Understanding dielectric readings - Water vs Apple Sauce (for example)
chirp! is a capacitance sensor, not a dielectric measurement. The designer/author of the writeup seems to have made a slight error when they spoke of the water in the soil changing the dielectric. Rather, the sensor appears to have its own dielectric coating, using the soil/water/applesauce/milk as the second "plate" of the capacitor. As such, generally speaking, electrolytes with a higher dissolved ion concentration should register a higher capacitance, due to the "double layer effect" created by the ions (similar to that used in modern EDL supercaps).
This would explain why milk/applesauce (high ion concentration) had higher readings than tap water (low ion concentration), distilled water (very low ion concentration), or air (extremely sparce in both matter &amp; ions)

EDIT: see https://en.wikipedia.org/wiki/Electric_double-layer_capacitor for more info on ionic double-layers &amp; effect on capatance.