Nitrates the most efficient source of nitrogen for crops

There are three main forms of nitrogen in mineral fertilizers: urea, ammonium or nitrate nitrogen. Irrespective of the type of nitrogen fertilizer applied, plants absorb by far most of their nitrogen as nitrate. This is because of the rapid microbial conversion of urea and ammonium into nitrate in the soil (Fig. 1). At temperatures of 15 - 20°C the conversion of urea or ammonium into nitrate takes only about 1 to 2 weeks.

Specific soil microorganisms oxidize ammonium to nitrate (nitrification) in order to cover

their energy requirements. Thus, in an aerated soil any nitrogen form will be quickly converted into nitrate. Plants can then easily take up the water-soluble nitrate in high quantities together with the soil water. This has practical implications. Plants with rapid growth rates like vegetables, require high N uptake rates in the short term, only possible with the application of nitrate based fertilizers.

However, under water-soaked conditions this can be different. For example, paddy rice grown under flooded conditions take up most of its nitrogen in the form of ammonium because under anaerobic conditions the soil bacteria are unable to perform the nitrification due to lack of oxygen.

Ammonium is preferentially absorbed by soil microbes (immobilization), fixed to clay minerals or can be lost as gaseous ammonia (volatilization). These processes reduce the availability of ammonium and explain why nitrates are more efficient and often give superior results in terms of crop production when compared to ammonium or urea based products (Fig. 2).

Fig. 2: Average of 15 field trials with winter wheat (UK, 1994-1998)

Nitrogen immobilization
Just like plants, soil microbes (bacteria, fungi) need nitrogen for their growth. Therefore, they compete with the plants for nitrogen. The uptake of nitrogen by soil microbes is called microbial N immobilization. Immobilized N is not available to plants. Unlike plants, however, soil microbes prefer ammonium as the source of nitrogen. Therefore, if a fertilizer contains ammonium, a larger share of its nitrogen is absorbed by microbes, generating a loss in terms of plant production. This is one reason for the higher efficiency of nitrates.

Nitrogen fixation
Nitrogen fixation is the adsorption of the positively charged ammonium (NH4+) ion to negatively charged clay particles in the soil (see photo). Ammonium fixation reduces nitrogen availability for plants. The negatively charged nitrate (NO3-) ion is not fixed and remains fully plant available. Ammonium fixation depends upon the clay content and occurs fairly rapidly. The higher the clay content of the soil the higher is the rate of ammonium fixation.

Ammonia volatilization
Nitrogen is lost from urea and ammonium fertilizers mainly as gaseous ammonia (NH3). This process is called ammonia volatilization. Besides environmental conditions and soil types also the nitrogen fertilizer type determines the amount of ammonia volatilization. The higher the nitrate share in a fertilizer the lower the risk of ammonia losses. Ammonia losses do not occur from nitrate, but can reach up to 50% if urea is the nitrogen source. 50% loss means that only half of the nitrogen applied as urea is finally available for the crop. This effect can be visualized with the ammonia volatilization kit developed by Yara. Ammonia turns the initially yellow column blue. The more ammonia is formed, the more yellow has changed into blue.