Fertilisers can account for up to 40% of the variable costs of growing a crop. Hence it is well worth investing money in accurate soil testing and agronomic advice to help balance the potential financial benefits with the risk of short-term investment in fertiliser product that may occur in dry years.
(Note: this article applies to the northern grain region of Australia, predominantly Black and Grey Vertosol soils)
- A deficiency of ONE nutrient may limit the economic response to other applied nutrients.Check ALL relevant soil nutrient levels
- All soil nutrient levels are declining and responses to potassium (K) and sulphur (S) are appearing (especially when stubble is removed).
- Nitrogen (N) fertiliser requirement will be approximately 1.7 times the N removed in the grain
- › Measure soil N to rooting depth especially in less predictable situations e.g. after a long-fallow, double-cropping or after pulse crops, and top up with inorganic N
- › Up to 40% of soil N can be lost by denitrification when there is: waterlogging, high soil N, warm temperatures and fresh crop residue
- Potential response to P fertiliser is determined by soil P level.
- › Soil test for both Colwell and BSES P in both 0 to 10cm and 10 to 30cm soil layers.
- › Response likely if Colwell P < 25mg/kg in higher-yielding areas or <15mg/kg in lower-yielding environments
- › The BFDC (Better Fertiliser Decision for Cropping) database can help determine the critical level per crop per region
- Soil K exchangeable levels < 0.3 to 0.4meq/100g are often responsive to K fertiliser. Chickpeas are good indicators of deficiency.
- S deficiency is not widespread and mostly seasonal.
- Strategic use of Zn fortified starter products. In general, levels of 0.8mg/kg on soils pH> and 0.4mg/kg on soils with pH<7 are the critical levels
- Consider plant tissue and grain analysis to determine sufficiency of nutrient supply.
The first place to start in planning a nutrition program is not fertiliser, but water.
Useful questions are:
1. How much water is stored in your soil?
The amount will greatly affect the yield potential and may influence your fertiliser rate decision. See the link to the CropARM software which is the web-based version of WhopperCropper www.armonline.com.au . With this tool you can create ‘what-if’ scenarios that will show the effect of stored soil water (and many other factors including nitrogen fertiliser), on your crop of interest at a district near you.
2. How much rain will I need to plant?
Depends on the intensity of the rainfall and the amount of cracking. As a rule of thumb about 20 to 25mm of rain will be required to replenish each dry 10cm soil layer.
3. What are the soil phosphorus levels in the 0 to 10cm AND 10 to 30cm layers?
Sorghum is very efficient at extracting P from the soil so in water-limited situations, grain yield responses may not occur even though a response in dry matter may be seen. Sorghum exports approximately 3kgP/t of grain with 5kgP/t taken up in the whole plant. In soils with a low yield potential or a 3t/ha crop, approximately 15kg/ha P will be taken up, of which, 2 kg/ha comes from surface soil. Therefore a fertiliser rate that supplies 6kg/ha of P (e.g. 30kg/ha MAP) will be an adequate ‘Starter’ rate but the crop will draw on P reserves at depth to supply the remaining 9kg/ha. In higher yielding areas (6 t/ha) a similar starter rate may be warranted as well as, additional P fertiliser placed at depth.
Hence it is VERY IMPORTANT to test the soil in the 10 to 30cm layer. If the Colwell-P is < 10mg/kg it is likely that a yield response of up to 25% will occur if secondary roots develop effectively (see Table 1).
4. General critical levels for Starter and deep P
Table 1 below shows an example of critical P values used to determine likely response or drivers of P availability in Darling Downs soils) Source: Mike Bell 2012.Table 1
Critical levels for crops and soils in other areas may differ from those in Table 1. GRDC funded research is currently underway to fill in the current knowledge gaps.
5. What are the soil nitrate-N levels to the depth that roots will explore
Sorghum removes approximately 15 kgN/t grain (9.5% protein) and requires 25kgN/t to grow.
The nitrogen fertiliser rate required can be calculated using one of the many budgeting tools (e.g. NitrogenARM www.armonline.com.au) that calculate the difference between the soil supply of N (calculated from a soil test) and the expected demand. The nitrogen demand is calculated by:
N demand = grain yield (t/ha) * grain protein % * 10/6.25 * 1.7
e.g. N demand = 5 * 9.5 * 10/6.25 * 1.7
Note: the factor 1.7 is used for the ‘optimum’ grain protein of 9.5%
= 129 kgN/ha (76kgN/ha in grain)
Subtracting the available nitrate-N in the soil will give the fertiliser rate required. That is why soil testing (preferably to 90cm or the depth of effective moisture extraction) prior to planting is suggested.
In the fallow between summer crops, approximately 30 to 50 kgN/ha could be released from a moderately fertile soil (1.2% organic carbon).
After a long fallow, soil nitrate-N levels may be high, especially if following a pulse crop such as chickpeas. A prior chickpea crop may supply up to 50kgN/ha or more to the subsequent cereal and increase yield 20% to 100% depending on other rotational benefit`s gained e.g. disease break.
6. Crop grain removal
The following table summarises the grain removal quantities and can be used as base for rates of nutrient replacement for nutrients such as P, K and S.
By Howard Cox (Senior Agronomist DAFF Qld) with edits by Kaara Klepper and Chris Dowling.
Photos courtesy of Howard Cox and the GRDC.