New Farm Research Report
Compost tea research enters its second year
Study aims to shed light on current debates over the safety and efficacy of compost tea as an organic material

By Laura Sayre

Is compost tea the new compost?

The use of compost teas--liquid fertilizers made by soaking compost in water--is probably almost as old as the use of compost itself. In recent years, however, a new generation of farmers has begun reporting dramatic results from using compost teas to boost plant health and help manage plant pathogens. Scientists have been seeking to identify the constituent microorganisms and nutrients present in compost teas and to explain how and why they impact agroecosystems. Entrepreneurs have been busily inventing and marketing new contraptions for brewing the stuff. And last but not least, the National Organic Program (NOP) is struggling to develop official language to regulate its use in organic production. (Recommendations from the National Organic Standards Board were published last week--read more in this week’s featured news story.)

Sound familiar? Compost went through--and indeed, is still going through--a similar process, from traditional input to subject of scientific investigation. A growing number of peer-reviewed research articles have reported on compost's disease-suppressive qualities, but its use continues to be attacked in the mainstream media as a potential source of bacterial contamination in food. And while the NOP regulatory text now defines compost and specifies the conditions of its use, opinions differ about the proper application and underlying logic of those rules.

The Rodale Institute™ has been a leading advocate of composting for decades, testing different farm- and garden-scale composting systems, evaluating the practicality and nutrient profile of a variety of types of composts, and sharing the results through field days and publications. So it's not surprising that researchers here have also gotten interested in compost tea.

In the spring of 2003, The Rodale Institute (TRI)and Pennsylvania State University launched a two-year study funded by a grant from the Northeast Sustainable Agriculture Research and Education program (NE SARE). Lead TRI investigators Matt Ryan and Dave Wilson explain that the goals of the project are twofold: to gather hard data on the compost tea's effectiveness for stimulating plant growth and suppressing disease, and to educate farmers and extension agents about its potential benefits--and hazards--as an organic material.

"There's a lot of excitement right now about compost tea--more and more growers are using it, and there's a lot of anecdotal evidence about its ability to suppress plant diseases," says Ryan. "But there's a real lack of independent scientific evidence. Our hope is to start filling that void."

It's all in how you brew it

Compost tea can be 'brewed' according to two basic methods: passive and active, or aerated. Making passive compost tea is a low-tech proposition requiring nothing more than a barrel, a mesh bag, compost and water. You put the compost in the bag, submerge the bag in the barrel of water (TRI researchers suggest a ratio of 1 part compost to 5 parts water), wait a week or two, and you have your tea.

Aerated compost tea is a little more complicated, and involves supplying oxygen and nutrients to the microbial community in the compost solution. A number of compost tea brewers are now commercially available, or you can build your own. (This project is using the Earth Tea Brewer 22 [ETB-22], manufactured by EPM, Inc., of Cottage Grove, Oregon, but Ryan and Wilson emphasize that good compost tea can be obtained from any of a number of commercial models, or from home-made brewers.)

To better characterize the compost tea produced and used in the trials, the researchers are keeping tabs on the quality of the vermicompost (compost made with the benefit of earthworms) going into the tea, the measurable microbial biomass in the finished tea, and the microbial coverage on leaf samples after tea application. Compost tea samples are also being tested for E. coli, the bacteria at the center of concerns that compost tea use might spread human pathogens. One of the researchers' primary objectives is to develop reliable guidelines for on-farm production of safe, quality compost tea.

Ryan and Wilson began with fairly standard recipe for aerated compost tea based on vermicompost, with dry molasses, humic acid, soluble kelp, and fish hydrolysate as added nutrients and a small amount of peanut oil to reduce foaming. Preliminary tests showed that 'weed' microorganisms, including E. coli, were propagating under these conditions, however; so a revised recipe excluding molasses--the simple carbohydrate source that feeds rapid microbial reproduction--was developed. "We've decided to try to extract more [nutrients from the compost] and propagate less," explains Ryan. "These are opportunistic bacteria, and by encouraging them you may be lowering the overall diversity of the mixture."

Compost tea shows potential for disease management in grapes

Undertaken in collaboration with three area farmers and Pennsylvania State University plant pathologist Dr. James Travis, the field segment of TRI's compost tea study is focused on three crops: wine grapes, potatoes, and pumpkins. These crops were chosen for their profitability, susceptibility to fungal diseases, and consequent high use of fungicides under conventional management. As many of our readers will remember only too well, 2003 was a wet year in the Northeast, offering good growing conditions for plant pathogens and therefore good test conditions for the use of compost teas.

The Rodale Institute farm has no vineyards, so the wine grape trials were conducted at three commercial vineyards in the Northeast: Shinn Vineyards in Mattituck, New York; Roth Vineyard in Fairfield, Pennsylvania; and Wright Wine Works in Barto, Pennsylvania. Barbara Shinn of Shinn Vineyards had already been using compost tea as a part of her management system; Phil Roth has been using compost in his vineyards but had not yet experimented with compost tea.

The vineyard experiments include three treatments: a weekly, foliar application of compost tea beginning in mid-May, a pesticide control, and a no-spray control. In 2003, results here were the most dramatic out of the three crops, with compost tea suppressing powdery mildew (Uncinula necator) by approximately 50 percent on Chardonnay grapes. The tea also appeared to help control the spread of gray mold (Botrytis cineria), but this result was not statistically significant. Trials showed no detectable effect, finally, on black rot (Guignardia bidwellii) or Phomopsis (Phomopsis viticola), and use of compost tea actually seemed to encourage infection by downy mildew (Plasmopara viticola). (Vineyard managers resorted to fungicides to control the latter diseases in late June and early July.)

"These may be the kinds of results that are going to be more exciting for plant pathologists than for farmers, at least for now," comments Rodale researcher Matt Ryan. "Fifty percent control of powdery mildew is not an adequate level of control for most grape growers. But on the other hand, it's a big response. There's definitely something going on out there."

Pumpkins and potatoes react very differently

Mixed reviews: Potato plots at The Rodale Institute Experimental Farm, 2003. Disease levels were low in all three treatments; plants receiving regular compost tea applications produced 18 percent more marketable potatoes than the control. Above: Compost tea applications were not effective in suppressing disease in pumpkins in 2003 trials at The Rodale Institute.

Results in the vegetables contrasted starkly with those found in the grapes. Powdery mildew in pumpkins is caused by a different fungus (in fact, by two fungal species, Erysiphe cichoracearum and Sphaerotheca fuliginea) than powdery mildew in grapes, and the compost tea applications showed no effectiveness here, with high levels of infection across all treatments. In the potato plots, on the other hand, disease levels were so low overall that no significant differences could be found between the three treatments.

The spuds did show a yield response to compost tea applications, however. Plants receiving regular doses of compost tea produced larger, better potatoes than both the nutrient-ingredient-only and the untreated control plants. Marketable yields in the compost tea plots were between 18 and 19 percent higher than in the untreated plots and about 15 percent higher than in the nutrient-only plots. Compost tea-treated plants also produced tubers that tested higher for a range of nutrients, including iron, boron, potassium, and manganese. Iron showed the biggest response, with levels an astonishing 1700 percent higher in plants receiving compost tea than in untreated plants.

Overall, the data underscore how much remains to be learned about the on-farm use of compost tea, whether in organic or conventional systems. The widely divergent results in the three crops studied here suggest that it is difficult, if not impossible, to generalize about the efficacy of compost tea for disease suppression across all crop species--different crops have different leaf architecture, which means they will receive sprays differently, not to mention the differences in physiology and phylogeny.

Field conditions, moreover, are always more challenging to assess than controlled laboratory environments. "Researchers have been studying compost tea in the lab," says Ryan, "and coming closer to a theoretical understanding of how these microorganisms interact in the phyllosphere and the rhizosphere" (that is, on and with the leaf surfaces and the root surfaces). "But to demonstrate those actions in the field, that's another matter entirely." Hopefully, Ryan adds, the National Organic Program's eventual ruling on organic farms' use of compost tea will permit farmers and researchers to continue to work together in pursuit of a more complete answer.