Nutrition
The natural fertility of cropped agricultural soils is declining with time and grain growers need to continually review their management programs to ensure the sustainability of grain production. Pasture leys, legume rotations and fertilisers all can have important roles in maintaining the chemical, biological and physical fertility of soils.
With more frequent opportunity cropping, improved farming techniques and higher yielding varieties, paddock nutrition programs should be reviewed regularly. Paddock records, yield monitoring, remote sensing images, fertiliser test strips, crop inspection and monitoring, benchmarking water use efficiency, and soil, plant tissue and grain testing can assist in formulating efficient, sustainable cropping programs.
Whilst rotations with grain legumes and ley pastures have an important role to play in soil fertility, fertilisers remain the major source of nutrients. Nutrient input programs should aim to supply a balance of the required nutrients in the amounts needed to achieve a crop’s yield potential. Nutrient removal in grain is the main pathway for fertility rundown, with high-yielding crops exporting large amounts of nutrients.
Common nutrient deficiencies in Queensland´s broadacre grain areas are nitrogen (N), phosphorus (P), potassium (K) and zinc (Zn), while sulfur (S), copper (Cu) and molybdenum (Mo) may be also be lacking in some soil types and growing areas.
Nutrient removal
Ultimately, nutrients removed from paddocks will need to be replaced at some point to sustain production. In irrigated cropping, large quantities of nutrients are removed and growers need to adopt a strategy of programmed nutrient replacement, but dryland growers should also consider this approach. Table 1 below gives the average nutrients removed by various crops in both grain and hay.
The yield potential of a crop will be limited by any nutrient the soil cannot adequately supply. Poor crop response to one nutrient can often be linked to a deficiency in another nutrient or other management technique. Crop response can also be linked to soil constraints such as acidity, sodicity or salinity, problems with beneficial soil microorganisms or pathogens such as nematodes.
To attain optimum yields, an adequate supply of each nutrient is necessary. However, it is important to realise that only a small proportion of the total amount of an element in the soil may be available for plant uptake at any one time. For nutrients to be readily available to plants, they must be present in the soil solution (the soil water), or easily exchanged from the surface of clay and organic matter particles in the root zone, and be supplied when and where the plant needs it.
Temperature and soil moisture content will affect the availability of nutrients to plants, and the availability of nutrients will also depend on soil pH, degree of exploration of root systems and various soil chemical reactions, which vary from soil to soil. Fertiliser may be applied in the top 5-10cm, but unless the soil remains moist the plant will not be able to access it. Movement of nutrients within the soil profile in low rainfall areas is generally low except in very sandy soils.
Lack of movement of nutrients combined with current farming methods (e.g. zero till) is resulting in stratification of nutrients, where concentrations of nutrients are building up in the surface of the soil where they are not always available to plants, depending on the seasonal conditions. Often, on the Western Downs and in Central Queensland, wheat is deep sown in moisture that is below the layer whereby nutrients have been placed or are stratified, which has implications for management and fertiliser practices.
