Through a combination of variety and agronomy upgrades crop yields are rising, especially when growing conditions are good.
For many farmers two consecutive high yielding years (2020, 2021) has resulted in an enormous export of nutrients per hectare via these crops. Big drawdown in soil nutrients, especially nitrogen. phosphorus, potash and sulphur are particularly significant.
Two consecutive high yielding wheat grain crops, for example 7.0 t/ha, at 12 percent protein, results in loss or export off farm of 320 kg/ha nitrogen.
Even two modest wheat yields, for example 4.0 t/ha, at 12 percent protein, results in export off farm of 184 kg/ha nitrogen.
A 3.0 t/ha canola crop, followed by a 7.0 t/ha wheat crop would have removed around 281kg/ha nitrogen. These are big numbers.
Dual purpose grazing and grain crops, use even higher nutrient levels.
For example two consecutive wheat crops providing 4.0 t/ha grazing drymatter, plus 6.0 t/ha grain, each season would use around 532 kg/ha of nitrogen. 312 kg/ha of this nitrogen would have been exported off farm via grain and meat.
Phosphorus removal via high yield isn't as startling as nitrogen, but is high. Two consecutive 7.0 t/ha wheat crops remove around 42 kg/ha phosphorus and two 4.0 t/ha crops remove 24 kg/ha. A 3.0 t/ha canola crop plus 7.0 t/ha wheat crop also removes around 42 kg/ha phosphorus.
Potassium is also an element used in big quantities by crops, but so far not generally regarded as deficient in most cropping areas. Two 7.0 t/ha wheat crops would remove around 21 kg/ha potassium and a 3.0 canola crop plus a 7.0 wheat crop would also remove around 21 kg/ha. Sulphur is also an element to watch in long term higher yielding cropping programs. Two consecutive 7.0 t/ha wheat crops would remove around 21 kg/ha and a 3.0 t/ha canola crop followed by 7.0 t/ha wheat crop would remove around 22 kg/ha sulphur.
Other factors may well play a role in assessing fertiliser needs for 2022 crops. Waterlogging can especially contribute to less available nitrogen and sulphur as both can be quite leachable. Denitrification (loss of nitrogen) can also be an issue in water logged soils, especially those long waterlogged. At the other extreme crops lost through flooding commonly rot down into the soil with nutrients recycling.
Long term cropping only rotations commonly remove more nitrogen than is being returned to the soil via fertiliser input and the periodic pulse crop.
High yielding pulse crops, especially those with good bulk (e.g. faba beans and sometimes peas and lupins) can often contribute good but not necessarily high amounts of nitrogen. Declining nitrogen can be an ongoing challenge and not only affects yield but also soil carbon levels.
However many long term cropping systems are covering well for phosphorus with current fertiliser application rates. Especially in northern areas assessing soil phosphorus not only in the top soil, but also in the subsoil is important. Zero till cropping also seems to be an important factor for assessment as more is likely to be stratified in the top soil layer.
Clearly with fertiliser costs so high at the present, and likely into 2022, as accurate assessment as possible will be important for determining most economical fertiliser rated for the coming year. If good subsoil moisture reserves can be maintained, even an ordinary year could spell another potentially good crop season in 2022.
Soil testing via Australian accredited laboratory's is a good starting point for assessing likely fertiliser need for the coming winter crop. Both top (0 - 10cm depth) and sub soil (at least to 50 - 60cm) testing is especially important for nitrogen assessment. Nitrogen can be mobile, especially on lighter soils. Sometimes available nitrogen is leached down the profile, with a spike at a subsoil layer.
Estimates of soil nitrogen also needs to take into consideration variable rate of soil nitrate conversion (from the total nitrogen pool) during fallowing and crop development. It can depend on aspects like quality and quantity of organic matter, temperature as well as soil moisture.
Next week: Planning pasture fertiliser needs with high fertiliser costs.
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