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Tasmanian Institute of Agriculture

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Published: 1 Nov 2018

Researchers at the Tasmanian Institute of Agriculture (TIA) have discovered that just like your own piping hot cuppa, not all compost teas are created equally.

Though compost tea has been considered an important low-cost tool in protecting crops and improving soil fertility for decades, TIA studied how compost tea’s ingredients and application play a role in fighting disease.

The “tea” is produced by fermenting compost in water, which is then sprayed on soil or plants to add beneficial organisms and nutrients or control disease. TIA’s research identified two key factors which contribute to the ideal tea to fight disease.

“It’s all about diversity and timing,” said TIA lecturer Dr Karen Barry.

“The more different kinds of organic material fed in to the compost, the better the tea.”

That’s because the more diverse compost is, the more diverse microorganisms it will have. Microorganisms like bacteria can be key to fighting off diseases partly because they form a biofilm, a kind of natural shield against invaders like funguses.

As part of a student exchange, TIA researchers targeted their study to smallholder potato farms in Ethiopia and discovered that compost tea made from agricultural waste—a good mix of plant material and animal waste—reduced disease more than tea made from worm farm compost or household waste compost.

Though the study was aimed at family farmers in a developing nation, the same concepts may apply in other regions as well. Which means your backyard compost full of last night’s veggie scraps may make a better tea if, say, you also mixed in some leftover leaves from the potato farm down the road and a bag of manure from the neighbour’s chooks.

However, the material used in any tea must be well composted, otherwise there is a risk of introducing pathogens such as E. coli. All compost should be properly monitored to make sure it reaches the right temperature.

“Before it’s safe for tea, compost needs to reach higher temperatures—the thermophilic phase—to kill off dangerous pathogens,” said Dr Barry.

Dr Barry also noted the quest for different compost ingredients could present an interesting opportunity for remote producers to share resources.

“Let’s say you’re a small potato farmer located near a vineyard. You could trade your plant waste for their leftover crushed grapes and both of you are going to end up with more diverse compost.”

And remember: Better compost makes better tea.

After building a diverse compost, the next most important contributor to successfully using compost tea against disease is when and how it is applied.

In treating a problematic area of soil prone to root disease, drenching the soil is probably better than spraying plant leaves. Applying the tea when the odds of disease outbreak are highest is also important. With fungal pathogens, for example, the best time to drench the soil can often be during rainier, hotter periods.

In addition to drenching soil, spraying compost tea on leaves may help treat disease as well, but Dr Barry noted that multiple applications are key.

“It can get washed off by rain, it can get fried by the sun. It’s a living thing. You have to keep it alive and keep spraying.”

TIA’s researchers also found that adding myrrh gum just before applying the tea can help the compost form a protective film, creating a better barrier against disease.

Though it can require more effort than commercial treatments, compost tea can act as a low-cost, organic alternative to fighting plant disease.

A more scientific look at TIA’s compost tea findings is available in the Journal of Applied Microbiology.

This article appeared in The Examiner and The Advocate newspapers on 1 November 2018.