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 5.5 – 7.5 (CaCl2). Maintaining pH above 4.8 in the subsurface will prevent aluminium toxicity which restricts root growth down the soil profile. Application of agricultural lime is effective in treating soil acidity and maintaining appropriate soil pH (see Soil Acidity and Making Sense of Chemical Indicators fact sheets).
Subsoil compaction can reduce rooting depth of plants by slowing the rate of root penetration. This means roots are unable to access leachable nutrients such as nitrogen. This can result in poor nitrogen use efficiencies and higher rates of soil acidification. Deep ripping is an effective method of decreasing subsoil compaction. Evidence suggests large areas of WA soils are responsive to deep ripping (table 1). Using deep working points at seeding time can also relieve some of the compaction (see Subsurface Compaction and Controlled Traffic Farming fact sheets).
Table 1: Average increase in wheat grain yield after deep ripping. (Source: Steve Davies DAFWA, Crabtree 1989, Davies et al., 2006, Jarvis, 2000.)
Average increase in yield
Various, mostly sands
Yellow loamy sands
Duplex with A horizon < 30 cm
Duplex with A horizon > 30 cm
Nitrogen fertilisers are a significant expense for broadacre farmers so optimising use of fertiliser inputs can reduce this cost. There are four main sources of nitrogen available to crops: stable organic nitrogen, rotational nitrogen, ammonium and nitrate. 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 “Select Your Nitrogen” (DAFWA) or NuLogic (CSBP).
Stable organic nitrogen is a significant supply of nitrogen to cereal crops during the growing season. Approximately 2 % of the stable organic nitrogen in soil is made available to the crop each year. This is greater in years when there is summer rainfall, because rainfall increases the period over which nitrogen is mineralised (table 2). Most nitrogen models estimate the amount of stable organic nitrogen in soil from the amount of total organic carbon in soil.
Table 2: The estimated amount and value of nitrogen supplied by stable organic nitrogen in soils with different amounts of total organic carbon in years with and without summer rain (based on the price of urea being $500/t, derived from “Select Your Nitrogen” model DAFWA).
Total organic carbon (%)
No summer rain period
Legumes in the rotation can supply significant nitrogen to cereal crops for up to 4 years after they are grown (table 3). 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. Growers need to take this nitrogen into account when comparing the benefits of a legume in rotation.
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.
Table 3: The amount and value of nitrogen supplied by legumes in the rotation in sand soils in Western Australia (derived from “Select Your Nitrogen” model DAFWA).
Crop biomass (t/ha)
Amount and value of N in years after legume in rotation
Crabtree WL (1989) Cereal grain yield responses to deep ripping on duplex soils. Australian Journal of Experimental
Agriculture 29: 691-694.
Davies SL, Gazey C, Gilkes R, Evans D, Liaghati T (2006) What lies beneath?—Understanding constraints to productivity below the soil surface. In ‘Geraldton Crop Updates, 2006’. Department of Agriculture and Food, Western Australia.
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.
Jarvis R (2000) Deep tillage. In ‘The Wheat Book: Principles and Practice’ (Eds WK Anderson, JR Garlinge) pp 185-187. Agriculture Western Australia Bulletin 4443.
Author: Richard Quinlan (Planfarm Agronomy Pty Ltd).
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