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Fertiliser Types: Nitrogen (N)

Artificial Nitrogen Fertilisers can be in either of three forms or indeed in combinations of more than one form:

  • Nitrate Nitrogen
  • Ammoniacal Nitrogen
  • Ureic Nitrogen.

Nitrate Nitrogen is the most readily available form of artificial nitrogen fertiliser for plant roots. The other types have to be converted to this form by soil acting bacteria before they can be utilised by crops. Nitrate N is very easily washed out of the soil. Because they are quick acting, fertilisers containing nitrate nitrogen are often used as top dressings on growing crops.

Calcium Ammonium Nitrate (C.A.N.) contains 26-27% nitrogen - half of it as nitrate nitrogen and half as ammoniacal nitrogen. This means that half the nitrogen is readily available while the other half is slow acting. This makes it a suitable fertiliser for grass, especially for spring grazing. It is suitable for any crop or soil and may be applied at sowing time or preferably as a top dressing.

Ammoniacal Nitrogen is a form of nitrogen based on ammonia, which is a nitrogen rich substance. It is slower acting than the nitrate form. It must be converted by soil acting bacteria to nitrate nitrogen before it can be absorbed by plant roots. It is not as easily lost from the soil by leaching.

NH4 ----------> NO3 --------------> Plants
(Ammonium) (Nitrate)

It can take from 3-5 weeks for the ammonium nitrogen to be converted to nitrate nitrogen by soil bacteria depending on soil moisture and temperature. Urea converts to nitrate N in 2 to 4 days depending on soil conditions.

Sulphate of Ammonia contains 21% nitrogen. Its nitrogen is in the ammonium form and hence is slower acting than C.A.N. It is useful in areas of low soil sulphur content.

Ammonium sulphate nitrate (ASN) contains 2^% N and 14% S. This is a mixture of ammonium nitrate and ammonium sulphate. The ammonium nitrate converts rapidly to nitrate N and the sulphur is also in a plant available form and immediately available to the plant.

Ureic Nitrogen is obtained from the substance called Urea which is also a nitrogen rich compound. In order to be absorbed by plant roots this form must be converted firstly into the ammonium form and then to the nitrate form by soil acting bacteria. For this reason it is slow acting and can be affected by low soil temperatures and dry soil surface conditions. Water must be added to Ureic Nitrogen to convert it to Ammonium Nitrogen. This can only occur in moist soils. This explains why urea is used early in the season and is not used during the dry summer months.

Urea contains 46% nitrogen. It is commercially used on grassland and as a cereal top dressing. It is not safe to combine with seed as it can severely inhibit germination. It needs to be applied on moist soil to be fully effective. It also gives best results when used in warm conditions. Urea should not be applied to soils which have been recently limed.

Urea is a cheap form of nitrogen. It is often found to be as good as C.A.N. but in some cases is found to be less efficient. Urea, like nitrate fertilisers, is easily leached when first applied to the soil. This would be a big disadvantage if heavy rain were to follow application. After application, urea is rapidly converted to ammonium carbonate, this may lead to losses at or near the surface of the soil or do damage to young roots or germinating seeds. This is the reason for caution about including urea in compound fertilisers.