Compost Tea for Cannabis

Emerald Queen Farms uses compost teas

Cannabis farmers are always searching for ways to optimize productivity.  Larger yields, higher quality and ways to cut costs top the directives. Within this industry however, there seems to be an overuse of synthetic fertilizers with little understanding of the biological systems involved in nutrient uptake and disease control.  Many inexperienced growers overuse synthetic fertilizers, hoping that more nutrients means higher yields.  All that really happens is a massive salt buildup, which leads to dead microbes, nutrient lockout, and a lot of flushing.  Many others overdo synthetic fungicides without really targeting “the root” of the problem.

Over the last 20 years, compost tea has been clearly gaining traction as an important variable on organic and sustainable crop production.  Anecdotal evidence, now coupled with considerable scientific research now proves various types of compost teas can suppress plant pathogens and diseases while also boosting yields and quality through microbial activity, effectively eliminating the need for hazardous agrochemicals.

What is Compost Tea and How Does it Work?

Technically, compost tea is where beneficial microorganisms are extracted from compost, humus or vermicompost (worm compost).  When provided with the right food source, their populations can effectively multiply into the billions.  Jeff Lowenfels, the author of Teaming With Microbes, reports that the bacterial population in 1 teaspoon of compost can grow from 1 billion to 4 billion in an aerated compost tea (ACT).  So when compost tea is brewed, you are literally creating life by facilitating the population growth of diverse groups of microorganisms.  High quality compost tea will have all of these organisms working synergistically in the soil to optimize conditions that facilitate nutrient uptake and plant health.  Many nutrients used in compost tea recipes function as a fertilizer for the plants as well.

Some benefits from using compost tea include:

• Enhancing plant health while suppressing disease
• Using less fungicides, fertilizers & maximizing nutrient uptake
• Working synergistically with biological pest controls
• Recharging the old soil with fresh microorganisms
• Increasing water holding capacity of soils
• Building farmable horizons of poorly structured soils

Compost Tea Dynamics

When you utilize compost tea, your soil and plants receive a concentrated dose of microorganisms that you have created in a bucket.  Luke A. Besmer, a biologist who runs a business out of Humboldt County called TeaLab, breaks it down a bit by saying, “A good compost tea is a living soup of bacteria, fungi, protozoa, and nematodes that help to provide your garden with essential nutrition, and also protect it from disease.  Some of the microbes directly benefit the plant, while others are there to help create and maintain a complete and healthy soil system.”

Foliar Spraying

The ATTRA Sustainable Agriculture Program provides helpful insights into the benefits of foliar spraying.  They explain, “When compost teas are sprayed onto the leaf surface, these beneficial organisms occupy spatial niches on the leaf surface and gobble up leaf exudates that pathogenic organisms would otherwise feed on to prosper; other microbes directly interfere with pathogenic organisms through antagonism. Pathogenic organisms that land on the leaf surface simply cannot compete with the beneficial organisms and therefore have a greatly reduced chance to initiate disease in the first place.”

Root Drenching

Root drenching with compost tea allows microorganisms go to work in the soil and provides a similar method of antagonism as the foliar spray. There is an amazing process of communication between the plants roots and the microbiology found in the growing medium.  The plants roots exude (release) a variety of molecules into the rhizosphere (root zone). These “exudates” include things like acids, sugars and enzymes, which create an optimal chemical environment where the bacteria and fungi intermingle.  There is an exchange that takes place.  Mycorrhizal fungi, for example, bring water and nutrients to the plants roots in exchange for certain exudates. Protozoa eat bacteria, releasing nutrients that the plants roots can readily uptake. The relationships of these microorganisms work synergistically with the plants root systems to modulate nutrient uptake and optimize plant health. The best way to describe it is “probiotics for plants”!

Bacterial vs. Fungal Tea

Using the right tea depends on what you are trying to achieve with your soil or phyllosphere (above ground plant matter). High sugar teas will produce teas high in bacterial populations. Teas brewed with humic acids, kelp and bokashi will produce more of a fungal dominated tea. Albeit that certain soil food web scientists, like Scott Skamnes of Crescive Soil, would argue that fungal teas are not really capable of being brewed because of the delicate nature of the fungal hyphae. “Fungal dominated” teas might be better described as pre-biotics, innoculating the soil to provide the proper structure for the beneficial fungi. A well-balanced tea facilitating both bacterial and fungal populations would provide sensible ratios for cannabis. If you are planting in native soil, different ratios will be more beneficial depending on soil type.

Types of Compost Tea and Parameters

Today when industry talks about compost tea, they are typically referring to aerated mixtures. These are called aerated compost teas (ACT). There are other types of “compost teas” less commonly used. They are called non-aerated compost tea, anaerobic compost tea, manure tea, compost leachate and fermented plant teas.

This article will solely address the use of ACT.

Base Ingredients

Dr. Elaine Ingham is an American microbiologist and soil biology researcher and founder of Soil Foodweb Inc. and is considered by many to be the pioneer into microbial analysis of compost teas. She clearly stipulates the most important ingredient in compost tea is the quality of the compost since all the organisms in compost tea come from the compost itself.

The typical base ingredients used in brewing compost tea are: Compost, Vermicompost (worm castings) and Humus. The high-end organic versions of these provide the highest ratio of beneficial microorganisms with the highest quality and cleanest inputs. A general rule of thumb is to find OMRI listed compost. Vital Earth makes an excellent OMRI certified compost. Alaskan Humus Soil is another favorite.

Foods & Nutrients

Food sources are what facilitate the explosion of microbial populations. Many ingredients that are food for your microbes actually double as a nutritive fertilizer for you plants. A typical list of ingredients being used throughout Humboldt County and the Pacific Northwest include: Fish Hydrolysate, Kelp, Insect Frass, Alfalfa, Glacial Rock, Azomite, Bokashi, Humic/Fulvic Acid, Molasses, Yucca and Guanos. The ingredients will also determine whether your tea is bacterially dominated vs. fungal dominated. There are many decent recipes found on the web and in hydroponic stores that carry compost tea. There is also a handy app called the Compost Tea Calculator that provides recipes and instructions.

Oxygen

Properly controlling oxygen input into the water is critical.  Dr. Ingham describes, “A large problem in making highly beneficial teas is when microbial growth rapidly uses up a significant portion of the oxygen such that anaerobic conditions ensue, and materials that are toxic to plant growth are produced in the tea.” Oxygen probes are desirable for larger operations that can afford to invest in the equipment.  Aerated compost teas thrive at dissolved oxygen levels above 6 ppm.  Most of the brewing kits provide large enough air pumps to meet the 6ppm threshold.

Temperature and Time

There is a temperature range that works best for compost tea and it is around 65ºF-85ºF. Any colder and the microorganisms will take longer to populate or not even reproduce. Any hotter and the tea may bloom too fast and have heavy dieoffs, making the tea go anaerobic and prone to pathogen growth.  The general rule of thumb is the warmer the tea the faster it will brew. Typical brewing times extend from 24-48 hours depending on temperature.

Management Plans

With the right management program integrating compost tea, you can use ½ to 1/3 fewer nutrients and still get increased yields, higher quality and clearly superior flavor. 

Application Rates

A very successful regime observed in Humboldt County involves transitioning between traditional fertilizers, watering, and compost tea applications.  So one would use their liquid fertilizer on the first watering, the second watering would be the compost tea and the third watering would be plain water. You can repeat that schedule or mix and match in different patterns depending on other variables your dealing with. Some growers dilute the tea and some apply it as full strength. Some growers simply pre-amend their soils with organic fertilizers and then apply water and compost tea periodically throughout the growing cycles.

Supplies / Equipment

Buying the right equipment can save lots of time in the long run.  The first step is to access how much tea you will need to treat your farm and whether you are going to be foliar spraying or root drenching. A 5 gallon batch of compost tea used for foliar spraying can cover 1000 square feet, where it could take 50-200 gallons of compost tea to root drench.  As you scale up in systems, so do the costs. Air Blowers, Large Tanks, Pumps, cleaning equipment are typical purchases when operating +100 gallon systems. A couple companies that make really nice larger kits are Vital Garden Supply, BioLogic Systems, Synergy and Microbe Makers. Time and resources spent cleaning the equipment should also be considered. Small systems 5-50 gallons are fairly streamlined and easier to wash. Larger systems, like 100-1000 gallons can require pressure washers or full hydrogen peroxide rinses.

Considerations

If brewed improperly, aerated compost tea can actually do more harm than good by adding detrimental pathogens to the soil. Teas that smell sour, putrid or rotten typically have gone anaerobic. Discretion is warranted and sometimes teas need to be discarded. Without the use of microscopes and knowledge of microbiology, the compost tea brewing process has a level of subjectivity to it. The smell of teas is complex and depends on the input ingredients. A tea brewed with just molasses and worm castings will smell much different than a tea made with fish hydrolysate, kelp and compost. Having a microscope to view growth provides precision to know when your teas peak in populations.  Careful management is integral in producing high quality teas.

Foliar spraying compost teas during flowering should be avoided. As the industry evolves, analytical laboratories are becoming more stringent with their microbial residue testing, with good cause. Compost tea should not be applied to cannabis sold as flowers. If cannabis is grown for concentrate production, studies need to be performed to determine that spraying throughout flowering will not impact microbial counts in analytical tests. Ideally it would only be foliar sprayed during vegetative growth and used as a root drench during flowering. More studies need to be conducted to determine if compost tea can be applied in the first several weeks of flowering.

Compost teas are considered to be an integral component of many ecologically minded cannabis farmers. The bottom line is that using compost tea will save you money by cutting down on expensive nutrient lines. It will help reduce fungicide use, intensify the terpene profiles and when used properly with other organic fertilizers, it will synergistically boost yields beyond what standard practices could do alone.

By: Cannabis Horticultural Association