Two old problems
Moisture release curves have always had two weak areas: a span of limited data between 0 and -100 kPa and a gap around field capacity where no instrument could make accurate measurements.
Between 0 and -100 kPa, soil loses half or more of its water content. If you use pressure plates to create data points for this section of a moisture release curve, the curve will be based on only five data points.
And then there’s the gap. The lowest tensiometer readings cut out at -0.85 MPa, while historically the highest WP4 water potential meter range barely reached -1 MPa. That left a hole in the curve right in the middle of plant-available range.
New technology closes the gap
In 2008, UMS (now METER Group AG) in Germany released the HYPROP, an instrument capable of producing over 100 data points in the 0 to -100 kPa range. This solved the resolution issue with more than 20 times the data behind that section of the curve.
In 2010, Decagon (now METER Group) released the redesigned WP4C water activity meter. Significant accuracy and range gains allow the WP4C to make good readings all the way up to tensiometer range.
Using HYPROP with the redesigned WP4C, a skilled experimenter can now make complete high resolution moisture release curves.
Not easy, but easier
Creating a full moisture release curve can still be challenging. Measuring water potential with the HYPROP is an art, and using the WP4C to make measurements up into tensiometer range requires care and skill, but with good measurement techniques, it is finally possible to get a complete, high-resolution soil moisture release curve.
For in-depth information about how to make full soil moisture release curves, download the Moisture Release Curve App Guide.