How to


How to Make Compost Tea and What is it for?

Compost teas are a form of liquid organic fertiliser mix that provides supplemental nutrients and beneficial microorganisms that help cannabis plants reach their full potential and bulletproof soil health. Growers can’t go wrong using compost teas as part of their regular nutritional programme, or if their crop requires a boost against infection or during delicate periods of transition as it is a sustainable, organic way to provide nutrients. 

Compost tea can be brewed at home in a fairly straight-forward process, using easily accessible ingredients and methods, and then applied to plants by watering into the soil and/or as a foliar spray.

The What and Why of Plant Nutrition

Marijuana plants uptake most of the nutrients needed for growth from the soil. The most important nutrients are nitrogen (N), phosphorus (P), and potassium (K). Growers will find these three nutrients listed as ratios of NPK on labels of fertiliser and nutrient products.

The plant’s demand for these three critical nutrients will depend on the stage they are in during their lifecycle. Plants require more N during the vegetative phase before they form flowers, as N is crucial to the formation of healthy plant cells, which results in strong stalks and healthy leaf growth. N is also an essential ingredient in chlorophyll, which the plant uses to photosynthesise light into proteins and sugars.

P is needed when during flower formation and stem growth. Growers can achieve desired higher yields by making sure their plants have enough phosphorus during transition and flowering. It can also help the plant withstand environmental stress by improving root development.

K is needed during flowering for full flowers, dense trichomes, and potent terpenes to develop. K is needed for water transportation functions inside a plant (most of the volume inside plant cells is water). The resinous, dense buds associated with high-grade flowers are expressions of water retention facilitated by optimal K uptake.

NPK is found primarily in soil and will need replenishing as the plant develops and uses it up. Hydroponic fertilisers, solid organic top dressings, liquid organic fertilisers, and compost tea are all ways of delivering these essential nutrients to cannabis plants. The bonus of compost tea is the beneficial microorganisms found in a brew that improves soil health, defends against pathogens, and encourages plant growth by improving uptake of NPK in several ways:

Applied to eaves as a foliar spray

When applied as a spray to the surfaces of leaves, the beneficial microorganisms in compost tea increase the amount of time the plant’s pores (or stomata) are open, allowing for more nutrients to absorb. The rate of photosynthesis is also increased as more carbon dioxide enters the plant through open pores. Stomata are also entryways for bacterial pathogens, and when applied to leaf surfaces, beneficial microorganisms will increase plant resistance to disease and combat potential infections.

Applied as to soil as a liquid 

Growers watering their compost tea into the soil of their plants will improve the rate of growth by increasing nutrient availability in the root system and nutrient retention in the soil.

Compost tea will also break down solid organic top dressings over time, slowly releasing even more nutrients into the soil and root systems and replenishing nutrients washed out during regular watering. Predators such as nematodes can be included in a brew to counteract fungus gnat larvae and other soil pathogens.

Basic Ingredients for Compost Tea

The ingredients for compost tea take the form of solids or liquids. They can be sourced at home or in most garden stores.

Compost (solid): The foundation for it all, healthy compost is the culmination of the natural decomposition of organic matter such as fruit, vegetables, garden waste, and eggshells. Locally sourced compost will contain microorganisms that are used to the local pathogens that may be found in planting soil. High-quality compost contains a multitude of beneficial microorganisms such as bacteria, nematodes, and fungi, which are extracted into the tea solution during the brewing process.

Worm Castings (solid): Also known as vermicast and worm excrement, castings are an outstanding source of nutrients, minerals, and beneficial bacteria. They are a rich source of N (but not so much that it will cause nutrient burn), humic acid (which boosts nutrient uptake) and make the soil more absorbent.

Kelp (liquid): A sustainable and organic source of NPK and micronutrients, microorganisms found in fermented kelp solutions increase nutrient availability in soil by binding nutrients to soil particles in the root zones, minimising nutrient loss.

Molasses (liquid): While everything else in the tea will feed the plants, the microorganisms need some love too! Molasses are a rich source of food for bacteria living in soil, as well as for the bacteria brewing in the solution, which will need to eat while the tea brews for 24 hours.

These four ingredients, brewed together, will create a compost tea brew that growers can apply to their plants. Growers looking to super-charge their teas can look at the following ingredients to add to the core four mentioned above:

Additional Ingredients for Health and Pest Resistance

Granular Humus (solid): Essentially dried compost, humus acts as a soil conditioner that improves water retention, helping soil stay moist. It also promotes root development by trapping oxygen and preventing erosion.

Fish Hydrolysate (solid or liquid): Pulverised fish and crustaceans can be added to a compost tea brew to increase N content and feed beneficial fungi, feeding both the plant and beneficial microorganisms in the soil.

Growers looking to boost their plant’s immunities to pathogens may consider adding beneficial nematodes and the bacteria bacillus thuringiensis (or Bt) to their mixes, preferably after the brewing process is complete and before watering. Nematodes are an organic, non-toxic solution to pest management as they prey on larvae of soil pests, eliminating the need for broad-spectrum insecticides that harm beneficial microorganisms and damage soil health. Bt is a naturally occurring bacteria that has been used since the 1950s and controls certain pests by damaging their stomachs when consumed. In its natural form, it is non-toxic to birds, fish, and mammals.

Compost tea serves as beneficial nutrition for cannabis plant
Cannabis plants of all strains and grow habits can benefit from compost tea applications.

Brewing Aerated Compost Tea

Compost teas brewers can be built at home using a 5-gallon bucket, aerators such as those used in aquariums, and an air pump. Aerating the tea as it brews will oxygenate the solution, which is necessary for beneficial bacteria to develop and survive. A mesh bag (400 microns) to hold the dry solid ingredients (compost, castings, humus) will also be required – this is the “teabag” that is suspended in the brew.

How to Brew Aerated Compost Tea

  • Fill the mesh bag with the dry solid ingredients (compost, castings, humus) and make sure it is properly sealed or closed to prevent its contents from spilling into the solution. For a 5-gal bucket, use approximately 100ml of each ingredient.
  • Add liquid ingredients to the water inside the bucket. For a 5-gal bucket, use approximately 60ml of each ingredient.
  • Using a metal rod, suspend the bag of solids in the bucket, making sure it is completely submerged but not impeding the aerators.
  • Place aerators inside the bucket. These will agitate water from the bottom of the bucket to the surface, causing bubbles to rise and pop, distributing oxygen throughout the bucket. Make sure bubbles are visible.
  • Let brew for 24 hours. Keep away from direct sunlight.
  • Add any additional nutrient solutions if desired.
  • Ph to 5.8-6.0
  • Add nematodes and beneficial bacteria if desired.
  • Water into the soil or apply to plants as a foliar spray. Dilute using a ratio of three-to-one tea to water if applying as a spray.

Compost teas have a limited window of effectiveness. As it is full of living microorganisms, it has a relatively short shelf-life and should be used within 6-8 hours of being brewed. Otherwise, the microorganisms perish, resulting in a sour, rotten, fermented scent, and the tea will be unusable.

Post author
Martin is a production horticulturist with experience in commercial cannabis cultivation and sustainable farming from his time with Emerald Cup Award-winning farmers Esensia Gardens in northern California's Emerald Triangle.
See more from Martin

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