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17 Sept 2021

A ‘green manure’ experiment at Holmwood

Written by Juliet Turner, Gardener at Holmwood
Gardeners Martin Cuthbert and Sean Argue (from Pollok House) stand in the garden at Holmwood in front of fresh soil beds, as they help to dig in crops and cover them with manure
Gardeners Martin Cuthbert and Sean Argue (from Pollok House) lend a helping hand with digging in crops and covering them with manure
Our garden team at Holmwood ran an experiment over several months in 2021, with the aim of enriching the soil in the kitchen garden ready for heritage vegetables throughout 2022.

As gardeners we are constantly thinking about keeping our plants healthy, and sometimes we can neglect to keep our soils healthy too. This year at Holmwood, we decided to put our garden soil into its own lockdown, and give it a chance to rest and rejuvenate until spring 2022.

Instead of planting vegetables, this year we had a ‘green manure season’ and sowed a variety of nitrogen-fixing crops from the Fabaceae family, which included alfalfa (Medicago sativa), black medick (Medicago lupulina), fenugreek (Trigonella foenum), bird’s foot trefoil (Lotus corniculatus), crimson and white clover (Trifolium incarnatum and Trifolium subterraneum), narrowleaf lupin (Lupinus angustifolius) and winter tares (Vicia sativa).

These fast-growing, foliage-dense crops cover the soil, prevent weeds from growing, and help to stop soil and nutrients from being washed away by the rain. They also provide habitat and shelter for insects.

A close-up of bright yellow bird's-foot trefoil flowers, growing on leafy plants.
Bird’s foot trefoil (Lotus corniculatus)

We had a very dry, hot summer at Holmwood and while some of the crops flourished – such as the red and white clovers, bird’s foot trefoil and winter tares – the alfalfa, the black medick and the lupins struggled a little to germinate, and these crops were less lush.

We are now at the point of returning the plant biomass to the soil: digging the roots in lightly; improving its structure; encouraging the formation of pore spaces which allows movement of air and water; better drainage; and unrestricted root growth.

Digging in the plant biomass also replenishes one of the most important macronutrients: nitrogen. Nitrogen is involved in many important plant processes, including helping the plant to photosynthesise, and supporting healthy cell and tissue growth.

The beds will be covered with organic manure, followed by a weed fabric which will allow water to permeate but prevent weeds from growing. This will be removed in spring 2022, and we will then begin work on producing heritage vegetables once more!

A view of the kitchen garden on a sunny day at Holmwood, with the house in the background. Several beds are laid out and surrounded by flowering borders. Currant frames can be seen at the centre, and several scarecrows stand in various beds.
A view of the kitchen garden on a sunny day at Holmwood

Nitrogen is one of the most abundant elements in the atmosphere. This sounds like good news for plants, but unfortunately it exists mainly in the form of atmospheric gas (N2), which plants can’t use. N2 must be changed into the ammonia form of nitrogen (NH3) through the process of nitrogen fixation, and the nitrogen-fixing plants work in a symbiotic or mutually beneficial relationship with soil bacteria called rhizobia, in order to bring about this transformation.

The rhizobia bacteria enter the root of the plant. Small root nodules form, which are initially white or grey inside, and then begin to turn pink or red once nitrogen fixation begins – this is a good indication of whether the plants are successfully fixing nitrogen. In return for food, the bacteria turns N2 into NH3 which can be used by the plant. If the plant is allowed to set seed, it will divert energy to the developing seed instead of the bacteria at the nodule, and nitrogen fixation will slow or stop.

Baskets of fruit and vegetables on a table.
Hoping for a bumper crop

Using nitrogen-fixing ‘green manures’ means that we can maintain healthy soils, improve structure, protect from weeds and erosion, and restore important nutrients. We don’t need to resort to using expensive chemical fertilisers (which are energy intensive to produce, can leach out of the soil and poison waterways, and which don’t support microbial soil life) – we can instead utilise relationships which already occur in nature. One small adjustment in the way we manage our soils can make an important contribution to mitigating climate change, as well as help us to use energy and resources more efficiently and develop a more symbiotic and sustainable relationship with nature.

We look forward to seeing the benefits of this experiment in 2022 – and are hoping for a bumper crop in the garden!

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