The aim should be to rarely have bare soil on your plot. If you do, nutrients will be washed away and it will gradually lose its texture and organic content. Green manures are fast-growing plants that are grown on bare soil to improve its quality. When the ground is needed for planting they can be lightly dug in, or cut down with a hoe and left on the surface as a mulch, and/or covered with a further mulch (garden compost, for example).
Green manures can be sown directly as seeds as soon as you have harvested a crop from an area, or even sown under larger crops that are spaced apart, such as tomatoes, pumpkins or sweet corn. The foliage of the green manures will also suppress weeds and provide cover for beneficial animals such as beetles, which will control pests. Some, like clovers, will also have the benefit of flowering and attracting bees and other pollinators. The green manures can be left until the area is needed again (although some may die off naturally over winter), or planted as a ley (an area of ground sown and left for a season or two to improve its fertility – see the Crop Rotation section).
One of the most important nutrients for plants is nitrogen. It is a major component of chlorophyll, the compound by which plants use energy from sunlight to produce sugars from water and carbon dioxide during photosynthesis, and is also an important part of proteins. Green manures can help maintain or increase nitrogen in the soil either by nitrogen lifting (absorbing surplus nutrients so they are not washed away) or by nitrogen fixing (absorbing nitrogen from the air and storing it in the roots after it has been converted to non-gaseous nitrogen compounds by bacteria) which then improves nitrogen availability for future crops.
Common green manures are:
- Crimson Clover – sow April-September – weed-smothering foliage, flowers attractive to bees.
- Trefoil – sow March-August – tolerates shade, good for sowing under larger crops.
- Lucerne (Alfalfa) – sow April-July – deep rooting, can be left for years.
- Mustard – sow March-September – quick growing for short-term use.
- Buckwheat – sow April-August – quick growing, regrows if cut.
By steeping well-rotted garden compost in water, you can create compost tea which can be applied as a plant feed either direct to foliage or by watering the soil around plants. The steeping extracts beneficial microorganisms and nutrients from the compost, making them more immediately available to plants than by applying compost as a mulch or within a potting medium.
There are various methods for making compost teas: a simple one is to fill a bucket ¼ full of compost, top up with rainwater and leave for three or four days, stirring occasionally, then strain the mixture (putting the solids back on the compost heap) and dilute the liquid 1:10 before using immediately.
Other methods use an airstone and air pump (such as used in aquaria) to create aerobic conditions in the water so that the microorganisms multiply rapidly. To do this, place a handful of compost in a bucket of rainwater, add 4tbsp of molasses (to feed the microorganisms) and aerate with the pump for one or two days. Strain and use immediately (certainly within four hours) on foliage or soil.
Other homemade plant feeds can be made using foliage from other plants such as stinging nettles or comfrey. Their leaves are steeped in water and release nutrients as they decompose. Nettle provides nitrogen, potassium, magnesium, sulphur and iron and is good for feeding young vegetables and watering containers. Comfrey is particularly rich in nitrogen, but also provides potassium, iron, calcium and phosphorus and is good for feeding fruit crops such as aubergines and cucumbers.
To make a liquid nettle feed, harvest nettles (young ones in spring are highest in nutrients, but any will do) and pack them tightly into a bucket or barrel to within 15cm of the top. Pour in water until the nettles are just covered and leave for approximately two weeks. During this time a sort of fermentation will occur and the liquid may start to froth. It will also start to smell unpleasant, so place it away from the house and neighbours!
When the fermentation has died down, strain off the remaining stems and leaves – you can use a soil-sieve on top of a bucket and just tip the mixture in – and there you have it: a bucket of free, natural, concentrated plant food. Just throw the strained-out solids on the compost heap. To use this liquid, dilute 1:10 with rainwater and water pots and beds every week or two during the growing season.
To make a feed from comfrey, follow the same instructions but allow to ferment for four weeks.
Other Vegan Organic Fertilisers
If you are unable to make your own plant feeds, there are organic options available to buy, but you must make sure they haven’t used any animal products. The most common is seaweed extract, which can increase yields of fruit and vegetables by producing healthier plants with a good root structure as well as extending cropping periods. It will come in concentrated form, so dilute as instructed before spraying onto foliage or direct on to soil with a watering can. It can also be used as an organic fertiliser for houseplants.