The fertility of soil on organic farms is maintained through a number of methods:

• Growing crops that add nutrients to the soil as part of a crop rotation
• Growing crops that are ploughed back into the soil (green manures)
• Applying composts and farmyard manure - encouraging life in the soil
• Applying certain mineral fertilisers and supplementary nutrients (such as ground basalt)

A healthy soil has many microbes living in it. One of their jobs is to turn organic matter (like dead leaves) into humus which allows the plant to absorb nutrients at a natural rate.

Organic farming does not rely on artificial fertilisers. These kinds of fertilisers contain only the elements needed to make the crops grow not all of those that are needed to ensure good health. They can also suppress the important soil microbes which deliver all the nutrients the plants need to be healthy.

Crop rotations

Crop rotations have many important functions:

• Helping to control pests and diseases
• Maintaining soil fertility
• Maintaining soil organic matter levels and soil structure
• Ensuring that enough nutrients are available
• Ensuring that nutrient losses are reduced

Crop rotations are at the heart of organic farming. They involve changing the type of crop grown in one area on a regular basis. Organic farmers plant alternate groups of plants (roots, cereals, brassicas, legumes) to add fertility and prevent pests and diseases from building up. Some plants, like clover, add nutrients to the soil - while wheat and potatoes use up nutrients.

While using crop rotations might sound like quite an old fashioned thing to do, it is actually a much more effective and sophisticated system than relying on chemicals. Using chemicals to fertilise the soil often only provides crops with the three basic elements that they need to grow, rather than providing them with all the nutrients they need to actually be healthy.

These three elements are nitrogen, phosphorous and potassium. While these are essential to enable the crops to grow, plants actually need a minimum of 13 different nutrients and minerals.

This means that conventionally-produced food may not be as nutritious as organic. Crops growing in soil depleted in nutients from long term reliance upon artificial fertilisers are more vulnerable to attack by pests and disease.

Healthy soil
Crop rotation prevents the build up of pests and diseases, which help organic farmers to avoid the use of pesticides. When a farmer plants the same crop in one field year after year, the pests and diseases that attack the crop increase in numbers. This is because the crop that they need in order to survive and reproduce is readily available.

When the farmer alternates the crops grown in one field this pest and disease build up can be prevented. This is done by choosing crops that are not vulnerable to the same pests and diseases. In this way the farmer does not need to rely upon pesticides, which can harm wildlife and leave residues on our food.

Companion planting

Companion planting (planting two different types of plant in the same area) is also used by organic farmers to increase the amount of wildlife. More wildlife is useful because it means higher levels of natural predators to keep pest numbers down. Companion planting also helps reduce disease problems by preventing them from easily spreading across the field. For example planting marigolds at the end of a row of vegetables discourages pests.

Recycling nutrients
Organic farmers aim to recycle nutrients, which keep brought-in manures to a minimum. Soil fertility normally comes from grass/clover leys, legume crops (peas and beans) and livestock manures. The crop rotation balances fertility-creating crops with those which use up lots of nutrients.

Clover
Clover leys form the foundation of any organic farm. They have nodules on their roots that 'fix' nitrogen into the soil from the air, providing the nutrition that enables crops to flourish. Sometimes you may see clover being grown under cereals. This undersowing provides a certain amount of nutrients for the current crop and a head start for that ley.

Artificial fertilisers
Factory-produced fertiliser can cause environmental damage: the manufacturing process uses a lot of oil and nitrogen sprayed on fields is easily washed away, polluting rivers and streams. Many of the chemicals required on non-organic farms are produced overseas and require the use of fossil fuels to transport them to the farm.

Some studies estimate that only half the amount of artificial nitrogen fertilisers is actually used by the plants. The rest can 'leach' into rivers and streams, causing pollution. Nitrate levels are beginning to exceed levels set by European law - so it is important to use alternatives to chemical fertiliser, such as clover.

Green manures

Green manures are crops grown specifically to be ploughed into the soil. This ensures that the soil contains healthy levels of decomposed and partly decomposed organic matter (humus). The humus maintains nutrients in the soil when a crop isn't being grown so more nutrients are available for the next crop. It also reduces the chance of these nutrients leaching out of the soil into streams and rivers, potentially harming wildlife. Green manures are also beneficial because they support insect populations. So that's more food for birds and more predators to reduce the number of insect pests in the crop.

Farmyard manure

The amount of manure that can be used is restricted. This ensures that levels of nitrogen in the soil do not reach levels that could lead to leeching and damage wildlife and waterways.

Fresh manure may be used on grassland, but cannot be spread untreated onto horticultural crops during the growing season.

If there is a good reason why they need to, and only with our permission, organic farms can use non-organic manure. In order to do this it must be composted or stacked to make sure that it contains no harmful residues. We also have very strict rules about what kinds of non-organic farms this manure can come from. Before we decide whether or not to give permission for this, the farmer has to provide us with lots of information explaining what kind of manure they would like to use and why. We may ask them to give us a soil or manure analysis before we decide whether to allow its use.

The farmer would either stack the manure for six months, or compost it for three months. As it decays, the compost heap heats up - often to over 60°C. The heat kills off any diseases. The manure may then be used to fertilise the soil, but organic standards set time periods that must have passed before the crops grown in that field can then be harvested. Non-organic pig and poultry manure must be stacked for one year or composted for six. Organic farmers cannot use manure from intensive farming systems such as battery hen farms.

Compost from household waste can be used but only if it meets all legal requirements. If you want to read these requirements you can do so by going to the Defra website.

If a farmer wants to use a compost which we have not certified or approved, they have to provide us with a heavy metal analysis so that we can make sure that the levels do not exceed those in our rules.

Mineral fertilisers and supplementary nutrients

All of the farms certified by us must have production systems which are planned to minimise the need for any brought-in nutrients.

With our permission farmers can use:

• natural rock phosphate
• calcined aluminium phosphate, but only where the soil pH is greater than 7.5

However the cadmium level must be no more than 90mg per kilogram.

With our permission supplementary potassium (potash) can be used, but only where soil analysis has shown severe deficiencies.

Dried or liquid seaweed, provided that it is free from ingredients we don't allow, can be used, but calcified seaweed is banned.

Our rules list those minor minerals that can be used by farmers if they get our permission. The farmer would need to be able to provide us with a suitable justification for their use.

Stone meal, such as ground and liquid feeds made from plants produced on the farm basalt can be used, but only with our permission and with an appropriate justification.

If the farmer finds severe deficiencies, with our permission they can use the following supplementary nutrients:

• Sulphur
• The trace elements boron, copper, iron, manganese, molybdenum, cobalt, selenium, zinc, sodium (in the form of granular rock salt)
• Basic slag

Farmers cannot use meat, blood, bone, hoof and horn meals on the land, but they can add it to compost for use in pots to grow plants from seeds. However this can only be used on farms where there are no cattle or sheep.

Farmers can also use the waste wool left after shearing sheep, but only if this will not come into direct contact with the crop, and lime from sugar production.

In compost for growing seeds, in glasshouses or for plants that live for more than one growing season, farmers can use fishmeal as a fertiliser.

If a farmer has a problem with a disease in apple trees called bitter pit, they can use calcium chloride, but under no other circumstances.

If a farm has severe deficiencies they can apply to us for permission to use commercial fertilisers and liquid feeds, provided that they are suitable for organic use. In this case the farmer would need to tell us the ingredients before we would give permission.

Peat cannot be used on the land as a fertiliser. It can only be used in pots for growing plants from seeds, before they are planted out into the soil. We ask our licensees to try to use alternatives to peat wherever possible, which are made from sustainable UK produced materials.

We do not allow

• Sewage sludge, effluents or sludge-based composts cannot be used.
• Farms cannot use residues or manure from livestock systems that do not meet our standards.
• Artificial fertilisers

Factory-produced fertiliser can cause environmental damage: the manufacturing process uses a lot of oil and nitrogen sprayed on fields is easily washed away, polluting rivers and streams. Many of the chemicals required on non-organic farms are produced overseas and require the use of fossil fuels to transport them to the farm.

Some studies estimate that only half the amount of artificial nitrogen fertilisers is actually used by the plants. The rest can 'leach' into rivers and streams, causing pollution. Nitrate levels are beginning to exceed levels set by European law - so it is important that all farmers use alternatives to chemical fertiliser, such as clover, in the same way that organic farmers already do.

Introduction | Previous: The importance of soil | Next: Weed control

This document is not to be used by Soil Association licensees as a substitute for the Soil Association organic standards. The standards are available to buy for £30. To order a copy please call 0117 914 2406, or email goorganic@soilassociation.org.