Nitrogen in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 27

can, however, be translocated in the plant from senescing leaves to seed production
as old leaves die and fall off, which will partly make up for the low nitrogen
availability in the soil.
The form in which nitrogen is available to plants can inf luence plant growth.
Organic nitrogen that is available over time (slow release) will provide nitrogen
over the various growth phases of a plant's development. This can effectively
increase plant yield as there are no stages of the growth of the plant that are limited
by insufficient nitrogen at a point in time. If highly soluble chemical fertilisers are
used, the nitrogen tends to be available for a relatively short time only, and is at
risk of leaching if applied in excess to the plant's immediate needs.
Multiple applications of chemical-based fertilisers can achieve the same thing
as slow release organic fertilisers, but may not be economically viable and also runs
the risk of damaging existing soil microbiology.

The nitrogen cycle
Soil and plant nitrogen levels are dynamic, and vary over time according to the
inputs and outputs from the natural environment and from changes brought about
by farming practices. Figure 10 illustrates the basics of this dynamic cycle.

Free nitrogen in air

Fertilizer manufacture

Volatilisation
e.g. from urea,
NH 3 and urine

Nitrogen from
air fixed by
rhizobium in
nodules

Animal life

Grass

Legume

Animal excreta

Dead leaves
and roots

Dead leaves
and roots

Decaying organic litter

Soil organic matter
(or humus)

Absorbed by
grass roots

Ammonium
nitrogen

Denitrification

Nitrate nitrogen

Losses by leaching

Figure 10 The nitrogen cycle. (Price 2006)