Accurate and efficient measurement of soil water in dryland systems: issues and experiences in replacing probes with EM38

Author: Byron Birch (Rimanui Farms Ltd), Andrew Smart (Precision Cropping Technologies) and Neil Huth (CSIRO) | Date: 25 Feb 2016

Take home message

  • EM38's are a mobile soil moisture monitoring tool
  • They remove the hassle of installing, maintaining and removing permanent probes
  • They're able to monitor soil moisture at multiple sites with the one device - ideal for large broad acre paddocks
  • They fine-tune WUE (water use efficiency) calculations and monitor water infiltration in soil

Foreword

This summary outlines issues and experiences of using probes over several years on black cracking clays on a dryland broad acre farm, which employs contractors and casual staff. The EM38 has been widely used for recording conductivity variance across paddocks and only in recent years it has been modelled, by the CSIRO, as having a tight correlation to soil water. The EM38 is a scientific device and can be likened to a highly accurate quantitative push probe. 

Capacitance and neutron probes are ideally designed for irrigation

  • Probes are highly accurate tools used to measure soil water within close proximity of the probe site (1-10cm). This is great for smaller more uniform irrigation paddocks where you want to know how quickly the soil moisture is being used and when to schedule irrigation
  • These probes are usually installed post-emergence in an area of uniform plant establishment and are able to settle into the surrounding soil after the first irrigation.

Issues with capacitance and neutron probes in dryland farming

  • Dryland farmers want to know how much soil moisture they have prior to planting in order to make decisions on whether or not to plant. Thus the probe needs to be installed during the fallow
  • Probes work best when they are have settled into the surrounding soil. This increases the risk from damage to spray rigs, planting rigs and headers. Also, operators have a tendency to drive around or lift over probes with planting rigs which results in probe readings being inconsistent to the rest of the paddock
  • Capacitance probes are too expensive to be placed in every paddock in large broad acre farming systems yet for low rainfall years moisture status of each paddock is very important. While neutron probes are a lot cheaper they still require installing and time specific management during busy operational periods such as sowing and harvest
  • Farmers who are trying to measure accurate WUE want to know how much moisture is left at the end of each season. Paddock soil moisture is often estimated at the end of a crop cycle due to the properties of dry soil i.e. soil being air dried around probes as localised cracking forms or the inability to penetrate push probes through the dry top soil
  • Probe placement is subject to multitude of unforeseen variables such as localised runoff areas flowing to probe sites or vice versa running away.

Why change over to an EM38 to read soil moisture

  • The EM38 readings have a tight correlation to capacitance and neutron soil moisture curves
  • Like probes, the EM38 requires a known range before the readings have any useful meaning; i.e readings for wilting point and field capacity need to be recorded usually from a repeatable GPS location or certain soil type
  • In years where the profile is not at field capacity and there is doubt over sufficient stored water to grow a crop, the EM38 has the ability to measure what moisture has been stored (even in uneven cracked soils, which have been filled from the bottom up)
  • An EM38 has the ability to take accurate readings anywhere in a paddock with the identical EM classification. This requires an EM survey map of the paddock along with known full and empty readings for that classified EM soil
  • As the EM38 is non-intrusive, there is a limitation to measuring the exact depth of the moisture front. The EM38 has two ranges, 0-75cm and 0-150m, however holding the device above the ground at known heights can manipulate the depth of reading
  • In conjunction with an EM38 it can be useful to verify readings against neutron probes and capacitance probes.
  • The EM38 has the ability to give a tangible figure, which can be put into a spreadsheet much like the probes.

Advanced use of EM38 in broad acre cropping

  • As farmers strive for greater production and take on farm based trials the EM38 is handy tool to measure differences in soil moisture between a trial and control (i.e. farmers questioning the infiltration rates in-between tram lines compared to the rest of the paddock as repeated harvest operations deposit strips of chaff behind the header.

EM38 in practice

  • The EM38 is quite daunting at first when trying to calibrate however there are several sources of information including PDF’s, YouTube and precision farming consultants
  • Calibrating readings for soil moisture should ideally be conducted with a soil corer at the same time as the readings
  • Leave the device turned on when transporting between paddocks  to avoid variations in readings while the machine is warming-up
  • Familiarise yourself with the instruction manual for the do’s and don’ts  of operating an EM38.

Contact details

Byron Birch
Rimanui Farms Ltd
Ph: 0438 549481
Email: Byron@rimanui.com.au

Andrew Smart
Precision Cropping Technologies
Ph: 02 6792 2638
Email: Andrew@pct-ag.com