Nitrogen is essential for plant growth. It also plays a fundamental role in “feeding the world”, as nitrogen is a key component of protein.
There are two forms of nitrogen, organic and inorganic.
Organic forms are found in the soil and need to be converted into the inorganic forms ammonium or nitrate for plant use. This process is called mineralisation and it is a complicated process involving soil microbes, pH, temperature and moisture. It is a significant process as it represents about 98% of soil nitrogen. The conversion of organic substances into NH4+ is referred to as Ammonification, similarly the conversion of NH4+ to NO3- is referred to as Nitrification.
Legume crops fix large quantities of organic nitrogen that become available to future crops as it mineralises during the season. However, as most legume crops as grown for seed, much of the nitrogen fixed is removed from the soil. As a rough rule, the nitrogen remaining after a harvested legume crop is usually less than half the requirement of the subsequent cereal crop.
Nitrogen contained in residual plant material (stubble) is readily recycled into the soil. (see Nutrient Removal).
Plants take up most nitrogen in the ammonium (NH4+) or nitrate (NO3-) form. Urea is the most common form of fertilizer nitrogen used in Australia. Urea also needs to be converted before plants can take it up.
Urea ----> Ammonium(NH4+)+Ammonia (NH3+[gas]) ----> Nitrate (NO3-)
The rate of conversion depends on the same processes which control the mineralisation of organic nitrogen forms, however urea needs special attention as it is also converted into ammonia gas. Under the correct conditions of pH, temperature and moisture, urea will rapidly convert to ammonia gas and be lost to the atmosphere if not applied correctly.
Table 1. clearly shows the possible loss if urea is left on the soil surface.
|Nitrogen Form||Autumn Application||Winter Application|
|After 4 weeks||After 7 weeks||After 4 weeks||After 7 weeks|
Table 1. % N lost by volatilisation from surface applied nitrogen on pH 7.0* soil (courtesy CSIRO) *NB Volatilisation of N increases when pH increases (ie when soils are more alkaline) It reduces when soils are more acidic.
The other common form of nitrogen is ammonium (Amsul, DAPSZC, MAPSZC, VIGOUR). Ammonium does not have the same application restrictions as urea as it is not as subject to volatilisation (Table 1). As discussed previously, ammonium will convert to nitrate over a period of time – the rate of which is determined by environmental conditions.
The form in which nitrogen is present in the soil is also an important consideration. Urea, ammonium and nitrate all express differences in soil mobility. Urea and nitrates are very mobile in the soil and can be readily leached on lighter soils. Ammonium is relatively immobile in the soil and generally will not leach. As a general rule, a split application of urea is suitable in the high rainfall regions (>500mm) to minimize the loss of urea and nitrate out of the root zone, before a crop can utilize it. The ammonium nitrogen in products such as DAPSZC and MAPSZC® are generally stable until they are converted to nitrate (Figure 2).
Figure 2. Diagram showing the differences in mobility between nitrogen sources.
The availability of nitrogen to cereals is critical for setting yield potential, in the first six weeks of growth . Cereal yield is the sum of the number of grains per hectare and the weight of the grain. Grain numbers are influenced by the number of plants per m2 X number of tillers per plant X the number of grains per head.
The maximum number of tillers and grains per head are set by the time the plant is approximately six weeks old. Therefore it is important that the correct amount of nitrogen for the expected yield result is available to the plants within this time period. An inadequate nitrogen supply can result in the loss of tillers. As a rule, each tiller aborted reduces yield by approximately 200kg/ha.
Nitrogen deficiencies are usually easy to identify. Nitrogen is very mobile in a plant. A deficient plant will begin to go pale from the tips of the oldest leaves and then move to down the leaf blade. As the chlorosis moves to the middle leaves the lower, older leaves will turn very pale and die. Tiller abortion will occur. Enough nitrogen needs to be available to support yield potential. Cereals that tiller well can run out of nitrogen later in the season, also resulting in tiller abortion. This is common in cereals grown after lupins with no additional nitrogen supplied.