Abstract: Soil moisture sensor apparatus includes a housing 10 having an expansion chamber 13 and a piston 20 slidably mounted within the housing 10. Formed through the wall of the housing are electrical lead take-outs 15 through which pass cable tails 16. The piston 20 has a rebate 21 adapted to allow the piston 20 to pass partly by a protruding portion of a fixed contact 22 embedded in the wall of the housing at the upper take-out 15. Bearing on the lower surface of the piston 20 is a moving contact 24, the contact 24 being urged against the piston face by a spring 25 which also serves to bias the piston 20 towards the open end of the housing 10. Disposed across the expansion chamber 13 is a flexible membrane 26 retained in position by an end cap assembly including an annular end cap body member 27, an outer mesh screen portion 32 and a dirt excluding inner fabric portion 33.