Nitrogen is considered the most important component for plant growth, being a core element of many plant structures and for both their internal and external metabolic processes.
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, which represents about 98% of soil nitrogen.
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 are 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.
As demonstrated in this nutrient removal table, nitrogen contained in residual plant material (stubble) is readily recycled into the soil.
Forms of nitrogen
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, but it needs to be converted before plants can take it up. The rate of conversion depends on the same processes which control the mineralisation of organic nitrogen forms, however, urea needs special attention. Under the correct conditions of pH, temperature and moisture, urea will rapidly convert to ammonia gas and can be lost to the atmosphere if not applied correctly.
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).
Table 1. Possible loss if urea is left on the soil surface
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 (i.e. when soils are more alkaline) It reduces when soils are more acidic.
Nitrogen in soil
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, whereas 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 minimise 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.
Importance of nitrogen to cereals
The availability of nitrogen to cereals is critical for setting yield potential in the first six weeks of growth. It’s important that the correct amount of nitrogen for the expected yield result is available to the plants within this time period, as an inadequate nitrogen supply can result in the loss of tillers. As a general rule, each tiller aborted reduces yield by approximately 200kg/ha.
Nitrogen deficiencies are usually easy to identify as nitrogen is very mobile in plants. Symptoms may include: