Compost tea beats liquid fertiliser in grass growth study

A cheap brew made from compost and seaweed appears to be better for grass than liquid chemical fertiliser, according to research conducted by an undergraduate student at Scotland's Rural College.

Penelope Wright. Image: SRUC
Penelope Wright. Image: SRUC
SRUC horticultural technician and student Penelope Wright is studying Garden and Greenspace Design after working for 15 years as a landscape professional in the USA.

Her final year dissertation compared the effects of water, purchased fertiliser and aerated compost tea on grass growth. The six-month trial found compost tea created a far better root structure, more even grass growth and improved the soil's moisture retention.

Wright developed her tea recipe with the help of staff at the Boston Greenway, a ribbon of US parks which are maintained organically. The brew began with green waste compost provided by the Royal Botanic Gardens Edinburgh, with humic acid and seaweed (in this case kelp) added 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.

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 soil fed only water suffered poor growth both at the surface of the soil and at the roots.

However the compost tea grass grew evenly with turgid stems and had a far greater root mass than the liquid-fertilised grass. Unexpectedly, the tea-treated soil also retained water far better than the other two treatments.

While working in the USA Wright often saw compost tea used to improve soil at prestigious green spaces such as the grounds of Harvard and Princeton and the Boston Red Sox baseball field. She now hopes compost tea could be more widely used in UK green space management.

Applying compost to grass areas or urban trees is impractical, and fertiliser can be expensive. But compost tea is "efficient, inexpensive, sustainable and environmentally friendly", Wright said.

"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."

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