The Neutron Probe
These tend to be used by consultants providing irrigation scheduling services to growers. Extremely accurate but expensive, hence few are privately owned. Very repeatable and still considered the "Benchmark" against which all other instruments are judged  -  for technical information click here

Capacitance Probes (Delta-T Profile Probe)
Used by consultants providing a services or owned privately by growers. Technical data click here
The Profile Probe pictured and installation kit are available from Agri-Tech
P.O.A

Time Domain Reflectometry (Theta Probe)
The Thetaprobe from Delta-T is extremely accurate. Useful for shallow rooted crops or crops grown in the pot / bag environment. Used either with hand-held recorder or can be attached to a data logger for continuous data. Technical data click here
P.O.A

Electrical Resistance Measurement  (Gyspum blocks, Watermark sensors)

A cheap solution which can provide useful information portraying good trends. Can be buried at varying depths. Can be connected to a hand-held recorder, more commonly connected to data logger / weather station
Click here for technical data
P.O.A

Crop Modelling using weather Station Data & ET figures

A useful guide when used in conjunction with a tool to measure soil
Moisture
Click here for technical data
P.O.A