Soil water measuring and managing: know what lies beneath
Author: Melissa Williams | Date: 20 Jun 2014
- Capacitance soil water probes can provide useful information about soil water data when used with adequate professional support.
- Computer simulation models predict soil water information based on soil tests, soil characterisations and climate/rainfall data.
- Both systems contribute to risk assessments of achieving target crop yields.
- Integrating on-site rainfall information and weather station data with either system will improve the accuracy of results.
Soil water levels and availability can be estimated using capacitance soil water probes and/or computer simulation modelling.
Geophysics tools, such as electromagnetic (EM) induction maps, are primarily used for soil mapping but can also provide crude estimates of soil water when multiple measurements are taken over time.
Soil water monitoring sites using capacitance probes usually have an associated weather station to track climatic conditions in the crop near the immediate site.
The increasingly reliable soil water data generated at these sites could then be used in crop yield forecasting models.
These can guide optimal top-up fertiliser, spraying, grazing and grain marketing decisions as the season unfolds.
An alternative to using soil water probes is to use simulation systems – such as the Agricultural Production Systems simulator (APSIM) and its online interface Yield Prophet®.
This model simulates stored soil water, crop water use and nitrogen use during the growing season and provides a risk assessment – through probability – of achieving a target grain yield.
Researchers are investigating the best ways to integrate information from soil water probes to the Yield Prophet® system for data validation and improvement.
Regardless of the tools used, to get the best out of measuring and monitoring soil water it is valuable to understand the underlying soil measurements.
The GRDC booklet ‘Estimating Plant Available Water Capacity’ contains a comprehensive step-by-step guide to measuring soil water and undertaking soil characterisation. It can be found at: www.grdc.com.au/GRDC-Booklet-PlantAvailableWater.
Soil water - what to measure
Drained upper limit (DUL) or field capacity
This is the amount of water a soil can hold against gravity. It can be measured opportunistically if the season is wet and the profile is full, or with controlled application of water at a soil characterisation site.
Crop lower limit (CLL)
This is the amount of water remaining after a crop has extracted all the water available to it from the soil. A soil core is taken at crop anthesis to provide data about the interim soil water status, which is then compared with information collected at crop maturity. Differences in these measurements provide data about rooting depth, water extraction at depth and potential chemical and physical constraints to root growth.
Bulk density (BD)
Bulk density is an indicator of soil structure and is usually expressed in grams/cm3. It is calculated as the dry weight of soil divided by its volume. This volume includes the volume of soil particles and the volume of pores among soil particles. Bulk density is measured from core samples using a bulk density testing kit. Samples are usually taken in conjunction with DUL sampling.
Plant Available Water Capacity (PAWC)
This is the difference between DUL and CLL in the crop root zone and indicates the total amount of soil water available to crops that a soil can store. Rooting depth and patterns are affected by soil properties and subsoil constraints.
Soil characterisation is a measurement of the PAWC of a soil at a representative point in the landscape.
It is usually representative of a broad section of the landscape that is considered to be a similar ‘soil type’.
Soil characterisation is about measuring the ability of the soil to hold water for use by a particular crop – or the size of the soil water ‘bucket’ – and takes into account potential constraints that affect the soil’s ability to store water and plant’s ability to extract it.
The importance of characterising soils
Soil characterisation data can be used to:
- Add to growers’ intuitive knowledge about soil water
- Fine-tune management of crop inputs
- Provide input into simulation yield modelling
- Calibrate soil moisture sensors.
How to characterise soils
- Soil characterisation includes information about:
- Soil chemistry (including pH, Aluminium (Al), Electrical Conductivity (EC), boron, sodicity and cations)
- Soil physical properties (including particle size, porosity and penetration resistance)
- Soil classification (WA soil group and Australian soil classification).
Soil characterisation can be determined by:
- Calculating PAWC from measures of DUL, CLL and BD
- Laboratory testing of soil core samples
- Estimating PAWC based on knowledge of water holding capacity of a particular soil
- Multi-year interpretation of soil moisture probe data.
Characterisation of WA soils
Across WA, with GRDC funding, CSIRO researchers, the Department of Agriculture and Food WA (DAFWA) and local grower groups and consultants are working to characterise local/regional soils that are not yet well characterised. This will include 105 sites across the State during the next three years through the new GRDC ‘Measuring and Managing Soil Water’ project. Heavy, gravel and duplex soils that have been hard to characterise in the past are being targeted initially.
Selecting soil water monitoring sites
Monitoring sites should be selected to represent the important soils of a property or a local area.
This requires good local knowledge and can backed-up by spatial tools, such as EM maps, and support from soils experts.
It may be necessary to characterise several sites in a landscape to represent soil variability.
Soils are highly variable and characterisation of a ‘soil type’ for PAWC will only ever be a good estimation at that point and a reasonable estimation of the surrounding soil.
Sites should be selected based on whether:
- The soil is of regional importance or of particular interest to a grower or consultant group
- There is the likelihood of local logistic support
- There is sufficient land area to enable measurement of DUL and CLL at the same site
- There is a distance of at least two to three tree heights from any trees
- There is opportunity to add to existing data sets
- Note that soil moisture probes require mobile phone coverage sufficient to send/receive text messages to transmit data back to a web-based server.
Caption: Capacitance soil water probes can provide soil water information when used with professional support.
GRDC Project Code RAI00003, CSP00170, CSP00111, RCSNEsp/Alb12/13JointProj1, SDI00015, LIE00006, CSA00023, RAI00003, ERM000002