A hygrostat system employs a hygroscopic element of a coherent polymer suitable for burying in soil near a plant (for example, a pot-plant). The hygroscopic element directly contacts the soil and has no lost motion as it expands and contracts so that the soil does not interpose itself at the ends of the element. In one embodiment, the hygrostate system has an upper chamber shaped like an inverted cup closed by a diaphragm and a lower frame shaped like an inverted stirrup having a lower arch and having an integrally-molded upper snapping ring, which fits inside the rim of the upper chamber. The upper chamber has a water inlet/outlet spigots extending therefrom. The hygroscopic element is strip shaped and is clipped or keyed at its lower end to a key-stone protrusion formed at the bottom of the lower frame. A pair of guide rails, which serve to stiffen the hygrostat element against buckling, extend from each side thereof. The upper ends of the rails are joined together by a foot to which the upper end of the hygroscopic element is attached and attached to the diaphragm by pull-though studs. The lower ends of the rails are free to slide over the keystone, and positioned laterally adjacent thereto.