3.4 Nitrogen for Pastures
Role in plant
Nitrogen (N) is essential for pasture growth. Nitrogen
also improves pasture quality as protein content is directly related to
its nitrogen content. Whole plants with adequate nitrogen will contain
between 2.5% and 4% N in spring (18% to 25% crude protein) for legumes,
and between 1.5% and 3% N (9% to 18% crude protein) if a grass.
Most nitrogen is in the organic form. The soil contains
large amounts of nitrogen as organic matter. For example, a permanent
pasture with 4% organic matter in the top 10cm, will contain over
5000kg/ha total nitrogen.
This organic nitrogen has to be mineralised into inorganic
mineral nitrogen (ammonium - NH4+ and nitrate - NO3-) before plants can
use it. Less than 2% of the total nitrogen is available each year.
Mineralisation occurs with moisture, temperature and
biological activity and mostly occurs in autumn/early winter following the
opening rains, and again in spring, or during summer under irrigation.
Legumes fix around 20kg N/tonne dry matter per year - but
most of this goes into the organic nitrogen pool. However, the amount of
mineral N available to plants in autumn/early winter will increase in
proportion to kg/ha legume dry matter the previous spring. The conversion
of atmospheric nitrogen to organic nitrogen is called fixation.
However, more nitrogen will move into mineral N when the
pasture is eaten by animals, as dung and urine contain mineral N.
As plants mature they move root N into seeds and leaves -
thus most mineral N will be in seeds and leaves, whilst the organic N will
be in roots and stalks.
The conversion of organic nitrogen to ammonium and nitrate
is called nitrification.
Grazing animals and soil fauna/bugs are important for
Losses of nitrogen
Leaching - mostly nitrates - NO3- due to water
movement through the soil, especially sandy soils.
Volatilisation - evaporation of ammonia NH3 gas into
atmosphere - especially on alkaline soils.
Product losses - mostly seeds and removed forage
Denitrification - conversion of NO3- into atmospheric
N - due to waterlogging combined with warmth (soils above 10°C)
Immobilisation - NH4+ attached onto organic
Fixation - NH4+ attached onto clays. The most
important losses in high rainfall SA pastures are due to
volatilisation, leaching, product losses and fixation.
Identification of N status
Soil N tests are of little value unless they can detect
either ammonia/ammonium N or nitrate N and have been calibrated against a
plant response. A nitrate N soil test using soil sampling to 60 cm to
extract nitrate N is being used in cropping paddocks, but not in pastures.
Leaf analysis of total N can be used to indicate a deficiency - but plants
translocate nitrogen in the nitrate form - a sap nitrate test has been
developed for cereals, but not for pasture grasses.
All perennial grass dominant pastures will respond to
nitrogen in high rainfall SA.
Expected responses are Autumn 10-15kg DM/kg N Winter 5-10
kg DM/kg N Spring 20-25 kg DM/kg N Even though spring pasture responses
are greatest in spring, the value of the pasture produced is higher in
How much nitrogen? apply 25 - 30 kg N/ha per application
When to apply?
Nitrogen should be applied two - four weeks before you
anticipate a feed shortage!
Apply nitrogen to grass dominant actively growing
pastures - preferably at around 4 - 8cm height.
Apply nitrogen to fertile (adequate P and K), and well
Nitrogen best applied to moist soil - with a few mm
rainfall within a couple of days of application!
Nitrogen applied after a rain can volatilise if no
further follow up rainfall. Dews can increase volatilisation losses.
Nitrogen can be cost effective if used to produce early
winter feed from a responsive pasture, particularly if there is a late
break and the paddock can be spelled following N application. 50 kg/ha
urea will cost $20/ha (on the paddock) and may produce 250kg/ha dry matter
of extra pasture. This extra pasture will then cost 8c/kg ($80/tonne dry
matter) or 0.7c/MJ ME* compared to around 1.3c/MJ ME for barley or similar
for purchased hay.
What form of nitrogen?
DAP/MAP(di and mono - ammonium phosphate)
Ammonium and nitrate
Urea is usually cheapest source of nitrogen - but
the most volatile - as it is converted into ammonia gas before being
further converted to ammonium and then nitrate. Mixing into soil greatly
reduces loses due to ammonia volatilisation.
Ammonium sulphate, DAP and MAP are less
volatile than urea because less ammonia is formed when applied to soil.
Also, sometimes the nitrogen in DAP/MAP is cheaper than in urea. However,
these fertilisers will acidify the soil at faster rate than urea.
Ammonium nitrate is the less volatile than
DAP/MAP/Ammonium sulphate fertilisers as only half the N is ammonium - the
remainder is nitrate. Acidification is also lower than for urea. There is
however increased potential for leaching and denitrification under
Calcium nitrate. No losses due to volatilisation -
but losses due to leaching and denitrification may occur. Because
nitrification (conversion of ammonium to nitrate) doesn't occur, and due
to calcium content, calcium nitrate can increase soil pH. Calcium nitrate
is expensive and difficult to justify for pastures.
3.4 Nitrogen for Pastures
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