For high moisture environments and shallow depth applications the gypsum sensors developed for Vinduino can be improved for responsiveness at <20 kPa soil moisture. Reducing the (gypsum) body volume of the sensor will likely give an improvement.

I came across a publication "Development of Mini-Gypsum Blocks for Soil Moisture Measurement and their Calibration to Compensate for Temperature" by A. Keyhani, that describes a easy to reproduce and small body volume/low hysteresis gypsum sensor.

Below picture shows the acrylic mold used to make this sensor by stacking layers of gypsum, using stainless steel mesh as electrodes.

"Mini-gypsum blocks were constructed using a clear acrylic mold capable of making four blocks at a time and prepared in four layers (57 mm × 160 mm) . The base layer (6 mm thick) was solid and each of the other three layers (3 mm thick) had four openings of the dimensions 3 × 15× 17 mm.For each gypsum block, two probes of stainless steel mesh screen No. 14 (0.64 mm) with the dimensions of 10×10 mm were used."

This looks very promising, definitely worth further investigating. Maybe this project is not done yet .......

Development of Mini-Gypsum Blocks for Soil Moisture ...