Accurate and efficient measurement of soil water in dryland systems

Take home message

  • EM38’s are a mobile soil moisture monitoring tool
  • Removes the hassle of installing, maintaining and removing permanent probes
  • Ability to monitor soil moisture in more than one site in large broad acre paddocks
  • Fine tune WUE calculations and monitor water infiltration in soil       

Introduction

This summary out lines 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 been modelled by the CSIRO as having a tight correlation to plant soil water use. ‘Why change over to an EM38 to read soil moisture’ has a few assumptions, which need to be verified by the CSIRO and during initial commercialization of the EM38.

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.
  • 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 limitation to the accuracy of where the soil water is; the EM38 has two ranges 0-75cm and 0-150cm
  • In conjunction with an EM38 push probes, neutron probes and capacitance probes can be used to verify the readings.
  • The EM38 has the ability to give a tangible figure which can be input into a spread sheet much like the probes.
  • 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.

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 quiet daunting at first when trying to calibrate however there are several sources of information including PDF’s, YouTube and precision farming consultants.
  • Calibrating for soil moisture readings should ideally be completed over a medium where there is little variation in soil moisture over a season i.e. on a machinery or silo bag pad
  • Transporting between paddocks it is easiest to leave the EM38 running as 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