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
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
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
The vegetable crop trials were based on a half-acre, randomized complete
block design plot located at The Rodale Institute Experimental Farm,
and also included three treatments: compost tea applied once at planting
as a soil drench and then weekly as a foliar spray; a non-compost
tea application containing the nutrient ingredients but not the compost
found in the tea; and a no-spray control.
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
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.