Guide written by:
John, Passionate gardener, Cambridge
Plants draw nutrients from the soil in order to develop and grow. With this in mind, it may be necessary to replenish your soil from time to time using one or several methods including applying an organic or mineral fertiliser, mulching or sowing green manure seeds.
The most direct way to fertilise soil is by adding manure or compost once a year. Both manure and compost work as amendments to improve the physical properties and structure of the soil. They are also fertilisers which help to enrich the soil with nutrients such as nitrogen, phosphorus, potassium, trace elements, and so on.
It's best to use manure that has been sitting in a compost pile for at least three months. Simply spread the manure over your soil at the end of autumn. Turn the soil lightly in springtime to get rid of any remaining manure. You'll need about 10 kg of composted manure per square metre.
If you decide to add raw manure, do not bury it: manure needs oxygen to break down and may release toxins to roots if buried.
A few weeks before you plant your crops, add some mature compost to the soil and turn your soil lightly. Be careful not to go overboard: around 5 kg of compost per 5 square metres should be plenty for medium-rich soil.
Generally speaking, your soil should only be fertilised once a year. However, you may find yourself adding more fertiliser on occasion; for example, in poor weather conditions, following a pest infestation, to crops that require a lot of nutrients or to balance out certain soil deficiencies.
You can of course use a synthetic fertiliser but it is better for the soil to use natural products like organic fertiliser or plant slurries.
Natural fertilisers can be added to the soil before you plant, as you plant or when your crops start to grow, depending on how quickly the fertiliser gets to work. Here are a few examples of organic fertilisers:
- Crushed and roasted horn: rich in nitrogen; releases nutrients gradually (roasted horn especially).
- Guano: made from accumulated bird excrement; very rich in nitrogen, phosphorus and trace elements. This is one of the quickest-acting and most effective natural fertilisers.
- Dried blood: rich in nitrogen, dried blood is also known for its quick action on crops.
- Bone powder: good for flowering plants, especially those with heavy flowering (e.g. fruit trees).
- Castor cake: to be used with caution, toxic to animals.
Home-made nettle and confrey slurries can also be used as fertilisers when diluted with water. They provide a large amount of easily absorbable nutrients (nitrogen for nettles and potassium for confrey).
Why use organic fertilisers?
Chemically manufactured mineral fertilisers provide easily absorbable nutrients directly to plants. Often high in nitrogen, these fertilisers boost growth but also make your plants more susceptible to disease and pests like aphids, especially when applied in large doses. The nutrients are very soluble and will quickly get carried away by rainwater and pollute groundwater and rivers.
On the other hand, an organic plant- or animal-based fertiliser will nourish all soil life and microorganisms, and work to gradually release minerals to plants.
In short, it could be said that chemical fertilisers will nourish plants while organic material works to feed the soil and help it to sustain life. But be careful: even natural fertilisers can be overused.
Choosing a fertiliser for your plants
Some plants require highly fertilised soil (e.g. 5 kg of compost per square metre); others are less demanding and some are even able to grow in fairly poor soil.
High nutrient demands
Average nutrient demands
Low nutrient demands
Artichokes, aubergines, celery, cabbage, cucumbers, squashes, spinach, leeks, peppers, potatoes, tomatoes
Beetroot, carrots, parsnips, chard, lettuce, salsify
Garlic, onion, shallots, broad beans, beans, peas, turnip, radishes, lamb's lettuce
The mulch we use to cover the earth between plants or different crops doesn't just help the soil to retain moisture in the summertime and prevent weed growth. A mulch mixture made of organic material (straw, grass clippings, sawdust, etc.) can also help to fertilise the soil by releasing minerals as it it is slowly broken down by animals and microfauna.
Weed control covers
Sowing green manure seeds
Green manure seeds are sown, as the name suggests, to fertilise the soil. As they break down, the seeds provide minerals to the surface which are then brought down into the soil by the plant roots. Some of the most common green manures include rye, alfalfa and peas. What's more, green manure helps to inject air into the soil, protecting it from poor weather conditions. They also provide humus to the soil which improves its structure.
Guide written by:
John, Passionate gardener, Cambridge, 57 guides
When I was young, I was already working in the family garden. Perhaps that is where my interest in plants and gardening came from. So, it was logical for me to study both plant biology and agronomy. At the request of various publishers I have, over twenty-five years, written many books on the subject of plants and mushrooms (a subject that is close to my heart).They were mostly identification guides at first, but shortly after they were about gardening, thus renewing the first passion of my childhood. I have also regularly collaborated with several magazines specializing in the field of gardening or more generally in nature. There is no gardener without a garden, I have cultivated mine in a small corner of Cambridge for the last thirty years and this is where I put into practice the methods of cultivation that will I advise you in as well.