Irrigation scheduling involves applying the right amount of water, in the right place at the right time, in order to maximise production and improve water use efficiency.

Growers like Andrew Parkes of "Keytah", Moree, and Von Warner, manager of "Bullamon Plains", Thallon, Queensland, commonly use soil moisture monitoring tools to assist with their scheduling decisions.

These tools need to be placed in the part of the field most representative of the majority soil type. A moisture probe placed in the wrong spot can result in a field being over or under irrigated.

Traditionally, growers like Von and Andrew sited their probes visually, based on experience or gut feeling. However, today technology to look below the surface is available and allows growers to build a more defined and accurate picture of the majority soil types.

Electromagnetic Induction (EM or EMI) surveying, used in conjunction with soil sampling, can be used to map soil variations across fields and farms. It does this by measuring the soil’s apparent electrical conductivity, which is related to factors such as soil texture, soil moisture and salinity.

Ground truthing is essential to calibrate the instrument. This involves collecting and analysing soil samples from known positions and relating the results to the EM readings.

An EM survey can give an indication of texture changes over the field. Analysis of the data provides maps of similar soil types and consequently can be used to locate the "majority" soil type within a field.

Andrew and Von are convinced about the benefits of EM soil surveys on their farms. Both growers have used calibrated EM maps to examine soil variability across their fields, to position moisture probes in sites that are representative of the majority soil type, year in and year out.

"Using EM surveys to assist with siting moisture probes has given me more confidence with my scheduling decisions," Von said.

Moisture probes are just one tool he uses to schedule irrigations.

"Keeping a close eye on weather forecasts and visual inspection of the crop is still vital," he said.

To further enhance probe placement, an EM soil variability map can be overlayed with a slope map to analyse variations from a perfect plane (to make sure the probe is not placed in a hollow or a ridge).

It can also be overlayed with a cut and fill map if the field was laser levelled in the last two to three years. These layers of data can then be combined to produce a map which best represents majority soil type, closest to majority slope and in some cases removal of areas of high previous cuts. This map is then used to site the probe in the field.

In conjunction with this type of map, Andrew Parkes reminds irrigators that ground truthing is still critical.

"You need to check your probe is placed in an average plant stand which is also representative of the rest of the field," he said.

Contact Cotton Communities CRC Water Team - Janelle Montgomery, David Wigginton (NSW DPI) and Emma Brotherton (QDPI), (02) 6750 6302.