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Most plant uptake, under normal pH levels is in the Nitrate form. This is then converted by the plant to ammonium. The Nitrogen transformations occur in several sites within the plant, which in turn convert the ammonium to simple soluble organic molecules. This leads to the formation of proteins.

Nitrogen is very mobile in the plant, and it is important in terms of potential crop production to maintain an adequate supply of the nutrient. Any deficiency in the younger parts of the plant will result in the mobilisation of Nitrogen compounds in the older leaves, ultimately leading to loss of chlorophyll, shown as yellowing of the plant (chlorosis).

Nitrogen is also the most important element involved in the natural breakdown and re-use of organic matter in the soil. It is important to remember that soils with limited amounts of Nitrogen available to support microbial action on organic matter will ultimately lead to poor conversion, and the immobilisation of soil Nitrogen. As an indication of this, experiments have shown that 1 tonne of wheat straw requires 10 kgs of Nitrogen to ensure rapid breakdown without immobilisation. (Note that this refers to 10 kgs of elemental Nitrogen and NOT 10 kgs of 34.5% formulated product).

The National Vegetable Research Station produced a leaching model that describes that in lighter, sandy soils, soil water and dissolved ions are forced down through the soil profile to be lost into ground water systems. This process can be greatly reduced by maintaining high organic matter levels in the soil, which will hold Nitrogen and other soluble elements within it, reducing nutrient loss by leaching and providing environmental benefits as well as agricultural benefits.

Sewage and other organic products will provide a useful amount of total nitrogen within the soil mass. As the effluent begins to degenerate by microbial activity, which will largely be temperature dependant, the less available forms of Nitrogen are released and become available to the crop and soil Nitrogen cycle.

It is advised that, during preparation of specific site and crop recommendations, consideration is given to the Soil Nitrogen Supply (SNS) index system contained in the revised edition of RB209. The use of this index will more accurately reflect the formulation of an application rate suited to the crop and soil conditions prevailing.

In normal cropping programmes, Nitrogen is supplied by the application of farm wastes and supplemented with mineral Nitrogen where required. Timing of additional Nitrogen application is manipulated to ensure that it corresponds with plant requirements. The application of effluent will therefore provide an increased soil Nitrogen level, which will provide a base dressing that can be supplemented if and when required. The use of the effluent can also significantly improve farm economics. There are important environmental benefits accrued also, by reducing the need for the production and transport of manufactured Nitrogen.

Most sewage products contain approximately 5% of dry matter. Of each tonne of dry matter applied some 5% will be Nitrogen and Nitrogen compounds. This means that an application of 25 tonnes (m3) per hectare will provide some 250 kgs per hectare of Nitrogen. This is equivalent to a dressing of almost 200 units per acre of Nitrogen fertiliser.

Many other organic effluents also contain substantial amounts of Nitrogen in both available and ‘slow release’ forms. These materials will behave in the same way as farm yard manures and sewage. Generally, they are better than purchased mineral fertilisers because release to the crop is controlled by soil temperature and microbial activity. This means that they come into solution as the crop needs them, and not as a sudden flush of Nitrogen, regardless of the crops ability to use it.