Over 98 % of all nitrogen (N) present in the soil is in an organic form, which is mineralised by microbes into inorganic forms (2 % of all N) such as nitrate and ammonia. Mineralisation occurs throughout the growing season providing a steady, continuous supply of nitrogen to the crop.
The positive charged ammonium ion (NH4+) is immobile in soil, does not leach, and is an intermediary in the conversion of organic-N to nitrate (NO3-). Nitrate on the other hand has a negative charge and is highly mobile in the soil. This mobility provides a nitrogen source that moves readily towards plant roots, but can also be leached out of reach of the plant root system (figure 1). Nitrate is used by the plant to make precursors to plant proteins.
Figure 1: Principle nitrogen cycling pathways in a mixed cropping/pasture system (adapted from Peverill et al., 1995).
As proteins make up much of the content of cells, nitrogen is needed in greater quantity that any other mineral nutrient. Nitrogen plays an essential role in the production of chlorophyll, and any deficiency is displayed as yellowing leaves and reduced tillering in cereal crops (figure 2). This ultimately leads to reduced yields.
Nitrogen is highly mobile within the growing plant allowing it to re-mobilise and move to tissues that can use it more effectively. As a result, older leaves tend to exhibit nitrogen deficiency symptoms first.
Figure 2: Nitrogen deficiency symptoms in wheat plants (right) (Snowball and Robson, 1988).
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:
Models that use this information are SYN (Select Your Nitrogen) from the Department of Agriculture and Food, Western Australia, and NuLogic from the commercial fertiliser company CSBP.
A simple method to determine whether the correct nitrogen regime was used is to measure grain protein following harvest. The optimum protein percentage in wheat grain varies between cultivars, however it is a useful tool for auditing nitrogen budgets. Increasing N rates to achieve protein levels above these thresholds results in lower economic responses, as protein payments and yield do not offset the extra cost of nitrogen. Conversely, if crops consistently achieve protein levels below these levels, they are forgoing optimum yield potential.
In the past, Western Australian growers have had little reason to test for NO3- and NH4+ in the soil, as little mineral nitrogen is present in the soil over summer. However, in some years summer rain results in mineral-N release from organic matter and soil testing helps in determining the worth of this available nitrogen. This can be viewed as a pre-emergent application and can be a significant N source in some years; however it is also prone to leaching if heavy rainfall occurs before crop establishment.
Once organic-N is converted to nitrate it is prone to leaching, particularly in sandy textured soils in high rainfall zones where soil compaction problems slow root growth. Other subsoil constraints, such as soil acidity, may also reduce the efficiency of uptake of
NO3- by the crop. Finer textured soils (e.g. red loams) are less likely to suffer significant NO3- to leaching, allowing efficient use of available nitrogen.
Grain yield improvements are mainly caused by increased tiller numbers and grains per ear, both of which are determined early in the life of a wheat plant. This places importance on a sufficient supply of nitrogen during crop emergence and establishment. Due to potential leaching, nitrogen use efficiency on sandy soils can be improved by delaying fertiliser application until the crops roots system is adequately developed. This can be 3 – 4 weeks after germination.
Later nitrogen applications can also have yield benefits through increased tiller survival, leaf duration and photosynthetic area. Delaying application however, reduces the chance that economic response to nitrogen will be achieved. An advantage of late applications (1st node visible) is that growers have a better idea of yield potential before applying the nitrogen.
Anderson WK and Garlinge JR (2000) The Wheat Book: Principles and Practice. Agriculture Western Australia Bulletin 4443, Agdex 112/01.
Bowden B Personal Communications. Department of Agriculture and Food, Western Australia.
Cook RJ and Veseth RJ (1991) Wheat Health Management. The American Phytopathological Society Press.
Peverill KI, Sparrow LA and Reuter DJ (1995) Soil Analysis an Interpretation Manual. CSIRO publishing.
Snowball K and Robson AD (1988) Symptoms of Nutrient Deficiencies in Subterranean Clover and Wheat. (The University of Western Australia).
Author: Richard Quinlan (PlanFarm Agricultural Consultancy) and Andrew Wherrett (Department of Agriculture and Food, Western Australia)
This soilquality.org.au fact-sheet has been funded by the Healthy Soils for Sustainable Farms programme, an initiative of the Australian Government’s Natural Heritage Trust in partnership with the GRDC, and the WA NRM regions of Avon Catchment Council and South Coast NRM, through National Action Plan for Salinity and Water Quality and National Landcare Programme investments of the WA and Australian Governments.
The Chief Executive Officer of the Department of Agriculture and Food, The State of Western Australia and The University of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.