The dry material of any crop consists largely of carbon, hydrogen and oxygen that have come from the air and from water as well as a number of essential nutrients that have been absorbed from the soil. The most important nutrients are based on the elements of nitrogen (N), phosporus (P) and potash (K) and their availablity to the crop depends upon existing reserves in the soil and the amount that is supplied by rain or by the farmer.

Fertilisers are substances applied to the soil to increase crop yields by providing one or more of the essential nutrients. In the UK farmers have been doing this for thousands of years by using wastes from crops and animal excreta. It was not until the early nineteenth century that it was discovered that when bones were dissolved in acid that the phosphates became soluble in water and a very effective fertiliser. Further discoveries followed and the foundation of the inorganic fertiliser industry was laid.

Today the most important fertilisers are manufactured. Ammonium nitrate or nitrogen as we generally refer to it, is illustrated to the right. It is produced by combining natural nitorgen from the air with water and methane to produce ammonia. When treated with nitric acid, ammonium nitrate is formed. Because nitrogen amounts to 70% of the earth's atmosphere, it is available in unlimited quantities it would be hard to describe the application of this fertiliser as unsustainable.

The next most important fertilisers are those containing phosphate and potash and the image illustrates a blend of these ready for spreading by the farmer. Phosphates are produced by mixing rock containing phospate with acid. The small phosphate granules are soluble in water and will dissolve slowly to become available to the plant. Know world reserves are equivilent to 40 billion tonnes of phosphate. Potash is extracted from natural rock deposits called evaporites which originate from dried up oceans. World deposits in Europe, Canada and Russia are enormous.

Fertilisers are usually supplied to the farm in half tonne bags and there are important rules and regulations concerning their handling, transport and storage. These rules are in place to minimise the risks to operators and the environment. The application of fertilisers in the field is also governed by a number of rules and regualtions. This is because the inapropriate application of nitrogen and phosphorous can have adverse environmental implications. Nitrates can leach into the water course and raise levels above the minimum permitted for drinking water standards while excess phosphate can cause eutrophication in still and sluggish waters.

Farmers are naturally concerned to avoid adverse environmental impact. This is not only because of concern for the environment in its own right, but because fertilisers are an expensive part of crop production. If fertilisers leach out of the soil and into a watercourse they are wasted. This is akin to pouring money down a drain which is not an attribute normally given to farmers! To avoid these problems farmers adopt a number of policies:

In the first instance an assessment of the crop's need is made. This involves monitoring nutrient availablitiy from the field and determining the crops requirement, factors dependant on the type and growth stage of the crop and its place in the rotation. Fertiliser applications must then be made accurately at the correct rate ensuring no spread into hedges or watercourses. In the case of nitrogen which is very soluble, applications are only made in the growing season when the crop can utilise the nutrient.

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