Story by Brian Getting
Photos by MG Imaging

Compost tea is a natural way to provide nutrients and beneficial organisms to your plants, and can even help battle diseases.
Hmmm, how about a nice big, warm glass of compost tea? Doesn't sound good, and if you are ever asked that at a coffeeshop, get the hell out of there. Compost tea is exactly that--a tea made from compost to be given to plants or soil, not to people. Compost teas have been used for soil boosting and foliar feeding for quite some time. Recently the teas have been shown to help fight disease as well. The development of brewing machines has made their production quick and easy, and also cost effective. Smaller versions are now being marketed to home gardeners.

A compost tea is created when some form of compost is steeped in water to get a broth of microorganisms that can be applied to the soil. Normal soil contains much more than just plant roots and nutrients. There are all kinds of bacteria, fungi, nematodes, worms, and many more living things that make the soil productive. Plants require specific forms of the nutrients that they need. Some of these forms don't stick to soil very well, and are washed away easily. Living creatures serve as a stockpile of nutrients, eating up different things and storing them in their tissues.

Some soil organisms process nutrients into plant-usable forms. Other organisms feed on smaller living things, and process them into plant-usable forms. The microbial ecology of soil is similar to the ecology of the macroscopic world. There are complex relationships, food webs and predator-prey interactions.

One of the uses of compost teas is to fight pathogens. There is a survival race among the microorganisms on and around a plant. Just as in the macroscopic world, there is a limited amount of food and spatial resources, for which multiple contenders compete. Using compost tea, which is a broth of microbes that we know are not harmful to the plants, we can introduce new competitors into this battle, and swing the outcome to our favor. Giving pathogens other competition can weaken them, and give the plant a chance to react with its own immune system. Also, exposure to a variety of bacteria and fungi keeps the plant's immune system active, boosting its own natural defenses.

The most traditional compost teas are actually a side product of composting. Newer and more specialized teas are made in brewing machines, with special ingredients. Let's start with traditional compost teas, the leachate from compost bins.

Composting requires water, and there is always a leachate, or runoff of liquid, that collects either in the bottom of the compost bin or below the bin in a catcher. This liquid is essentially the first compost tea. Over the course of the composting process, bacteria break down the organic material and turn it into usable compost. As some of the water percolates through the compost, it picks up nutrients and a sampling of the organisms in the compost that are leached out by the water. People have long been applying this liquid to their soils, with amazing results. The effect is to introduce the microbes into the soil, where they multiply, feed, and cycle nutrients as we discussed before. This adds to the productivity of the soil.

Nearly identical to this is the leachate that is obtained from vermicomposting. Vermicomposting is the term for making compost using worms. Worms eat organic matter, like newspaper and vegetable scraps, and turn it into a rich compost, called worm castings or vermicompost. This moisture sometimes collects and leaches out of the castings, again either to the bottom of the bin or into a catcher below. Just as before, this liquid is rich in nutrients and microbes, and can be used to boost soil.

Realizing that this liquid was beneficial to their plants, people began to take compost and steep it in water, hoping to get the benefits of rich compost into a wonder liquid that could be easily applied. One easy method has been to put some compost into a burlap sack, and place the sack into a vat of water. Over the course of a couple weeks, the microorganisms and nutrients from the compost seep into the water. If there is a sufficient food source, the microbes will multiply, making the tea richer. The length of brewing time depends on how fast the microbes grow, and how concentrated you want your tea to be. Since the microbes are really the hallmark of compost teas, speeding up the process would allow for larger amounts of tea to be made.

There are many variables limiting the growth of microbes in compost tea, such as nutrient availability, competition and oxygen availability. Bacteria, fungi and anything else that we would like to be growing in the tea are not photosynthetic, so they require oxygen and sugars to create energy. Just like us, they also need a source of amino acids, or some nitrogenous compound, to make proteins. One way to speed up the process of making compost tea is to provide all of these things.

This quest has led to many commercially available machines for making compost tea, though fundamentally they are no different than the vat mentioned above. The important thing to realize is that the ingredients that go into the tea are the determining factor for how effective the tea is, not the device used to brew the tea.



Our real goal is to make a broth that contains a diverse sampling of microorganisms that will be beneficial to our soil and not pathogenic to our plants (and ourselves). In order to do that, we need a diverse sampling of beneficial soil organisms. Then to grow those organisms we need to provide them with a carbon source (for metabolism), a nitrogen source (to make proteins), and sources of vitamins and minerals. Like us, they also need oxygen in order to be able to metabolize carbon into energy.

Where do we get the organisms from? Well, right now in American history is a bad time to be on the Internet looking at biotech companies trying to get your hands on otherwise benign bacteria. Fortunately, we can get a diverse sampling of good bacteria from compost. Good compost can be bought nearly everywhere, and most of the time it is laboratory-tested to assure quality. To ensure that there are no human pathogens and that the compost has a diverse population of microbes, insist on buying laboratory-tested product. The type of compost that you use depends on your application.

Ordinary vermicompost is good to use for a general soil-building compost tea. Only a small amount is needed, so don't go filling up the back of the pickup truck. Outdoor soil happens to contain all the goodies you need as well, but may need to brew a little longer, as it has a lower concentration of organisms than store-bought compost.

Next we need to provide the organisms in our compost sampling with a source of carbon so that they can grow well. There is some variation here, such as what organisms use what sugar sources. As a rule, bacteria can usually grow faster than fungi in a broth that they like. However, fungi have the advantage of being able to metabolize carbon sources that most bacteria cannot, such as lignin and cellulose. That difference allows us some control here about the end result of our tea. Theoretically, in order to have a tea that is heavier in fungi than bacteria, the sugar source should be nearly all lignin or cellulose, in order to select for fungi (there will still be bacteria, as the fungi release byproducts that bacteria can live on).

Conversely, if we use dextrose as a sugar source, bacteria will proliferate, and the tea will have much more bacteria than fungi (there will still be fungi, since fungi can metabolize dextrose as well). If you are wondering why this is important, fungal teas are more helpful in fighting fungal pathogens, such as leaf blight. Also, the soil biota play an important role in succession, so treatment of the soil may help in restoration efforts. To give an idea of what this means, Conifer-forest soil usually has more fungi than bacteria, due to the high amounts of lignin available and a lower soil pH. Grassy meadows usually have a much higher level of bacteria than fungi, because in the lack of wood sources, the bacteria out-compete the fungi. For production and garden soil, the ideal is equal amounts of bacteria and fungi. This may seem simple, but in reality, the bacteria tend to dominate.

Once the tea is applied, nature will determine which will make it and which will die. Cost is another issue, depending on the amount of tea being made. At the recommended concentration of 20% (weight/volume) carbon source, choosing your source wisely can save much money. Molasses is a cheap sugar source, and it goes into solution well. Raw cane sugar can be used too, as well as nearly any sugar source. Lignin and cellulose can be a little more difficult to find, but they are out there. The sugar source should be added to the water of the tea, and allowed to dissolve before brewing starts.

One very important point to make here is that human pathogens can grow in a non-selective carbon source. Though not likely, it is a possibility, and should be avoided. One way to avoid this is to use carbon sources that are complex sugars, which require special enzymes to metabolize, unlike sucrose. Such a carbon source helps to select for soil biota. A simple rule is to look at what the microbes would be eating anyway. In the case of using compost teas, plants are the common denominator. Vegetable and other plant extracts contain many compounds that are ideal for a carbon source. Since very little research is being done about this, the sky is the limit for combinations and discovery.

Soil organisms also need a source of nitrogen in order to be able to make proteins and carry out basic life processes. Meat sources of protein are not going to work here, so don't even think about it. In general, meat has no place in the composting process, because it can potentially harbor some nasty little things that are pathogenic to humans. Yeast extract is a very good source of nitrogen, as it is just dried yeast (lyophillized) that has been ground up into a powder. It also contains other vitamins and minerals that are beneficial. Nonfat dried milk is another good source of proteins, but it may take some extra processing, which may increase brewing time. Of course, flour, rice meal, wheat, etc. have protein in them, so there is a lot of room for experimentation in this area. A good amount for tea is to use 10% (w/v) of your nitrogen source, although I would use about half that if using dried milk.

In my opinion, the nitrogen should be from a non-animal source (yeast extract, rice flour, etc.), since it will be used on plants, and should be processed as little as possible (for example, bleached flour is not necessary). Another exciting factor to consider is that many of the grains also contain complex carbohydrates, which means that you can be adding carbon and nitrogen in one fell swoop.

In addition to the carbon and nitrogen sources that we have just provided, we may want to add some extra goodies to our tea. These can be things that will help the microbes grow better, or things that will benefit the plants or soil. Humic acids are a good example of something to add for the soil. Both humate and fulvate supplements can be purchased, and can help improve the water retention of soil, among other things.

Kelp meal is also great to add. It's made by harvesting kelp, drying it, and grinding it into a powder. This powder is rich in potassium and micronutrients that benefit the plants and the bacteria. It's very helpful in maximizing microbe growth, and is available commercially. One small problem with kelp meal lies in its solubility. The powder does not dissolve into water very well, and at best goes into a colloidal solution. That means it eventually settles out of the tea, unless it is stirred constantly. An advantage is that these colloidal particles have a relatively large surface area that acts as a substrate for some microbes.

I have found that one drop of castile soap per gallon of tea is effective at getting more of the kelp into suspension. Castile soap is a natural soap that will not harm the organisms. This can also be a problem for other ingredients where solubility is less than ideal. The small amount of castile soap should help out with solubility some, especially in the case of oils and hydrophobic compounds.

The sky is the limit as far as other additives. Many plants contain compounds that may be helpful, from insect repellents to vitamins. Make sure not to add anything to your tea that may have antibacterial or antifungal qualities, because that would hinder the microbes we are trying to grow. A few ideas of additives are things like dried nettles or other botanicals, VAM inoculates (beneficial endomycorrhizal fungi), cellulose or lignin, royal jelly (a bee product rich in vitamins, but also hard to get into solution), and much more. Compost tea use is relatively new, and more research needs to be done on the impact of tea ingredients to certain plants. The important thing to remember is that we are making a solution that will grow microorganisms, but can also contain things that we want to feed our plants.



So we have a basis for making a compost tea, and a beginners' recipe (20% sugar, 10% yeast extract, 10% kelp meal, compost). What do we do now? We said before that we could just put all this in a vat, and over a couple of weeks it will grow into a nice tea. However, now that we have added food for the microbes, we are only short on one thing, and that is oxygen. The microorganisms need oxygen to metabolize, and they get it from the solution around them. So if we could get more oxygen in the tea, it would grow faster. In fact, this is the center of the commercial compost-tea business: getting the oxygen into the water.

For the home enthusiast, a five-gallon bucket with a lid, rigged with a fish-tank air pump and air stone, is enough. Add your ingredients, make sure they are dissolved (as much as possible), and put your compost into a net sack, or some permeable enclosure which is steeped in the tea like a teabag. If you have no plumbing that can clog, like our 5-gallon bucket, we can just put the compost in the water. Turn on the air pump, and wait a couple days. The resulting tea can be diluted 1:10 for foliar applications or 1:5 for soil applications. In fact, 5 gallons of this tea can cover an acre of turf. Of course, that is up to you, just like the frequency with which you apply it. Regular applications will benefit the plants best, fight off pathogens and condition the soil.

What if you want to make larger amounts of tea, or you want to buy a home tea-brewing system? This is where the commercial industry comes into play. There are a few manufacturers of compost-tea brewers that are filling this new market niche. Of course, in order to ensure consistent, effective teas, these machines must tackle the same obstacles that we tackled above. There are many mechanical problems associated with tea brewers, as well as design features aimed at different applications. A good place to start your search is with Growing Solutions, located in Eugene, Oregon, or SoilSoup, out of Bainbridge Island, Washington.

Whether or not you need compost teas to fix a problem, or want to use them for their benefits, the ideas behind using them are useful to all growers. Caring for the soil, and not the plants, is not a common practice in American farming. Multiple applications of a natural product are not as appealing to us as fewer applications of poison to solve our problems. However. with topsoil erosion at an all-time high, drinking water quality at an all-time low, and our agricultural practices partly to blame, it might be time to rethink how we grow.

For the home or indoor gardener, these practices can make your plants less dependent, and keep you from worrying about when to fertilize. It can mean larger, chemical-free plants that use the nutrients that you provide them more efficiently. Compost teas can be used in place of fungicides and other chemicals that can leave residues. In fact, two common cannabis diseases, botrytis and fusarium, can be treated using compost tea. Most importantly, considering the cost of chemicals, it could save you money that you just may need if someone finds out what you are growing.