Using Crop Rotations. Crops Such As Clover and Beans Have Nitrogen-Fixing Bacteria in Their

Soils /
A healthy soil is the foundation of an organic system. To maintain this, no artificial chemicals are used. Instead, natural methods are used to maintain soil fertility and control pests and diseases.
Introduction
Like water and air, soil is an essential natural resource. It is the foundation of practically all agriculture in the world. It forms very slowly but can be lost very rapidly by erosion, or become contaminated by pollution. Soil is a complex, diverse and rich ecosystem where minerals, organic matter, water and air are worked on by a great variety of organisms to provide the right conditions for plants to grow. One crumb of soil may contain millions of bacteria, thousands of protozoa and possibly a kilometre of fungal hyphae. As the leading charity promoting organic farming in the UK, the Soil Association believes that a healthy soil is the foundation of a healthy farming system.
This section examines three aspects of the soil that are crucial for farmers: soil fertility, soil structure and the level of pests and disease. For each area, key elements of the organic and conventional approaches are set out and important issues are raised.
1. Soil fertility
Soil fertility is a measure of the amount of essential minerals in the soil that are necessary for plant growth. These minerals can be lost from the soil in different ways, including the growth and removal of crops (such as cereals), or erosion involving loss of soil in water run-off from fields or leaching of minerals into water courses.
ORGANIC APPROACH
Organic farmers replace valuable soil minerals and maintain fertility by:

• Using crop rotations. Crops such as clover and beans have nitrogen-fixing bacteria in their roots, which transfer nitrogen from the air into nitrates in the soil. This replaces minerals lost through the growth of other crops such as cereals and vegetables.
• Adding organic matter, particularly plant material and animal manure. This organic matter provides a source of nutrients and energy for the soil organisms (worms, beetles, fungi, bacteria) that breakdown organic matter and makes nutrients available to plants.

Both these processes support the natural recycling of minerals through decay and the cycling of minerals through soil and atmosphere.
CONVENTIONAL APPROACH
Conventional farmers replace soil nutrients by:

• Adding water-soluble chemical fertilisers that are immediately available for plant growth.

ISSUES

• More food - crops grown with chemical fertiliser grow faster and give higher yields than those grown under organic conditions.
• Long term health of the soil ecosystem - a reliance on chemical fertiliser rather than organic matter means there will be less food and energy for soil organisms, which will gradually decline in diversity and number. This will compromise the diversity and health of the soil and, therefore, its capacity to support food production over the long term. Some fertilisers used by conventional farmers may be directly toxic to soil fauna
• Financial cost - increasing cost of fertiliser to the farmer at a time when prices of farm produce have been falling.
• Environmental cost - the process of manufacturing fertilisers pollutes the environment, whether by accident or through leaching into water courses. Fertiliser manufacture also requires oil, which depletes our limited supply of fossil fuels
• Human health - possible damage to human health from high levels of nitrates in water supplies.

2. Soil structure
The structure of a soil refers to the way in which the soil particles are arranged in the soil. A well-structured soil will have a continuous network of pore spaces that allow the free movement of air and the unrestricted development of roots. Moisture and nutrients will be retained within the soil while excessive water will drain away via cracks, old root systems and earthworm channels. A poorly structured soil, by contrast, may consist of densely packed soil with few cracks. Clogged up pore spaces will inhibit the movement of air, water and root development.
The working of soils, particularly by machines, has damaged the structure of the soil and resulted in it being more exposed and susceptible to erosion.
ORGANIC APPROACH

• Adding organic matter to the soil - Organic matter, such as composted plant material and animal manure, provides a source of nutrients and energy for the multitude of soil organisms. Earthworms process soil and organic matter through their gut, thus mixing soil constituents and bringing up materials from lower down in the soil. The channels made by the worms facilitate drainage and allow exploration of deeper soil layers by plant roots. The activity of soil organisms is, therefore, fundamental to ensuring good soil structure.
• Crop rotations - Different crops grown in rotation can draw up minerals from deeper in the ground and help to break up the soil.

Also, strongly charged nutrients in the soil (e.g. Calcium ions) can attract particles of organic matter, thus helping to bind the soil together and stabilise it.
CONVENTIONAL APPROACH
Although organic farmers also use mechanical methods of correcting damage to soil structure, this method is likely to be more prevalent amongst conventional farmers. Examples of this include deep cultivations to loosen and aerate the soil and putting in drains to allow water to penetrate through a waterlogged soil.
ISSUES

• Long term health of the soil ecosystem - mechanical work to remedy problems of soil structure (whether by organic or conventional farmers) is ultimately a short-term measure. The capacity of the soil to support agriculture over the long term will depend on the biological activity of soil organisms helping to improve structure. This in turn will depend on sufficient applications of organic matter to the soil.
• Erosion of soils - this is an increasing problem. In the UK, erosion is largely associated with the shift to arable crops on soils that should have remained covered with grassland. There is evidence that organic farming practices including rotations with grass leys, green manuring and under sowing of crops can help reduce the potential for soil erosion.

3. Level of pests and diseases in the soil
ORGANIC APPROACH
Organic farmers control the level of pests and diseases by biological control methods, which do not involve the use of artificial chemicals. Examples of such methods include:

• Crop rotations - Pests and disease in the soil are often specific to particular plant species and their concentrations build up over time. By having good crop diversity and using rotations to ensure that no one crop is grown for too long in the same place, the level of pests and disease will be kept to a minimum.
• Companion planting - Certain plants when grown next to others can inhibit the build up of pests or disease, e.g. onions grown next to carrots can repel carrot fly.
• Physical methods - Physical removal of pests, though time consuming, can be very effective in their control, e.g. removal of growing tips of broad beans before bean aphid infestation will discourage build up of the pest.
• Encouragement of predators, parasites or diseases of pests - Organic farmers will encourage predators, parasites or diseases of pests or may even add them to reduce pest infestations. For example, ladybirds may be added to crops to eat aphids, parasitic wasps may be released to lay eggs in certain insect pests, or disease as spores may be sprayed in water solution, such as the bacteria, Bacillus thuringiensis, to kill cabbage white caterpillars.
• Adding organic matter to the soil - Once again, the application of organic matter to the soil is vital. Adding organic matter to the soil will promote a healthy level of soil biological activity and stimulate the complex of predatory organisms, which help to keep pests and diseases under control.

Ultimately, pests and diseases, both below and above ground, should not generally be a major problem in a well-established organic system. This is because a healthy plant, given optimal soil conditions and balanced nutrition will be better able to resist pests and disease.
CONVENTIONAL APPROACH
Conventional farmers use chemical pesticides to help prevent or kill pests and diseases.
ISSUES

• Long-term health of the soil ecosystem - direct toxicity of chemicals to soil fauna, disruption of food chains, weakening of immune systems, confusion of chemical signals by which many organisms communicate.
• Human health - some pesticides have been linked to cancers, leukaemia in children, birth defects, allergies and other disease. Some fat-soluble chemicals may be concentrated up food chains with increasingly toxic effects on animals at the top, including humans. A classic example of this is DDT, which, though its use is now restricted, is still persistent in food chains and, it is still present in many ecosystems, being manifest in human fat tissue.
• Environmental costs - the process of manufacturing pesticides pollutes the environment, whether by accident (e.g. Bhopal) or through routine emissions. In Britain £121 million is spent each year cleaning drinking water supplies of pesticide contamination. Pesticide manufacture also depletes our supply of fossil fuels
• Financial costs - increasing pesticide costs to the farmer at a time when prices of farm produce have been falling.