LJN Blog Posts

A good cup of tea is said to soothe the soul, and it seems that a special brew can perk up plant life just as easily.

An undergraduate student based at the Edinburgh Campus of Scotland's Rural College (SRUC) has discovered that compost tea is an effective and cheap way of making sure our cities’ grassy areas are kept healthy all year long.

In a six month trial which compared the effects of water, purchased fertiliser and compost tea on grass, the compost tea was found to create a far better root structure, more even grass growth and improved the soil’s moisture retention.

SRUC horticultural technician and student Penelope Wright worked as a landscape professional in the USA for fifteen years and there she found compost tea is commonly used as a soil improver on some of their most prestigious green spaces including ivy league university grounds like Harvard and Princeton, the Boston Red Sox baseball field, and New York’s famous Battery Park.

Penelope says: “In America compost tea is regularly used but in the UK after urban green spaces are planted up they are often left to fend for themselves.

"While applying compost to grass areas or around urban trees is impractical, and adding fertilisers can be expensive, using compost tea is efficient, inexpensive, sustainable and environmentally friendly. It is applied two or three times a year just like regular compost, and my research showed an improvement in the soil after only six months.”

For her final year dissertation Garden and Greenspace Design student Penelope decided to focus on the merits of aerated compost tea (ACT), comparing its effects on grass growth with those of a popular brand of fertiliser or just water. Simply put fertilisers feed the plants while compost feeds the soil.

In the short term fertiliser can help plant growth through adding required nutrients, but it can also have negative effects, for example reducing the number of microbes in the soil – whereas compost and compost tea increases the microbe count which helps create healthier more nutrient rich soil in the long-term. Compost also aids water retention, which again is good for plant growth.

To help design the recipe for her compost tea Penelope visited the staff of the Boston Greenway, a ribbon of parks built on an old highway tunnel, one of just a few organically maintained urban parks in the United States.

The process of making compost tea begins with soaking, or brewing, a bag of good quality compost (free from manure) in water for a number of days. Penelope’s brew began with green waste compost provided by the Royal Botanic Gardens Edinburgh, with humlic acid and seaweed (in this case kelp) added to increase the number of microorganisms found in the tea (these additions are known as ‘microbial starters’). The tea was then left for a few days before oxygen was added using a pump.

After a week the tea was teaming with microbes and deemed ready for soil consumption. One section of grass was treated with the tea while another was treated with the branded fertiliser (applied in recommended volumes). The final batch was fed just water.

The results showed that while the grass treated with fertiliser grew longer the growth was more uneven, the stems were less substantial and the roots weaker than those treated with compost tea.

The compost tea grass was even with turgid stems and had a far greater root mass than the liquid fertiliser grass, a clear indication that this soil was healthier. The soil fed only water suffered poor growth both at the surface of the soil and at the roots.

Penelope says: “The soil treated with compost tea also retained water far better than the other two treatments, which was an unexpected result, but is yet more evidence of the benefits of using compost tea. Overall the compost tea was the clear winner and I hope to see it used on our green spaces here in the UK.”