Stored soil moisture critical for summer planting opportunities
- Stored soil water can be measured while in-crop rainfall can only be estimated based on probabilities and historical climate data.
- Summer crop planting decisions should be based on what is known.
- Risk can be reduced by setting profitable crop yield targets, calculating the amount of water needed to achieve these yield targets and assessing that against available stored soil moisture and the probability of receiving additional moisture through in-crop rainfall.
Unseasonably hot weather and below average rainfall during October and early November has severely restricted spring planting opportunities and left growers weighing up their options for the summer cropping season.
Without significant and widespread rain, stored soil moisture promises to play a key role in dictating planting decisions and should be considered like “money in the bank” according to CSIRO senior research scientist Dr Jeremy Whish.
“Calculating available soil moisture is a risk management tool – avoiding a failed crop saves money now and saves stored moisture for future crops,” Dr Whish said.
Research conducted by Dr Whish and funded by CSIRO and the Grains Research and Development Corporation (GRDC), reinforces the benefits of setting profitable crop yield targets, calculating the amount of water needed to achieve these yield targets and assessing that against available stored soil moisture and the potential for in-crop rainfall.
“Having a target to aim for improves the decision making process, allowing the risk associated with planting under current conditions to be compared against future planting opportunities when conditions may be less risky,” Dr Whish said.
“Planting later increases the length of fallow allowing time to store water in the soil before planting but it has other flow on effects such as reducing options for double crops and reducing fallow length for the next crop. These issues must be considered from a whole of farm management perspective.”
To give an example, around 166mm of water is required to grow a tonne of sorghum in the Condamine region. According to Dr Whish approximately 100mm of this water will be used to establish the plant or lost as evaporation while the remaining 66mm will be used to produce grain at an average rate of 15kg per mm.
In some years this conversion will be better (around 28kg/mm) and in some years it will be worse (around 6kg/mm). The conversion rate is unknown because it depends on factors such as temperature, humidity and the timing of rainfall experienced by the crop.
“The water provided to the crop can come from water stored in the soil during the fallow or from in-crop rain,” Dr Whish said.
“When making a decision stored water can be measured but in-crop rainfall is unknown. This is why estimating a final yield based on the water stored in the soil plus a conservative estimate of potential in-crop rainfall is a good way of assessing the risk of planting a crop.”
While calculating potential crop yields based on total rainfall and soil water works as a simple estimate and guide for planting decisions, Dr Whish emphasised that it did not consider the impact of rainfall timing on crop demand like more sophisticated modelling tools such as APSIM and Yield Prophet®.
Using an example target yield of 2.4 tonnes/hectare on a Brigalow soil in the Condamine region, the amount of water required would be:
2400 (kg/ha) divided by 15 = 160 mm of water for grain + 100 mm for establishment and evaporation = 260 mm. Therefore the total water requirement is 260mm.
“If the current amount of water stored in the soil is 100mm (~60cm wet soil by push probe) then 160 mm needs to fall during the crop growing period. As a rough guide for a November planting at Condamine, the average rainfall during the growing season would be around 244mm and there is an 80% chance of getting 158 mm,” Dr Whish said.
“Water stored during the fallow has a higher value than rainfall because it is available when the plant needs it, provided that reserves are sufficient.
“In terms of risk management, the greatest value of stored water is in knowing the quantity available for crop production, before the crop is planted, when decisions are being made.
“It is also why managing fallow stubble cover and weeds is so important.”
For more information on the GRDC/CSIRO research visit www.grdc.com.au/Research-and-Development/GRDC-Update-Papers/2014/08/Sorghum-yield-risk-vs-starting-soil-moisture.
Caption: CSIRO Senior Research Scientist, Dr Jeremy Whish says summer crop planting decisions should be based on what is known.
Dr Jeremy Whish, CSIRO Senior Research Scientist
07 46881419 / 0428 763426
Sarah Jeffrey, Senior Consultant Cox Inall Communications
GRDC Project Code CSP00146; ERM00002; CSA000050