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Corangamite Region   'Brown Book'   - How to optimise your soils to enhance productivity
Do I need to apply nitrogen (N)?
Key Points
Understanding the question
Management
Other related questions in the Brown Book
Resources
References

Source: DEPI Victoria
Key Points
  • Nitrogen (N) is needed for crop growth in larger quantities than any other nutrient
  • Nitrate (NO3-) is the highly mobile form of inorganic nitrogen in both the soil and the plant
  • Sandy soils in high rainfall areas are most susceptible to nitrate loss through leaching
  • Other than when supplied with inorganic fertilisers, N is largely supplied by mineralisation from organic sources in the soil

  • The most common nutrient that pastures respond to is nitrogen
  • Urea and ammonium fertilisers need to be broken down to nitrate, the plant available form of nitrogen
  • Excessive N application can interfere with biological activity and dramatically reduce the efficiency and supply of nitrogen through the nitrogen cycle
Understanding the question
Why is it important to me as a farmer?
    Nitrogen (N) is needed for all growth processes, as it is the major component of amino acids, which are the building blocks of proteins, enzymes and the green pigment chlorophyll. Chlorophyll converts sunlight energy into plant energy in the form of sugars and carbohydrates.




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Why do you need nitrogen?
  • Over 98% of all nitrogen present in the soil is in an organic form (i.e. contains carbon in its molecular structure). Organic nitrogen cannot be taken up by the plant until it is mineralized by microbes into inorganic forms (principally nitrate (NO3) and ammonia (NH4))
  • Mineralisation occurs throughout the growing season providing a steady, continuous supply of nitrogen to the crop. This process can be stimulated significantly by cultivation, and is most rapid in warm moist soils
  • Retaining stubble provides more fuel for soil microorganisms, promoting a larger and more active population. This in turn can supply more nitrogen to a following crop
  • Improved pasture species allow a much higher stock-carrying capacity; but to maintain this productivity, they require a higher level of soil fertility than do native pasture species
  • Fertiliser applications are required to overcome the soil’s inherent nutrient deficiencies and to replace the nutrients that are lost or removed from the soil by pasture growth, fodder cropping or conservation, and animal products, such as milk or meat
The following table can be used as a guide to estimate the removal of nutrients from a dairy farm and the addition of nutrients brought onto the farm.

Table 1 - The percentage of nutrients removed or brought in by various pasture and animal components - Source: DEPI Victoria
  Product Nitrogen (N)
  Pasture (DM) 3.5%
  Milk (L) 0.6%
  Meat (kg LWt) 2.8%
  Cereal grain (DM) 1.8%
  Hay (DM) 2.2%
  Silage (DM) 2.4%
  Manure (DM)> 2.9%
  Urine (L) 1.2%


The following table provides some easy-to-use estimates for the amount of nutrients contained in a quantity of hay, grain and animal products

Table 2 - The weight of nutrients removed or brought in by particular quantities of hay, grain and animal products (kg) - Source: DEPI Victoria

  Product Amount Nitrogen (N)
  Fresh milk 1000 L 6
  Meat (cattle liveweight) 1000 L 28
  Hay (70% grass/30%clover) 1 t DM 20 to 35
  Lucerne hay or clover hay 1 t DM 30 to 40
  Oaten hay 1 t DM 20
  Pasture silage 1 t DM 30
  Maize silage 1 t DM 1.3
  Lupins 1 t DM 50 to 60
  Wheat, oats, barley, triticale 1 t DM 17 to 23
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How to recognise the need for nitrogen in the paddock?
  • Nitrogen deficiency symptoms include:
    • Stunted growth
    • Yellowing or light-green colour in pastures (very occasionally orange and red pigments may dominate)
    • Low protein content of grasses and crops
    • A lack of nodules or very small whitish nodules on clovers
  • Nitrogen is mobile in plants, so deficiencies show up in the oldest plant tissues first
  • Nitrogen deficiency in clovers has similar symptoms to a sulphur deficiency. However, sulphur is immobile in plants, so sulphur deficiencies show up in the youngest plant tissues first



    Figure 1 - Loss of colour in older leaves of wheat indicating nitrogen deficiency. – Source: DAFWA


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Management
Managing the application of nitrogen
  • In grazing systems, nitrogen fertiliser should be considered as a form of supplementary feeding, rather than a means of overcoming a nutrient deficiency. Dairy farmers apply nitrogen to reduce the amount of supplementary feeding required during winter
  • In sheep enterprises, nitrogen application should be considered, to boost pasture production:
    • where there has been a late break to a season
    • when the soil profile is much drier than normal at the end of inter, leading to a high probability of below-average spring growth
    • to allow earlier harvest and higher yields of hay and silage crops
  • In cropping systems, typically a deep soil N test (to rooting depth, usually 60cm) taken as close to sowing as possible will give an assessment of the at-sowing status
  • Depth, timing and handling of soil samples should be done according to guidelines provided by the soil testing service. The best rate of N will be one that matches crop demand
  • In dryland environments, crops can obtain up to 80% of total nitrogen requirement from biological sources within the soil when conditions are optimised
  • When moving from burning to cereal stubble retention, there can be an initial tie-up of soil nitrogen as the stubble decomposes, and starter nitrogen may need to be increased at sowing
  • Over two to three seasons the soil usually adjusts to the new conditions, and in the long-term is able to supply a greater amount of nitrogen for crop growth and yield
Fertiliser application – general
  • Too much nitrogen can actually slow plant growth because nitrogen not used by plants is washed (leached) out of the soil, contributing to soil acidity
  • It can also interfere with biological activity and dramatically reduce the efficiency and supply of nitrogen through the nitrogen cycle
  • Small frequent applications of fertiliser nitrogen will keep environmental and animal health problems to a minimum and prevent waste of fertiliser
  • Unlike fertiliser nitrogen, the nitrogen in poultry litter is released slowly and it can be applied as a single application
  • Apply urea on soils which are expected to increase their moisture content. Do not apply to waterlogged or dry soils
  • Only apply N when pasture is actively growing and can utilise the N
  • Avoid applying N fertiliser near streams/riparian zones and over drainage lines within a paddock
Fertiliser application – pastures
  • Apply N strategically at times of the year when pastures are deficient in N (periods of low clover growth)
  • Apply N at rates of 20 to 60 kg N/ha to actively growing pastures
  • Applications over 200 kg N/ha/yr are likely to reduce the N-fixing ability of clovers
  • Where annual N application rates exceed 250 kg N/ha per year, a proactive strategy of soil testing and liming may be required to prevent soil acidification. Usually 2.5 tonnes of lime per hectare will be required every 2 to 3 years
  • If possible, apply N immediately after grazing or 3 to 5 days before grazing
  • Do not apply urea when climatic conditions are warm and dry or cold and excessively wet
  • Avoid applying N when pastures are waterlogged. Denitrification or leaching losses can be significant, and uneconomical pasture growth responses will occur
  • Avoid grazing for 3 weeks after N application
  • Avoid graze until the pasture reaches the 3-leaf stage, if possible
  • Ensure that the extra pasture grown is utilised
Fertiliser application – Cropping
  • Nitrogen requirement of cereal crops is driven by yield potential, where every tonne of grain produced requires 40–50 kg-N/ha. Cereal crops access nitrogen from 3 major pools:
    • Stable Organic Nitrogen (SON) is released slowly throughout the season, and is by far the largest nitrogen source in the soil. Approximately 2% of SON becomes available to crops during the season
    • Residue Organic Nitrogen (RON) is mineralised rapidly into NH4 + and NO3-, and is highest following legume crops
    • Fertiliser Nitrogen is applied to a crop by growers where the above sources cannot meet the needs of the crop
  • Due to the number of different nitrogen sources accessible to the crop, it is best to use models to gauge nitrogen status in the soil. Most current models measure the following soil and crop attributes to determine soil nitrogen status/ requirement:
    • Yield potential—determines nitrogen demand of the crop
    • Total organic carbon (%)—gives indication of potential SON contribution
    • Rotation—characteristic of the last legume crop (RON)
    • Soil type—gives indication of the potential for leaching
    • Rainfall—determines RON breakdown and contribution to leaching


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Other related questions in the Brown Book


Brown Book content has been based on published information listed in the Resources and References sections below

Resources
References
  • Nitrogenous Fertilisers. Department of Primary Industries Victoria.
  • Fertilisers for pastures. Department of Primary Industries, NSW.
  • What Nutrients do Plants Require?. Department of Primary Industries, Victoria.
  • Bluett, C., and Wightman,B. (1996)Cropping in South-West Victoria, Department of Primary Industries Victoria.
  • Mineral Nitrogen. - factsheet - Soilquality.org.au.
  • Fertilising Pastures – (Chapter 5).Greener Pastures for south west Victoria (2006) – Department of Primary Industries, Victoria.
  • Norton, R. (2010) Nitrogen application of wheat in southeastern Australia. International Plant Nutrition Institute.
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This project is supported by the Corangamite Catchment Management Authority, through funding from the Australian Government’s Caring for our Country

Page Updated: September 2013
Produced by AS Miner Geotechnical