Optimising Soil Nutrition – Queensland
- Most soils contain reserves of nutrients that would otherwise have to be applied in fertilisers.
- Growers can minimise fertiliser inputs by optimising the use of these nutrient reserves by plants.
- Maximising root abundance and rooting depth means roots can take up nutrients and water in the subsoil.
- Use of soil reserves of nitrogen can be optimised by testing for the different forms of nitrogen and using the results when deciding on fertiliser application rates.
- Good yield estimates are critical when estimating nitrogen requirements.
Maximising root abundance and rooting depth
One way to optimise plant uptake of nutrient reserves in soil is to maximise root abundance and rooting depth. This means that crop roots can explore more soil and ensures they can follow nutrients and water down the profile. This is especially important for leachable nutrients such as nitrogen and sulfur and nutrients with significant reserves in the subsoil. Root abundance and rooting depth can be increased by maintaining optimum soil pH and minimising soil compaction and plant root diseases.
Nutrient availability is optimised in soils with topsoil pH levels in the optimum range pH 6.0 – 7.5 (CaCl2). Maintaining pH above 5.2 in the subsurface will prevent aluminium toxicity which restricts root growth down the soil profile (figure 1).
Figure 1: Roots of barley grown in acidic subsurface soil (right) are shortened by aluminium toxicity.
Application of agricultural lime and appropriate management of nitrogen fertilisers are effective strategies for managing soil acidity and maintaining appropriate soil pH (see Soil Acidity—Queensland and Making Sense of Chemical Indicators—Queensland factsheets).
Subsoil compaction can reduce rooting depth of plants by slowing the rate of root penetration (figure 2). This means roots are unable to access leachable nutrients such as nitrogen. This means roots are unable to access subsoil moisture and leachable nutrients such as nitrogen. This can result in poor nitrogen use efficiencies.
Figure 2: A distinct compacted layer in a sandy loam, note fractures in hard pan through which roots preferentially grow.
Avoiding traffic on the site when soils are moist or wet, restricting traffic to defined wheel tracks (controlled traffic farming) and/or minimising the number of tillage operations are all strategies that can be used to reduce or avoid subsoil compaction (see Subsurface Compaction and Controlled Traffic Farming fact sheets).
Plant root diseases and pathogens
Plant root diseases and pathogens can severely decrease root exploration and nutrient uptake. Root lesion nematodes, and crown rot are common in Queensland cropping regions.
The use of break crops to prevent the build up of disease/ pathogens (figure 3), growing tolerant crops to maximise yield when nematodes or diseases are present, and practicing good farm hygiene are some of the most effective strategies to manage plant root pathogens and diseases (see Root Lesion Nematode—Queensland and Crown Rot—Queensland factsheets).
Figure 3: Intolerant wheat variety showing symptoms of root leision nematode infection. There are no obvious symptoms in the chickpea and faba bean break crops on either side of the wheat (Photo by Kirsty Owen, DEEDI.)
Optimising nitrogen use efficiency
Nitrogen fertilisers are a significant expense for broadacre farmers so optimising use of fertiliser inputs can reduce this cost. There are three main stores of nitrogen within the soil with the potential to supply nitrogen to crops: nitrogen in soil organic matter, nitrogen in plant residues and mineral nitrogen (ammonium and nitrate) present in the soil. To optimise plants’ ability to use soil nitrogen, growers should first be aware of how much of each source there is. The best method of measuring these nitrogen sources is soil testing. These results can then be used to determine fertiliser rates with models such as WhopperCropper and ‘The Nitrogen Books’ (see Nitrogen—Queensland factsheet).
Nitrogen in soil organic matter
The release of nitrogen during the decomposition of soil organic matter is a significant supply of nitrogen to crops during the growing season. Most nitrogen models estimate the amount of soil organic nitrogen in the soil from the amount of total organic carbon present.
Nitrogen in plant residues
The decomposition of plant residues, and particularly legume residues, can supply significant amounts of nitrogen to cereal crops. Generally the amount of nitrogen fixed by legume crops is proportional to the amount of vegetative growth and the amount of grain removed (i.e. the harvest index). In loam or clay soils, legumes in rotation will have slightly higher levels of available nitrogen due to less leaching than in sand soils. An example of the amount of nitrogen in the soil following different types of legume rotations at a southeast Queensland trial site can be seen in Table 1.
Table 1:Effects of different rotations on soil total nitrogen (N) (source: Hossain et al. 1996).
|ROTATION||NUMBER OF||TOTAL SOIL N|
|WHEAT CROPS||0-30 cm (t/ha)|
|Grass/legume ley (4 years)||0||2.91|
|Lucerne ley (1-2 years||2-3||2.56|
|Annual medic ley (1-2 years)||2-3||2.49|
|Chickpeas (2 years)||2||2.35|
|Continuous wheat ( 4 years)||4||2.36|
Estimating yield potential
Estimating cereal requirements for nitrogen (and other nutrients) requires an effective method of estimating yield potential. For every 1 tonne of grain, a cereal crop requires 45 kg N. Currently there are a number of methods of estimating yield including the online Wheat Yield Potential Calculator based on the French Schultz equation (French and Schultz, 1984) and the Yield Prophet model by CSIRO/Birchip Cropping Group.
Further reading and references
French RJ and Schultz JE (1984) Water use efficiency of wheat in a Mediterranean-type environment. The relationship between yield, water use and climate. Australian Journal of Agricultural Research 35: 743-764.
Hossain SA, Dalal RC, Waring SA, Strong WM and Weston EJ (1996) Comparison of legume-based cropping systems at Warra, Queensland. I. Soil nitrogen accretion and status, Australian Journal of Soil Research 34: 273–287.
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.