What Happens in a Nitrogen Fertiliser Band in the Soil ?

One of the most common debates that circles cropping areas at some time or another is the differences between the  nitrogen fertiliser products. Urea and anhydrous ammonia (BIG N) are the two major products used  hence discussions usually involve the various merits of these two products.

In any discussion about differences and similarities between nitrogen fertiliser products, there is a need to clearly differentiate between the product, and the biological effectiveness of the product once in the soil.

The effectiveness of a  fertiliser product is related to  an number of factors that are not linked to the chemical make up  of the product. These include application efficiency (how much of the product remains in the soil after application compare to the rate applied) which is related to equipment and soil condition factors, timing of application (how  well does the availability of the product meet the crop demand) and placement  (is the  nutrient released from the fertiliser where roots can access it).

It is therefore difficult to conclude that a particular  fertiliser product is more suited to an crop or situation without  including reference to application method, timing and placement.

Once in the soil  at similar nitrogen rates, the comparisons between  nitrogen fertiliser products is relatively simple, even though the biological process that the N must go through  before plant uptake are complex.

The majority of the nitrogen that enters the soil is either as and ammonium form eg . BIG N, urea , DAP  and ultimately most organic forms,  or as nitrate  eg Nitramâ .

Once in the soil, the N in products that is not  in the nitrate  form is converted to nitrate  before  being taken up by the crop. Plants are not able to distinguish between the sources of nitrate taken up therefore, differences in efficacy of fertiliser products is usually as a result of a difference in  a parameter related to  application rather than any preference for one product or another.

A recent Incitec Fertilisers project based in southern Queensland involved measurement of the concentration of  soil ammonium in BIG N application bands and the dynamics of urea hydrolysis (conversion to ammonium) in  urea application bands.

The prime objective of  researching issues involving the interaction of product rate, row spacing, application depth,  timing and soil temperature,  was to identify products and practices to improve nitrogen efficiency, but  the research  also highlighted the similarities between urea and Big N.

Figures 1 and 2 show the   fluctuation in concentration of soil ammonium as a result of fertiliser N application as urea and BIG N. Figure 1 shows the change in soil ammonium over 38 day period as a result of a  hydrolysis of urea to ammonium and  mineralisation  of ammonium to nitrate  in soil band.  The arrow indicates the peak ammonium-N concentration  (~350 mg/kg)  from the 5.16 g/m urea application rate.

Figure 2 is the ammonium-N concentration  in soil bands resulting from the application of rates of BIG N. The arrow indicates the soil concentration from the equivalent of a 5 g/m urea application (~300  mg/kg).

As was expected, the results showed that  application of two products at the same nitrogen rates produced similar ammonium-N concentrations in the soil. The major difference between the two products was that the BIG N was converted to ammonium in the soil immediately whereas the urea took three days (six days in winter) to reach peak ammonium concentration.

The other aspect of N fertiliser that  is frequently discussed are “non target” effects. These are  the effects of the  N products on soil chemical, biological and physical characteristics as distinct from crop growth.

Non target  effects of nitrogen fertilisers are mostly related to the effects of ammonia/ ammonium.  Ammonium is present in soils at low concentrations (1 - 5 mg/kg) most of the time,  as an intermediate  compound in the mineralisation of organic matter. Ammonium-N concentrations of up to 50 mg/kg have been found in cotton paddocks after the incorporation of fresh trash.

The figures above clearly indicate that both BIG N and urea applied at the same rates have the potential for the same non target effects as albeit three days later in the case of urea.

The concentrations of ammonium in fertiliser bands are much higher than normal background levels.  The peak  ammonium concentration will depend on the rate of fertiliser applied, the application equipment used and the diffusion characteristics of the soil.

Ammonium concentrations greater than 400 mg/kg have been show to  temporarily depress microbial populations within the band however as the concentration  in the band decreases as a result of diffusion, the microbial population blooms (particularly nitrifying bacteria), stimulated by the ammonium.

Ammonium concentrations of 110 - 140 mg/kg are likely to depress germination of cotton hence placement of cotton  relative to  ammonium fertiliser bands  is an important consideration at sowing.

Photo graph 1 and 2

Similar to the effect on soil biology, the effects on soil chemistry and physical properties have been shown in long term studies to be localised, short term and non cumulative.

Another  facet in  this project was the opportunity to test products that modify the breakdown of N fertiliser products (Figure 3). By controlling the hydrolysis of urea, the characteristic of the product can be changed to fit into new  fertilising and farming practices. In Figure 3 the rate of breakdown of urea to ammonium has been slowed which has significant implications for  ammonia volatilisation losses and seed  placement “safety” of urea.

By revisiting some  aspects of the basic nitrogen fertiliser product research conducted over the last 30 years under Australian conditions , it is hoped that products and practices will emerge that will allow significant improvements in nitrogen fertiliser efficiency.

For further details contact your  local Incitec Fertilisers dealer or local Area Manager.

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