Compost tea is kind of like the tea we drink. It’s a liquid extract of compost produced by steeping finished compost in water in order to extract beneficial microorganisms and compounds. It is made in a variety of ways, including with or without aeration, and with or without adding supplemental nutrient sources. The theory is that compost tea can add precisely what a soil is lacking and it is supposed to be a speedy, cost-effective way of improving soil health and plant growth.
But does it work? And is it better than simply spreading compost, which can be difficult to do at a large scale, or using cover crops, that come with their own costs and benefits? Those are the questions that farmer and retired chemist Jim Tarnowski set out to answer with his North Central Sustainable Agriculture Research and Education (SARE) project.
It’s a complex topic with lots of variables. So we’ll start with getting to know more about compost tea and it’s use in this project as described by Jim in his final report to SARE. These are Jim’s words. We’ve just excerpted the report so that we can present it as a series of articles.
There is scientific evidence for a robust soil food web producing self-sustaining nutrient cycles and high biomass in native prairie and old growth forests. There is also strong evidence that this soil food web profile of organisms can be successfully extracted from compost into water and allowed to reproduce in an aerobic environment, thus greatly reducing the amount of compost necessary for soil application of the same number of organisms. Therefore, we chose to focus our efforts on investigating the efficacy and impact of using aerated compost tea on our own fields. Furthermore, we could not identify any scientific reports on agricultural fields after using compost tea, and no evidence of compost tea being applied on a farm scale in the Midwest. We decided our idea would be a good candidate for a SARE producer project.
Our main objective was to quickly improve and maintain soil health using sustainable, cost effective methods. Good quality compost is widely known to improve soil quality and health, as is cover cropping with deep rooted plants. However, the use of compost on a farm scale is a large investment of funds and locating a large enough supply to cover a field is an additional barrier to this approach. In recent years comments from some potato, grape, and strawberry producers in Western States, along with a growing number of gardeners, golf managers, horticulturalists and others extol the virtues of aerobic compost tea for improving soil and plant health.
Making and Using the Tea
Aerobic compost tea, made at the level of compost tea standards, contains many of the soil micro-biota necessary for a functioning soil system. If the use of compost tea can be shown to be effective at quickly restoring soil health, and the transition to adapting to and using compost tea is cost effective, then this may lead to a wider adoption among farmers. Restoration of soil health may then result in decreased reliance on pesticides and fertilizers, thus leading to a more sustainable farming system.
Compost tea standards and expert recommendations were closely followed for brewing of tea and for application rates to fields. We hired an expert compost tea consultant to help train us in making and application of compost tea. Baseline tests determined the soil in all plots to be lacking in beneficial fungi. Recipes for producing fungal dominated compost tea were made and thoroughly tested for quality and biological parameters to ensure we were applying high numbers of beneficial fungi and other soil organisms. Our test results show that we applied good to excellent levels of beneficial bacteria and fungi to the soil. Extracting larger soil organisms such as protozoa and beneficial nematodes has proven very difficult and inconsistent. With careful observation and changes in the recipe for the compost tea we were able to achieve fungal dominated teas with excellent numbers of organisms.
Using typical tank sprayers free of residues, the aerobic compost tea was immediately applied following the 24 hour brewing process at a 20 gallon per acre rate as a soil drench in fall and spring. The spraying protocol was to use low pressure and flood spray tips with screens removed to ensure survival of the living organisms in the tea solution. This rate of application was recommended by soil food web experts for quick restoration of the soil ecology. Following the baseline soil samples, tea was applied the first fall and then the following spring, and then repeated again for a total of four applications per treated plot during the study duration.
Test Set Up
There were no significant effects on soil health indicators or soil food web organisms from the application of aerobic compost tea. However, there were significant changes for several parameters of soil health and organisms when comparing all plots between the baseline samples of 2003 and the ending results from 2005. This result is even more surprising given that soil moisture levels were considerably lower and soil temperatures higher during the 2005 sampling. Baseline data from 2003 showed that all field plots were lacking in beneficial levels of fungi. Both beneficial and fungal properties of the soil greatly improved during the two year study. In addition, aggregate stability, infiltration rates, and cation exchange capacity of the soil all significantly increased when comparing baseline results from 2003 with final results in 2005. We believe this positive result is due to the increased attention we each gave to soil health by reducing or eliminating tillage and salt-based fertilizers and pesticides, while increasing cover cropping and the time that the soil had living plants growing.