Even sophisticated compost tea is no panacea for garden plants

By Adrian Higgins
Thursday, April 15, 2010

In the temple of organic gardening, you know you have become a true believer when your reaction to seeing a pile of well-made, finished compost is to roll up your sleeve. No earthly force can stop you from plunging your hand in to feel and smell this black gold.

We (the faithful) know that the least observed part of the garden, the soil, is the most important. Good compost, itself seeded with beneficial microbes, feeds the vast sea of microscopic life just below the soil surface. A plant whose roots are tickled by bacteria, fungal threads, nematodes and protozoa responds with vigor and health without the need for chemical fertilizer or pesticide. Is it so daft, then, to derive as much pleasure from holding crumbly, moist and earthy compost as in ogling a flower?

I plead guilty to that, which may be why at a trade show in Baltimore a few weeks ago I started to drool over a $1,400 contraption designed to brew compost tea. This was a ridiculous impulse on several levels, not least because this 30-gallon brewer was designed for commercial organic landscapers.

Historically, compost tea was little more than water steeped in compost or rotted manure and used to impart a quick hit and a drink to waiting plants. But for more than a decade, it has been a far more sophisticated product, one in which compost and organic nutrients are placed in a permeable sock in a tank of dechlorinated water and aerated for a day or more. Technically called actively aerated compost tea, the finished brew is typically diluted and applied as a foliar spray or soil drench.

The idea is to use the desired bacteria and fungi in compost to propagate many more, and to do so in an easy-to-deliver liquid form. A teaspoon of compost contains 1 billion bacteria, a teaspoon of tea 4 billion, according to Jeff Lowenfels and Wayne Lewis in their book "Teaming With Microbes" (Timber Press, 2010).

I didn't get the compost brewer, not just because of the cost or the fact I would soon run out of things to squirt in my smallish garden, but because when gardeners put aside their enthusiasm for organics, there is a lot of credible doubt about the value of this stuff.

Studies have shown that aerated tea can reduce powdery mildew on some plants and helps prevent damping off, a disease that causes seedlings to collapse and die. It has also reduced mealy bugs on greenhouse plants. But it doesn't reliably control most other fungal diseases, including black spot or wilt, or such common pests as spider mites and leafhoppers.

As for its increasing the microbial life of soils, Patricia Millner, a research microbiologist at the Agricultural Research Service, says that the existing populations are plentiful and well adapted to their environment. Adding compost tea to poor soils won't help them, either, because once the liquid dries, she says, "what do you have left for the microbes to live on?"

Casey Sclar, who runs the impressive, large-scale composting operation at Longwood Gardens in Kennett Square, Pa., is generally supportive of compost tea. "We have seen increased plant quality from compost tea applications," he says.

He also points out that you don't need to spend $1,400 on a commercial brewer; you can get good results with a five-gallon bucket, a couple of aquarium pumps, a length of perforated tubing and a nylon sock. (For full instructions, search "brewing compost tea" at http://www.finegardening.com.) What is vital for effective tea, he says, is good aeration and well-made compost replete with gung-ho microbes.

Good compost, itself the subject of entire books, is made from a careful blend of green and brown vegetative ingredients that are cooked by microbes on a high-oxygen diet. "If you're a lazy composter, you're probably going to make bad compost tea," Sclar says. There are other provisos. The shelf life of freshly brewed tea is short, a reason to be wary of buying the bottled brew at retailers. And while the tea may improve a healthy plant's immune system, it won't cure one under attack by pests or diseases. The most serious criticism of compost tea is that it can be a growing medium for E. coli.

Millner put this to the test by inoculating compost tea with the bacteria, "and it increased them," she says.

Brad Roeller, manager of outdoor gardens at the New York Botanical Garden, tested compost tea when he worked at the Cary Institute of Ecosystem Studies in Upstate New York. He found that the applications were not effective at controlling most diseases but made for more-robust plants. "I'm a firm believer in the positive impact that compost tea delivers in plant vigor and growth," he says.

Meanwhile I've stopped fixating on compost tea brewers and am thinking about the actual compost pile. I suspect I meet Sclar's unspoken definition of a lazy composter and need to move from a cool, passive compost pile to one that is well fed, well turned, well watered and well screened. Stay tuned.

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