The invention relates to, in a sensor comprising: an electrically conductive metal housing, said housing having an inside wall forming a transmission matrix cavity, said wall having perforations therethrough; a liner transmissible of water lining said inside wall and covering said perforations; a transmission matrix filling said liner; a tablet of slowly dissolving material atop said transmission matrix; a cap closing said housing forming an electrode matrix chamber; an electrode chamber matrix in said electrode matrix chamber; a pair of spaced-apart electrodes in said electrode matrix chamber in contact with said electrode chamber matrix; the cap passing electrical leads from said electrodes; an improvement comprising said liner comprising a layer of laid, thermally bonded fibers in a pattern such as to provide-passages of such size as to confine said transmission matrix but to permit flow of moisture to and from said transmission matrix and soil in contact with it.