Extracts of finished compost, sometimes called “Compost Tea”, have been demonstrated to possess potential plant-disease suppressive qualities. Eurpean research in the early 1980′s showed that unadulturated watery extracts were an effective means to obtain microbial populations from compost to be transferred by spraying or drenching soil and plant surfaces. The introduction ofthese organisms could in some cases reduce pathogenic fungal expression, thereby potentially reducing pesticide usage. The most successful applications have been in the control of Botrytis type fungi affecting wine grapes, strawberries and squash. In addition, fusarium and phythium “damping-off” disease of seedlings can be significantly controlled by compost extracts (although solid composts in the media are better).
Woods End has been active in the field since the late 1980′s and at one point worked closely with Friedrich-Wilhelm University Bonn (Dr. Tränkner), and AUC-Agrar Consult, in the study of compost quality for effective compost teas.
Early enthusiasm leading to confusion and misinformation has not brought recogition of success to the field of compost teas. The promulgation of compost tea methods where sugars and other “adjuvants” were added to “boost” or “brew” microbe populations, resulted in a significant increase of E. coli hazrads for teas, worsening of quality and increase in odors. This led to concern and ultimately restrictions especially within the organic-farming certified community on how and when compost teas may be employed safely. Check with your local ag representative before proceeding with a compost tea program for commercial crops.
Testing Compost Teas
The primary objective in analyzing compost extracts for microbiological parameters is to demonstrate:
- Absence of pathogens (E. coli) in the tea,
- Lack of toxicity to microbe and plants, and
- Presence of microbes known to confer disease resistance to plants.
Manufacturers should be aware of the USDA Compost Tea Task Force report on compost tea hygiene (listed below). The Woods End on-line news page contains stories related to quality and effectiveness of compost teas. In addition, if teas are being used to provide nutrients, then soluble mineral analyses may be useful (see section on compost testing).
Potential Quality Indicators:
- ACTIVE AEROBIC BACTERIA – Aerobic or Heterotrophic Plate Count test measures the total number of oxygen-utilizing bacteria. Greater than 100 million per gram/ml are desirable in properly mature compost tea.
- TOXICITY TEST- Using a highly sensitive toxicity-indicator plant or single-celled plantlet, the occurrence of toxins such as herbicides is ranked.
- VIABLE BACTERIAL SPORES -This facet of the bacterial population indicates the presence of several Bacillus species having the ability to suppress certain fungi responsible for plant disease. One gram of compost should contain about a million of these bacteria.
- ACTIVE FUNGI- The presence of actual growing fungi in amounts between 1,000 – 10,000/g or ml of compost or tea contribute disease resistance to certain crops and indicate overall tea health.
- FLOURESCENT PSEUDOMONADS -These bacteria impart fungi-disease resistance to plants. Approximately 1,000 to 1-million per g of compost or ml of tea are desirable.