root systems. Peat pots are small, compressed peat moss containers with an outer wall of expandable plastic netting. The flat pellets pop up into a seedling pot when watered.

Place a seed or cutting in a moist peat pot or root cube. If the little container does not have a planting hole, make one with a chopstick, large nail, or something similar. Set the seed or clone stem in the hole. Crimp the top over the seed or around the stem so it makes constant contact with the medium. On one to three weeks, roots grow and show through the side of the cube. Cut the nylon mesh from peat pots before it gets entangled with roots. To transplant, set the peat pot or root cube in a pre-drilled hole in a rockwool block or into larger pot. Clones and seedlings suffer little or no transplant shock when transplanted properly.

Check moisture levels in peat pots and root cubes daily. Keep them evenly moist but not drenched. Root cubes and peat pots do not contain any nutrients. Seedlings do not require nutrients for the first week or two. Feed seedlings after the first week and clones as soon as they are rooted.

Coarse sharp sand, fine vermiculite, and perlite work well to root cuttings. Sand and perlite are fast draining, which helps prevent damping-off. Vermiculite holds water longer and makes cloning easier. A good mix is one third of each: sand, fine perlite, and fine vermiculite. Premixed seed starter mixes sold under such brand names as Sunshine Mix and Terra-Lite are the easiest and most economical mediums in which to root clones and start seedlings. Soilless mix also allows for complete control of critical nutrient and root stimulating hormone additives, which are essential to asexual propagation.

Soil Amendments

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 cococnut 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.

Soil Mixes

Outdoor soil mixes that incorporate garden soil, compost, manure, coco peat, and rock powders grow some of the best plants in the world. Outdoor soil mixes can be mixed a few months early and left in the hole to blend and mature. Outdoor organic soil mixes are alive, and controlling the soil life is a matter of paying attention to a few details.

Indoors, outdoor soil mixes often create more trouble than they are worth. Too often misguided novices go out to the backyard and dig up some good looking dirt that drains poorly and retains water and air unevenly. The problems are compounded when they mix the dirt with garden compost packed with harmful microorganisms and pests. This lame soil mix grows bad dope. By saving a few bucks on soil, such growers create unforeseen problems and pay for their savings many times over with low-harvest yields. Avert problems with soil mixes by purchasing all of the components. Use garden soil or compost only if they are top quality and devoid of harmful pests and diseases. Use only the richest, darkest garden soil with a good texture. Amend the soil by up to 80 percent to improve water retention and drainage. Even a soil that drains well in the outdoor garden needs amending to drain properly indoors. Check the pH several times after mixing to ensure it is stable.

Solarize garden soil by putting it out in the sun in a plastic bag for a few weeks. Turn the bag occasionally to heat it up on all sides. Make sure the bag receives full sun and heats up to at least 140F. This will kill the bad stuff and let the beneficial bacteria live.

You can also sterilize soil by laying it out on a Pyrex plate and baking it at 160F for 10 minutes, or microwave it for two minutes at the highest setting. It is much easier and more profitable in the end to purchase good potting soil at a nursery.


Compost is outstanding. It solves most problems outdoors. Compost is an excellent soil amendment. it holds nutrients and moisture within its fiber. However, using backyard compost indoors is tricky.

Some growers have no trouble with organic composts, but others have bad luck and even lose their entire crop when growing in backyard compost. Good compost recipes are available from monthly publications such as Sunset, Organic Gardening, National gardening, ect, or from the companies specializing in organic composts. Outdoor growers love compost. It is inexpensive, abundant, and works wonders t increase water retention and drainage. It also increases nutrient uptake because of biological activity. Indoors, compost is not very practical to use in containers. It could also have unwanted pests. If using compost indoors, make sure it is well rotted and screened.

A good compost pile includes manure – the older the better. Manure from horse stalls or cattle feedlots is mixed with straw or sawdust bedding. Sawdust uses available nitrogen and is also acidic and not recommended. Look for the oldest, and most-rotted manure. Well-rotted manure is less prone to have viable weed seeds and pests. Fresh nitrogen=packed grass clippings are one of my favorites to use in a compost pile. Put your hand down deep into a pile of grass clippings. The temperature one or two feet down in such a pile ranges from 120 to 180F. heat generated by chemical activity kills pests, breaks down the foliage, and liberates the nutrients.

Build compost piles high, and keep turning them. Good compost pile recipes include the addition of organic trace elements, enzymes, and the primary nutrients. the organic matter used should be ground up and in the form of shredded leaves and grass. Do not use large woody branches that could take years to decompose.

Before using compost indoors, put it through 0.25 inch mesh hardware cloth (screen) to break up the humus before mixing with soil. Place a heavy duty framed screen over a large garbage can or a wheelbarrow to catch the sifted compost. Return earthworms found on the screen t the medium and kill cutworms. Make sure all composts are well rotted and have cooled before mixing with indoor soil.

Some growers mix up to 30 percent perlite into organic potting soil that contains lots of worm castings. Heavy worm castings compact soil and leave little space for air to surround roots. Adding perlite or similar amendments aerates the soil and improves drainage.

Growing Medium Disposal

Disposing of used growing emdium can be as big a problem as finding the proper soil. Most soilless mixes and soils contain perlite, which leaves white telltale traces when dumped anywhere. Grow soil is also packed with incriminating cannabis roots. The plug was pulled on more than one grow show because soil residuals were found in the backyard.

Dry soil is easier to work and transport. Press and rub dry soil through a 0.25 to 0.5 inch screen to remove roots, stems, and foliage. Screening also transforms the cast-container shape of the soil t an innocuous form. Once the roots are removed, dry, used soil can be bagged up or compacted. Pace the dry soil in a trash compactor to make it smaller and more manageable. Do not throw the spent growing medium into your trash can. Remember, it is not a crime for law enforcement in America to dig through your garbage. In fact, picking through a suspected grower’s garbage is a tactic often used by American authorities to secure a search warrant. Remove the depleted soil from the property. Take it to the dump or dispose of it in a very discreet locale. Never throw away the transporting bags with the used soil. Reuse the bags.

Used indoor growing mediums make excellent outdoor amendments when mixed with compost and garden soil. Do not reuse the depleted soil in outdoor pots. Many of the same problems that occur indoors will happen in outdoor pots.

Grow Medium Problems

These maladies are caused by growing medium problems but manifest as nutrient problems. The solution is found within the growing medium.

When water is abundant in the growing medium, roots easily absorb it. Roots use more energy to absorb more water as it becomes scarce. Finally, the point comes when the substrate retains more moisture than it surrenders, and the roots receive no water. A good growing medium readily yields its bank of stored water and nutrients to a plant’s roots. The more easily cannabis absorbs nutrients, the higher the yield.

The cation-exchange-capacity (CEC) of a growing medium is its capacity to hold cations that are availlable for uptake by the rots. The CEC is the number f cation charges held in 3.5 ounces (100gm) or 100 cc of soil and is measured in milli-Equivalents (mEq) or Centi-Moles/kg on a scale from 0-100. CEC of 0 means the substrate holds no available cations for roots. CEC of 100 means the medium always holds cations available for root uptake. Growing mediums that carry a negative electrical charge are the best.


Container preference is often a matter of convenience, cost, and availability. but the size, shape, and color of a container can affect the size and the health of a plant as well as the versatility of the garden. Containers come in all shapes and sizes and can be constructed of almost anything – clay, metal, plastic, wood, and wood fiber are the most common. Cannabis will grow in any clean container that that has not been used for petroleum products of deadly chemicals. Clay fiber and wood containers breather better than plastic or metal pots. heavy clay pots are brittle and absorb moisture from the soil inside, causing the soil to dry out quickly. Metal pots are also impractical for grow rooms because they oxidize (rust) and can bleed off harmful elements and compounds. Wood, although somewhat expensive, makes some of the best containers, especially large raised beds and planters on wheels. Plastic containers are inexpensive, durable, and offer the best value to indoor growers.

Rigid plastic pots are the most commonly used containers in grow rooms. Growing in inexpensive, readily available containers is brilliant because they allow each plant to be cared for individually. You can control each plant’s specific water and nutrient regimen. Individual potted plants can also be moved. Turn pots every few days, so plants receive even lighting and foliage will grow evenly. Huddle small, containerized plants tightly together under the brightest area below the HID amp, and move them further apart as they grow. Set small plants up on blocks to move them closer t the HID. Individual plants are easily quarantined or dipped in a medicinal solution. Weak, sick, and problem plants are easy to cull from the garden.

Grow bags are one of my favorite containers. Inexpensive, long lasting grow bags take up little space and are lightweight. A box f 100 3-gallon bags weighs less than 5 pounds and measures less than a foot square.

Grow bags are very easy to wash and reuse. Empty out the soilless mix and submerge bags in a big container of soapy water overnight. Wash each one by hand the next day and fill with soil. I like them much better than rigid pots because they are so practical.

The potting soil sack can be used as a container. The moist soil inside the bag holds its shape well, and the bags expand and contract with the soil, lessening the chance of burned root tips that grow down the side of pots.

Fiber and paper-pulp pots are popular with growers who move their plants outdoors. The bottoms of the pots habitually rot out. Painting the inside of the fiber container with latex paint will keep the bottom from rotting for several crops.

Set large pots n blocks or casters to allow air circulation underneath. The soil stays warmer and maintenance is easier. Planters should be as big as possible but still allow easy access t plants. The rots have more room to grow and less container surface for roots to run into and grow down. With large pots, roots are able to intertwine and grow like crazy.

Grow beds can be installed right on the earthen floor of a garage or basement. If drainage is poor, a layer of gravel or a dry well can be made under the bed. Some growers use a jackhammer to remove the concrete floor in the basement to get better drainage. An easier option is to cut a hole in the basement floor and install a dry well. Knocking holes in basement floors could cause water seepage, where water tables are high. When it rains, the water may collect underneath; the garden seldom needs watering, but plants are kept too wet.

A raised bed with a large soil mass can be built up organically after several crops. To hasten organic activity within the soil, add organic seaweed, manure, and additives. When mixing soil or adding amendments, use the best possible organic components and follow organic principles. There should be good drainage and the soil should be as deep as possible, 12-24 inches.

At the Cannabis College in Amsterdam, Netherlands, they are growing in large soil beds set on a concrete floor. The basement beds are below sea level and filled with outstanding Dutch cannabis. They are able to treat the beds similar to outdoor soil beds, but when watered heavily, water runs out on the floor and it must be mopped up. They also have a problem with ventilation. The ambient climate is naturally humid and the extra water in the large basement increases humidity to above 90 percent, day and night. They employ a large extraction fan and a relatively small intake fan. Air is pulled through the long narrow basement room quickly and efficiently to evacuate moisture and lower the humidity. Even with this much soil, they have to flush individual beds at least once every four weeks to avoid nutrient buildup. Much heat can be generated by decomposing organic matter, and it warms the room. Ventilation lowers heat, but it is a lot of work to replicate the great outdoors. Most organic growers opt for organic liquid fertilizers and a bagged commercial organic soil mix.

Another drawback to raised beds is that the crop will take a few days longer to mature than if it were grown in containers. But the longer wait is offset by a larger harvest.


All containers need some form of drainage. Drainage holes allow excess water and nutrient solution to flow freely out of the bottom of a container. Drainage holes should let water drain easily, but not be so big that growing washes out onto the floor. Containers should have at least two half inch holes per square foot of bottom. Most pots have twice this amount.. To slow drainage and keep soil from washing out of the large holes, add a one inch layer of gravel in the bottom of the pot. Surface tension created by the varying sizes of soil and rock particles cause water to be retained at the bottom of the container. Line pots with newspaper if drainage is too fast or if soil washes out drain holes. This will slow drainage, so be wary!

Put trays under containers to catch excess water. Leaving water-filled saucers under pots often causes root rot. To avoid water logging soil and roots, set containers up an inch or two n blocks when using trays.

Nursery trays used for rooting cuttings and growing seedlings must have good drainage throughout the entire bottom. Once clones and seedlings are in place in the tray, the tray should always drain freely with no standing water in the bottom.

Container Shape, Size, and Maintenance

Popular pot shapes include rectangular and cylindrical. Growers prefer taller pots rather than wide, squat containers because the cannabis rot system penetrates deeply. Of all the gardens I have visited, squat pots were few and far between. Growers I queried said squat pots may hold more soil for their stature, but they do not produce as extensive a root system.

The volume of a container can easily dictate the size of a plant. Cannabis is an annual; it grows very fast and requires a lot of root space for sustained, vigorous development. Containers should be big enough to allow for a strong root system, but just big enough to contain the root system before harvest. if the container is too small, roots are confined, water and nutrient uptake is limited, and growth slows to a crawl. But if the container is too big, it requires too much expensive growing medium and becomes heavy and awkward to move.

Marijuana roots develop and elongate quickly, growing down and out, away from the main taproot. For example, about midsummer, nurseries have unsold tomato plants that are still in 4 inch pots and one gallon containers. The stunted plants have blooming flowers and ripe fruit. but few branches extend much beyond the sides f the container; the plants are tall and leggy with curled down leaves and an overall stunted, sickly appearance. These plants are pot- root-bound. Once a plant deteriorates to this level, it is often easier and more efficient to toss it out and replace it with a healthy one.

Roots soon hit the sides of containers where they grow down and mat up around the bottom. The unnatural environment inside the container often causes a thick layer of roots to grow alongside the container walls and bottom. This portion of the root zone is the most vulnerable to moisture and heat stress and is the most exposed.

When soil dries in a pot, it becomes smaller, contracting and separating from the inside of the container wall. This condition is worst in smooth plastic pots. When this crack develops, frail root hairs located in the gap quickly die when they are exposed t air whistling down this crevice. Water also runs straight down this crack and onto the floor. You may think the pot was watered, but the root ball remains dry. Avoid such killer cracks by cultivating the soil surface and running your finger around the inside lip of the pots. Cultivate the soil in pots every few days and maintain evenly moist soil to help keep root hairs on the soil perimeter from drying out.

Do not place containers in direct heat. If soil temperature climbs beyond 75F, it can damage roots. Pots that are in direct heat should be shaded with a piece of plastic or cardboard.

A 1 to 2 inch layer of hydro clay mulch on soil surface keeps soil surface moist. Roots are able to grow along the surface, and the soil does not need to be cultivated. The mulch also decreases evaporation and helps keep irrigation water from damaging roots or splashing.

Green Roots

White containers reflect light and keep soil cooler. Always use thick, white containers so light does not penetrate and slow root growth. If roots around the outside of the root ball start turning green, you know they are receiving direct light. Remedy the problem by painting the inside of the container with a non-toxic latex paint.

Marijuana Horticulture

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