Improved soil water management offers productivity boost
Author: Sarah Jeffrey | Date: 14 Aug 2014
- A greater understanding of rainfall capture efficiency and soil water management will help dryland growers maximise crop productivity.
- Understanding how soils work in relation to water capture and storage, and the seasonal monitoring of soil water enables the setting of realistic decisions/goals on crop choice, inputs and yield potential.
- Tools are now available to better define the productive capacity of soils and for the routine seasonal monitoring of water resources for capture and utilisation by crops or weeds.
- Research suggests pre-crop management is more important than in-crop management in lifting the water use efficiency (WUE) and yield of wheat cropping systems.
Improvements in the efficiency of rainfall capture and soil water management promise to deliver valuable productivity benefits to growers in the northern cropping region.
Research being undertaken as part of the Grains Research and Development Corporation’s (GRDC) Water Use Efficiency Initiative is showing that pre-crop management is more important than in-crop management in lifting the water use efficiency (WUE) and yield of wheat cropping systems.
According to the research, it is the pre-crop practices of fallow weed management, rotation choice, long-term stubble retention and reduced tillage which are likely to have the greatest impact on fallow water storage and therefore WUE and crop yield.
Put simply, it is the combining of ‘best’ practices that will contribute to the more efficient storage and use of available water.
Addressing growers and agronomists at the GRDC Northern Research Updates in central western NSW earlier this year, CSIRO Researcher Neal Dalgliesh said while the benefits of stored soil water to crop production were well documented, there was less understanding over the processes involved in the capture and storage of rainfall for future crop production.
“There is a real need for farmers and advisors to have a good understanding of how soils work in relation to water capture and storage and why some soils have the ability to hold more water for crop use than others,” he said.
“Such baseline information collated in conjunction with the seasonal monitoring of soil water, using tools that can range from something as simple as a push probe to simulation modelling allows the setting of realistic decisions/goals on crop choice, inputs and yield potential.”
Tools to monitor stored soil water have been used by growers for many years however Mr Dalgliesh said additional tools were available to better define the productive capacity of soils and for the routine seasonal monitoring of water resources for capture and utilisation by crops or weeds.
He said those growers wanting to gather and utilise more detailed information for specific productivity projections and management decisions could easily calculate millimetres of water available according to soil type.
“The level of sophistication at which the grower invests in soil monitoring and management is at their discretion. However it should be remembered that while the use of higher level technologies can provide easier and more accurate real-time access to soil water information and to risk management tools such as simulation modelling, value can also be found in the more thoughtful use of simple devices such as the push-probe,” Mr Dalgliesh said.
“Tools for the characterisation of plant available water capacity (PAWC) and the monitoring of soil water have been available for many years although there are changes occurring which will impact on how water is measured and decisions made.”
PAWC is a measure of the ability of a soil to store water for later crop production and is affected by soil texture, soil depth as well as sub-soil chemical constraints such as salinity and sodicity and nutrient deficiency/toxicity resulting from high/low pH. These sub-soil chemical constraints are common in northern soils and impact a plant’s efficiency to access or extract soil water.
Twenty years of collaboration between CSIRO, the state departments, consultants, industry groups and individual farmers, with support from GRDC, has resulted in approximately 1000 Australian cropping soils being characterised for PAWC.
This data is provided to users through the APSoil database and its Google Earth derivative, available for download at http://www.apsim.info/Products/APSoil.aspx, and ‘SoilMapp’, an IPad application which is available from the Apple On-line Store. These data also form the basis for crop decision support using APSIM and Yield Prophet.
If relevant soil information was not available for a particular area, Mr Dalgliesh said one of the simplest methods to determine PAWC was for a consultant or farmer to make judgement calls on when a soil profile was fully wet to depth and additionally, when it was dry after the growing of a particular crop.
“Measurement of soil water content at these two points will give a reasonable estimate of the upper and lower limits of extraction and will allow a calculation of PAWC which will suffice for most uses,” he said.
A range of soil water monitoring devices was recently tested for practical applicability in dryland systems on both the cracking clay soils (Vertosols) and the southern rigid soils through a GRDC funded project.
The study concluded that no one device or field based technology was appropriate to all situations. It found in-situ capacitance or time-domain reflectometry (TDR) sensor probes were likely to provide a reasonable estimate of soil water in rigid soils whereas they faced difficulties in the shrink/swell clays due to soil cracking.
The surface based, EM38 (Electromagnetic induction) mobile device was found to be a more practical and accurate option for these soils.
Soil water modelling (APSIM model) was also shown to adequately predict soil water levels when compared against the outputs of the in-situ technologies and was recommended as an alternative to field based monitoring.
“There are positives and negatives associated with all of the tested devices and no one is likely to meet all requirements,” Mr Dalgliesh said.
“The final decision will be up to the individual and will be based on their views on accuracy, the frequency and timeliness of information delivery, soil type and salinity level and the investment and continuing commitment required.”
Management options to improve PAWC and WUE have been a focus of recent research through the WUE Initiative.
This research has shown that summer weeds are costing growers dearly in terms of available soil water and nitrogen for subsequent crop production.
An average increase of 60% in seasonal WUE was found across 21 trial sites in South Australia, Victoria and NSW where weeds were controlled during the summer fallow, a result of the improved efficiency of storage of summer rainfall and increased nitrogen supply to the subsequent crop.
In terms of the impact of stubble cover, the research indicates that stubble retention does not reduce fallow evaporation levels when present in the quantities normally seen in dryland agriculture and, as a consequence, affects neither the quantity of water stored during the fallow nor crop yield.
However, this does not discount the benefits of stubble in improving the efficiency of water infiltration and reducing water and wind erosion, Mr Dalgliesh said.
Controlled traffic (CT) farming also offers benefits in reducing soil compaction and increasing infiltration and fallow efficiency with the research showing an additional 63mm of water was captured annually in a system which had not been affected by compaction.
“Knowing something about a soil’s ability to store water can have impact on management. For example, knowing the storage capacity of a Red Calcareous soil may change thinking on fallow length, or knowing the capacity of a soil to hold water, or the amount of available water present at a particular seasonal decision point may change thinking on crop input investment such as fertiliser,” Mr Dalgliesh said.
“This can be taken further through the use of Yield Prophet which provides the grower with a means of exploring the riskiness of cropping options, both production and economic, based on knowledge of the soil resources present (water and nutrients) and the long term climate information.”
Neal Dalgliesh, Research Consultant CSIRO Toowoomba
Sarah Jeffrey, Senior Consultant Cox Inall Communications
GRDC Project Code CSP00170