BioCycle June 2005, Vol. 46, No. 6, p. 25

Napa Valley wine producer incorporates compost tea into its growing program – fine-tuning application methods and recognizing its potential to agriculture, large and small – conventional and traditional.

Kirk Grace

I CAME to Robert Sinskey Vineyards (RSV) with experience in both small and large-scale wine-grape farming, all of it in California’s Napa Valley. At the time RSV had already begun significant steps toward sustainable and organic farming, and it became my job to complete the transition to organic certification, which we accomplished in 2001. The opportunity to use organic practices on a larger scale was extremely attractive to me, and working with as progressive a company as RSV seemed to be just the place to pursue that interest. In recent years, much attention has focused on the obvious benefits of compost tea, and it soon became one of our standard vineyard tools. Nonetheless, there is a large gap between the practice and the science of compost tea.

If one thinks of the decomposition process of raw organic matter falling onto the ground and then decomposing through moisture, oxidation and microbial breakdown, and then being leached into the ground through the action of precipitation, that’s what compost tea is. Simply put, it is a liquid extract of compost made by immersing compost into a vat of water and allowing it to steep for a period of time.

There are two primary reasons for using compost tea: first, it is a way to apply the beneficial characteristics that is in compost such as its fertility and microbial populations and biological by-products and second, it can be an aid in disease management.

The process of making compost tea concentrates the beneficial characteristics of solid compost into a liquid medium, which can be applied to larger areas than is otherwise easily or economically feasible. While compost tea does not replace the use of solid compost, it serves as a bridge between normal applications of solid compost. From a recycling perspective, I don’t think compost tea represents a significant methodology of disposal for large volumes of green waste. Instead it represents a niche market in which the highest quality composts will be in the most demand for compost tea makers. It’s not just the N, P, K numbers that brewers are interested in. I want compost that is microbially alive and has a lot of humic texture.

When I spread solid compost in the vineyard, I usually spread it at a rate of one to four tons per acre. When I apply compost tea, I apply it at ten gallons per acre, which has been brewed with the equivalent of two pounds of compost. Compost tea, in itself is not high in chemical nutrition, especially when compared to most modern fertilizer materials, but most modern fertilizers use salt-based chemistry to provide nutrition, are extremely concentrated and therefore potentially toxic to the soil micro and macroorganisms. Instead, compost tea provides plants with a broad range of nutrients often lacking in modern fertilizers; it is also alive with the microbes from the compost, their metabolic by-products, and other organic compounds that are so difficult to quantify, but we know their qualities are essential to healthy soil and plant environments.

The second benefit of compost tea is its potential in the prevention and suppression of plant diseases. Compost tea has been widely touted for its disease suppressing qualities. But this is an area in which we need to wade carefully. Federal and California state pesticide regulations are very strict in what one can and cannot do with pest management recommendations and applications. I want to stress that compost tea cannot be legally recommended as a method of pest control. However, compost tea may provide optimal fertility, microbial populations and plant elicitors to encourage resistance to plant pathogens. The microbial populations of compost tea and the resulting biological products can be very valuable in disease prevention and suppression by creating a competitive and or hostile environment to plant pathogens. But, to solely rely on compost tea for pest management is foolhardy. Some people even feel that compost tea can serve as a food source for plant pathogens and as a vector of human pathogens. But there is no reason that compost tea should not be included with other disease prevention and suppression techniques.

There are two main types of compost tea and the methods to manufacture them. The most basic tea is commonly called anaerobic, although it should never be truly anaerobic. A more accurate term for this method would be passive brewing. It is a centuries-old technique. Passive tea consists of a vessel filled with water and inoculated with compost, sometimes suspended in a sack, and allowed to soak for a few days up to three weeks with occasional mixing to encourage extraction. The primary benefit of the extract is its supply of soluble nutrients, which can be used as a liquid fertilizer. Because passive brewing does not include the addition of food for microbes, the only microbes present will be those that were a part of the original compost and any incidental bacteria that establish themselves during the brewing. Anaerobic tea’s advantages are that it is cheap and low-tech, and it fits well into smaller agricultural and landscape operations. Disadvantages to anaerobic teas include bad smells, difficulty in using the technology in larger operations, and longer brew times.

The second type of tea is called aerobic tea. It is actively brewed in that the water is under constant circulation and air is injected. It typically consists of one part compost, suspended in 10-50 parts water, and the addition of food sources to encourage high microbial populations. Examples of feedstocks include kelp, fish products, humic acid, and rock dusts. A typical brew time is 24 to 36 hours, and the tea is usually then diluted to be used for foliar feeding or irrigation. The advantages of aerobic tea brewing include quicker brew times; it can be easily diluted because of higher microbial populations; it is more appropriate for larger operations, and offers a more consistent end product. The disadvantages of aerobic tea are that the equipment is more expensive, and there are more parts to break.

Regardless of methodology, compost tea quality is only as good as the materials used to make the tea. In both cases, the quality of the water, the compost, and feedstock all contribute to the success, or lack thereof, of the brewing process.

I primarily use tea that is aerobically brewed, although I do use both types of teas mentioned above. I spray compost tea on the vine canopy, and I inject it through the irrigation system. I apply aerobic compost tea at a rate of ten gallons per acre. The benefits I believe compost tea provide include: Helping plants tolerate environmental stress, especially heat; Provides nutrition and healthful elicitors to the soil and plant, and Helps to more effectively deliver other fertilizer materials such as K and P to the vine canopy when sprayed.

Depending on how one uses the tea, it is essential to filter it to avoid clogging equipment. As a drench or dip, a simple basket strainer, or sieve for small operations, is sufficient; When I use it for spraying on the foliage, I filter the tea through a 60-mesh strainer. Often I use a nylon stocking for this. The stocking is cheap and disposable if it becomes damaged. Something we are still learning is how spray variables such as time of day, spray pressure and droplet size, and velocity affect the survival rate of the microbes. It is also commonly felt that spraying in the hottest and brightest period of the day leads to microbial mortality and degradation of their metabolic products through the effects of heat, evaporation and exposure to ultra-violet light.

I primarily use compost tea for irrigation. I believe that this is where compost tea is most applicable, just as we see the benefits of compost on the ground. All of my irrigation is drip irrigation. In order to avoid clogging drippers, I filter the tea down to 100-mesh (many would recommend a finer filtration). What this does to the microbial population and diversity I cannot say. I can only imagine that this filtration makes the tea less effective than if it was not filtered to such a fine degree. I imagine that the filtration takes out some of the microbial diversity.

I have never used compost tea through sprinklers, although I can confidently say that it is not necessary to filter it to such a fine size. Because of the greater application rates of water through sprinklers, the tea would be delivered in a more dilute form, which may affect efficacy.

The use of tillage, fertilizers, and pesticides has brought about a revolution in food production. However, these practices when not done properly can also lead to loss of soil structure through over-tillage, salinization and toxic accumulation from fertilizers, toxic accumulation of pesticides, and pest resistance. There is no doubt in my mind that compost tea can be one valuable tool in helping to mitigate all of these negative effects. It is a mistake to blindly look upon compost tea as a silver bullet. After all, it is this same mentality that brought about many of the negative results of our modern agricultural practices. What we need to do now is to focus attention on compost tea research and practices, which will help perfect both the science and the art of agriculture.

First, we must learn how to produce a consistent product and ensure its safety. There is a legitimate concern that tea brewing may include human pathogens. Just as solid compost is regulated to avoid pathogenicity, compost tea should be made from compost that meets these criteria. The effect of various microbial feedstocks on microbial populations needs to be explored as well.

Second, it is essential we learn more about the disease fighting potential of compost tea and find which crops are most protected and which pests are easiest to control through the use of tea. This is an area where we know just enough to realize the potential of tea, but cannot fully define that potential yet.

Finally, if we are to realize the full benefits of this technology, I think it is essential that we learn how to brew for specific microbes and/or for specific biological metabolites. This is something that many large businesses are participating in right now, but if this technology could be transferred to the general population, the benefits would be extraordinary and lead to a general democratization of industry and agriculture worldwide.

For example, one of the major limitations to sustainable agriculture is the application of nitrogen for increased crop production. This is especially true in Third World countries where economies of scale and knowledge limit safe and efficient modern fertilizer and pesticide utilization. Wouldn’t it be amazing if we learned to brew for free-living, nitrogen-fixing microbes? In essence, with this one advance we could ensure the sustainability of food production indefinitely, and reduce our reliance on limited resources.

Another area to be benefited by being able to brew for specific microbes is in the field of bioremediation. We already know about many microbes, which break down toxins. If we could take this knowledge and consistently reproduce the microbes, we could reclaim areas once thought to be permanently impaired.

In conclusion, there is no doubt in my mind that compost tea has already proven to be beneficial to agriculture large and small, conventional and traditional. I will continue using it in my own agricultural endeavors, but without dedicated study, it will only exist as a generalist technology. I know that the potential of compost tea is great and that with more research and knowledge on this important subject, all of humanity could benefit.

Kirk Grace is vineyard manager at Robert Sinskey Vineyards in Napa, California. He can be e-mailed at

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