Most gardeners know about compost. You take your grass clippings, weeds, old leaves, maybe your kitchen scraps, and throw them into a pile or into one of those plastic "composters." Turn it a couple times maybe, and a few months, or a year later, there it is, you've made compost. Well, you and billions of microbes. It's good for the garden. You can use it like fertilizer to give your plants a boost, or work it into the soil to loosen it up, or you can use it like bark mulch to keep weeds down, moisture in, and general beautification.
What and Why Compost Tea?
Compost tea is a relatively new way to use compost. Old-timers may remember manure tea, where you put a few shovels of manure into a barrel of water, let it sit for a spell, and then ladled it out on your plants to feed them. This is not recommended anymore due to possible contamination with E. coli and other pathogenic organisms. It doesn't smell very good, either.
Compost tea is a modern version of manure tea, minus the stink. It uses a pump to inject air through a solution of compost and water to encourage a special mix of aerobic microscopic organisms to grow. These soil microbes are not harmful, but are in fact, very beneficial in a number of ways.
Some are important in creating a diverse soil microbe community, which is really the backbone of what we think of as healthy soil that plants grow well in. Some "fix" nitrogen, meaning, they grab it out of the air and turn it into a form that plants can use. And others — and this is the exciting part — help fight off plant diseases that might otherwise damage or kill plants. How this works is still not well understood. But certain microbes are antagonistic to others, and especially if they get there first, can hold off attacks by pathogenic (bad) microbes.
The Greatest Thing Since Sliced Bread?
The use of compost has been shown to help control some plant diseases. Over the last few years there has been growing interest in using compost teas, especially among organic gardeners and growers who have few ways of controlling plant diseases. There is some anecdotal evidence that these teas work, but unfortunately, very few scientific studies to prove it.
In many ways, compost tea is like a "black box." No one really knows what is going on in any particular batch because there are so many unidentified inputs. These include how the compost was made, the materials it was made from, whether it was "aerated" or not (more on this below), and most importantly, what microbes happen to be in your compost tea. Without knowing these things, there is no way to know for sure that one batch of compost tea will work the same as another.
Still, there is a building body of ancecdotal evidence that compost teas can be used with success as root drenches for both soil borne disease organisms like Pythium, and for foliar disease-causing organisms like Botrytis. Farmers claim success using them to bring back to life soils that have suffered under years of chemical treatments. Golf courses that use them claim they have good turf quality and fewer fungus problems. Vineyards say they see less powdery mildew. Organic growers use them to promote "healthy soil." Many gardeners claim that plants grown with them are more vigorous and taste better.
Aerobic vs. Non-Aerobic Compost Teas vs. Compost Extracts
The stinky manure tea from the old days was definitely anaerobic. Place any organic material into water and it will encourage anaerobic microbes because there just is not enough oxygen in it to support aerobic microbes.
In recent years, researchers and manufacturers have come up with devices to inject oxygen into the steeping tea, not unlike bubblers in a fish tank. These encourage aerobic microbes to flourish even in water. These aerobic microbes are the kind found in healthy soils with good structure. It is still not known whether aerobic or non-aerobic compost tea has more useful properties, but everyone is hoping that it is the aerobic (aerated) teas. The reason? The anaerobic teas just smell bad. So, when we talk about compost teas here, it is strictly about aerobic teas.
In addition, there is a new type of tea that is generating interest called compost extract. This is an unbrewed product where water is sprayed through compost to release the beneficial microbes. It is a much faster process that may have a longer shelf life than aerated compost teas. But because it is so new, there is even less of a track record for it. This may be an important new tool. Time will tell.
Where to Buy Compost Tea
The easiest way to try out compost tea is to buy it. Some nurseries and garden centers carry fresh brewed tea for around $3 per gallon. Compost tea has a short shelf life, so if you do purchase it, smell it first to make sure it hasn't gone sour. It should have a mild, pleasant earthy smell. Be sure to use it soon after you get it. Treat it with the same respect you would potato salad at a picnic.
You can also brew your own compost tea. Instructions below tell how to make good compost and a home-made brewing device.
Where to Apply It
Depending upon your needs, compost can be applied either as a root drench in the ground or in potted plants, or as a foliar spray. As an undiluted root drench, it may help protect against damping-off and other root diseases. As a foliar spray diluted at least in half with water, it provides a quick dose of nutrients and protection against foliar diseases.
Dr. Elaine Ingham, a pioneer in the compost tea field, uses compost tea in her own garden once a year in the spring. For people who don't have healthy, established organic gardens, she recommends using it every two weeks or once a month, at first. To prevent fruit tree diseases, it needs to be sprayed every 10 to 14 days starting just before bud break through petal drop.
Is It Worth the Effort?
As a method of foliar feeding and to encourage overall plant health, it seems hard to believe that compost tea would not be beneficial, in much the same way that taking vitamins is generally thought of as being good for people. Still, it is important to remember it is not a magic bullet. For building soil, fertility does not replace the use of cover crops, compost itself, and the use of rock minerals and bagged organic fertilizers.
It is a little more confusing when it comes to disease control. Results have been all over the map, and scientific efforts so few, that you are really in uncharted territory here. Over time, scientists will figure out what makes certain compost and compost teas disease suppressive, and will be able to provide reliable recipes so home gardeners can get reliable results. But for now, we are in the early stages and results will be hit or miss. For organic gardeners and commercial growers for whom most fungicides are not an option, compost teas are one of the more promising alternatives.
BREW YOUR OWN
Good Compost: The Essential Ingredient
To make compost tea, you need to start with high quality compost. If you have access to a good source of compost or worm compost, you are way ahead of the game. A mixture of "hot" garden compost and worm compost also works well.
Otherwise you can make your own compost. It is not difficult to make, but no bones about it, it takes work. The compost you need for compost tea must be from "actively managed" compost piles, not the piles that just sit in a corner of your yard for years and years.
The following compost instructions come from Dr. Elaine Ingham. Compost should be made with 25% (by volume) high nitrogen materials like manure, legume family plants, and the first grass clippings of spring. Another 45% will be other green materials such as grass clippings, coffee grounds, kitchen scraps. Weeds that haven't gone to seed are also fine. The remaining 30% should be woody and can include wood chips, sawdust, newspaper, or other paper products. The pile should be assembled at the same time and should be at least 1 cubic yard in size.
The pile will heat up "automatically" and needs to hold at 135 to 160 degrees F. for three days. This will kill most plant pathogens and encourage an optimal mix of beneficial organisms. Keep track of the temperature with a turkey thermometer or special compost thermometer.
There is no need to apply heat. The pile will cook itself as exo-thermal bacteria release heat as they grow. But you do need to turn the pile every day or two during the first week, and maybe a time or two in the next few weeks. You can let your gym membership lapse and save money! Turning the pile helps encourage bacterial growth and makes sure that the whole pile gets a good working over by the soil microbes. When your pile cools off and is barely warm to the touch, it is ready to use.
Some communities offer composting classes often associated with the Master Gardener programs at local Cooperative Extension offices. These are great resources for new composters.
Brew Your Own
Dr. Ingham also tells how to make your own brewing device and how to use it. You'll need a five gallon bucket or two, and some aquarium supplies, including three bubblers (air stones), a gang valve, a few feet of tubing, and a pump that can handle the three bubblers. (To protect your pump, also add a check valve to prevent backflow.)
The aerating components are easily connected with tubing. On three sections of tubing, attach one cut end to a bubbler and the other end to the gang valve. A fourth length of tubing connects the gang valve to the air pump, with a check valve added in between. (See photos below.)
Fill the bucket halfway loosely with compost. Bury the bubblers in the bottom of the bucket. Then add water almost to the rim. Plug the pump in!
At this point, Dr. Ingham's recipe calls for adding an ounce of molasses. Because new research has shown that the addition of molasses to compost tea encourages the growth of E. coli and salmonella, some researchers are recommending that molasses and other simple sugars be omitted from compost tea recipes altogether.
Now your tea is bubbling away. Stir it a few times each day. Don't forget to rearrange your bubblers after each stirring.
After three days, your tea is brewed. Strain the tea through an old stocking or pillowcase into another bucket. You can add some kelp, humic acid, or rock dust at this time. (These additions do not cause problem bacteria). Try to use your tea within an hour if possible as it can go anaerobic very quickly.
- Duffy,B. et al. 2004. Effect of molasses on regrowth of E. coli O157:H7 and salmonella in compost teas. Compost Science & Utilization. 12(1):93-96. [abstract : http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=168430]
- Golf courses implement compost tea program. "From The Ground Up" Newsletter/City of San Jose, California. Summer 2003.http://www.growingsolutions.com/home/gs1/page/116/13
- Camozzi, Rosemary. Sept. 7, 2003. Tea Time: A Eugene company's compost tea improves growing conditions. Eugene, OR: The Register-Guard.http://www.growingsolutions.com/home/gs1/page/68/13
- Compost tea purchased at Down to Earth Nursery, Eugene, Oregon. 5/17/07.
- Diver, S. Personal communication May 18, 2007.
- Ingham E. R. 2000. Brewing compost tea. Kitchen Gardener. Issue 29:16-19.http://www.taunton.com/finegardening/pages/g00030.asp
- Grubinger, V. 2005. Compost tea to suppress plant disease. (University of Vermont Extension)http://www.uvm.edu/vtvegandberry/factsheets/composttea.html
- Rodrigues, Kevin. 2019. https://gardeningmentor.com/compost-tea/
- Scheuerell, SJ & WF Mahaffee. 2004. Compost tea as a container medium drench for suppressing seedling damping-off caused by Pythium ultimum.Phytopathology 94(1):1156-1163.
- Welke, SE . 2004. The effect of compost extract on the yield of strawberries and the severity of Botrytis cinerea. Journal of Sustainable Agriculture. 25(1):57-68.
Stein, Dan, "Compost Tea." Eugene, OR: Northwest Center for Alternatives to Pesticides.