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 nitrification.

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 (hay/silage). 

• Denitrification - conversion of NO3- into atmospheric N - due to waterlogging combined with warmth (soils above 10°C) 

• Immobilisation - NH4+ attached onto organic matter. 

• 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.

Nitrogen responses 

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 autumn/winter.

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 drained pastures. 

• 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.

Economics. 

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? 

Figure 2: 
Nitrogen responses

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 wet/waterlogged conditions.

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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