Compost Tea

Compost tea.

What is it? How does it work? And are all the beneficial claims about compost tea true?

Let’s find out.

What is compost tea?

Compost tea is a brewed extract of bacteria, fungi, micro arthropods, nematodes, and protozoa made from compost. Traditionally, compost was placed into a permeable bag, which was then placed in a container filled with water until the water turned black. That water was then used to irrigate both indoor and outdoor plants as a liquid organic fertilizer. The idea behind it was that all the nutrients and microbes would end up in the water, easier for plants to absorb. Makes sense, right?

Compost (Ken Kerckx) Pixabay

That was the old-fashioned method of making compost tea and it resulted in a soup of anaerobic (without oxygen) microbes and nutrients. Modern marketing and manufacturing now offer compost tea brewing kits, or ready-made compost tea. These kits use energy to aerate the tea. The differences between aerated and nonaerated compost tea will be discussed in just a moment.

Compost tea claims

Compost tea is said to be a mild food additive for plants, much like our daily vitamin pills. Proponents of compost tea claim it makes plants and the surrounding soil healthier, reducing the need for fertilizer. Compost tea is also said to improve soil water retention, reducing the need for irrigation. The addition of all those microorganisms is supposed to improve soil structure, reduce compaction of heavy clay soil, and help sandy soil retain water and nutrients. But wait! There’s more!

Using compost tea is also said to stimulate root growth and counteract the negative effects of chemical-based herbicides, insecticides, and pesticides. And foliar (leaf) sprays of compost tea are said to stimulate the growth of beneficial microbes on leaf surfaces, reducing the chance of disease and pest damage.

Science and skepticism

Like most claims that sound too good to be true, compost tea does not hold up well to scientific testing.

First, the availability of nutrients is misleading. Unlike slow-release fertilizers and composts, compost tea risks dumping large (variable) amounts of nitrogen, potassium, and other nutrients into the soil all at once. Plants can only absorb so much at one time, so many of those nutrients (assuming there are any) end up in ground water causing eutrophication, or nutrient contamination.

Second, claims of disease prevention are greatly exaggerated. Just because something works in a lab does not mean it will continue to work in your garden. There are simply too many other factors at play.

It’s true. Some studies have shown that foliar sprays of nonaerated compost tea do, in fact, reduce the incidence of some diseases in laboratory settings. Disease severity is not altered, and results are less clear out in the real world garden. Also, some fungal diseases were reduced with foliar sprays of aerated compost tea. Those diseases were growing on Petri dishes. In greenhouse testing, the diseases were not affected. Other studies have shown that foliar sprays of aerated compost tea actually made diseases of apples and potatoes worse.

Like probiotics, the microbial and nutritional makeup of compost tea is extremely variable. It is almost impossible to say what each batch actually contains. In fact, some compost teas can contain human pathogens. [Can you say e. coli or Salmonella?]

The bottom line

According to the U.S. Environmental Protection Agency, it is against the law to sell unregistered substances for use as pesticides. Compost tea is not registered in this way, so any claims made about its effectiveness as a pesticide are false. Aerated compost teas have no documented ability to suppress disease.

Rather than wasting time and effort on something that lacks a scientific basis, use that compost the way nature intended. Apply a nice thick layer of aged compost as a mulch on your planting beds and let the worms, microorganisms, and other natural processes take advantage of billions of years of evolution.

The benefits of mulching with aged compost are well-documented. And if your plants need a little nutritional boost, I suggest fish meal.