ADLib Glossary (N)

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Nitrogen and Related Information

Total Nitrogen & Available Nitrogen

10% to 90% of the total nitrogen in livestock and other organic waste can be taken up by plants in the first few weeks after spreading. This is the available nitrogen. The amount of available nitrogen depends upon the type of manure. More information can be found in the DEFRA book on Fertiliser Recommendations. A computerised version of this is available in the Technical section of this package.

Nitrate

Nitrate, NO-3, is the main nitrogen containing anion occurring in the soil. It is very soluble and moves freely in water through the soil profile.

Nitrate in water is a pollutant above certain concentrations and can be a danger to human health. The main source of nitrate in water is agriculture although sewage discharges can also be an important factor.

Plant nutrients

To grow satisfactorily, plants need good supplies of the major nutrients: nitrogen, phosphorus, magnesium, calcium, and sulphur. Sodium is also required by some crops. Lesser quantities of the trace elements: iron, manganese, copper, zinc, molybdenum, boron and chlorine are required. Trace elements (for arable and vegetable crops) will generally be supplied by the soil, but may be supplemented by inorganic fertilisers and organic manures. A deficiency of any plant nutrients may cause a decrease in crop yields and quality.

Nitrate leaching

The 1980 EC Directive on the Quality of Drinking Water Intended for Human Consumption set a maximum of 50 mg/l of nitrate in drinking water. An increasing number of water sources currently exceed this limit. Agriculture is the main source of nitrate in drinking water where nitrate, having a high solubility, washes out of the soil and gradually, often over many years, reaches underground water sources.

Losses of nitrate by leaching depend on soil type and rainfall. The lightest arable soils only retain about 80 mm of water per metre depth; so nitrate in these, and the shallow soils which are so extensive in the UK are much more easily leached than nitrate in deep clay or silt soils which may retain more than 200 mm of water per metre. The amount of rain which is in excess of evaporation and crop transpiration and which therefore causes leaching, varies from about 150 mm per annum in the East to more than 300 mm per annum in some western and northern arable regions. In some grassland regions more than 1000 mm per annum is not unusual.

Nitrate cycle

Plants need nitrogen to grow, develop and produce seed. The main source of nitrogen in soils is from organic matter. Organic matter principally arises from plant and animal residues. The nitrogen in organic matter is largely in organic forms that plants cannot use. Bacteria found in soils convert organic forms of nitrogen to inorganic forms that the plant can use. Nitrogen is taken up by plant roots and combined into organic substances in the plant, such as enzymes, proteins and chlorophyll. Chlorophyll gives the plant its green color. When the plant dies, it decays and becomes part of the organic matter pool in the soil. The basic nitrogen cycle is illustrated in the diagram below. It shows nitrogen changing from organic matter in the soil, to bacteria, to plants and back to organic matter.

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