Disclosed are soil additives comprised of self-situating, stimuli-responsive polymer compositions and methods of their use. In one embodiment, the polymer composition or polymer network comprises permanent cross-links and non-permanent cross-links, wherein the non-permanent cross-links are capable of being removed upon application or exposure to a stimulus, typically located within the soil. In another embodiment, the method of increasing water retention in soils includes applying a soil additive, comprising polymer networks of the present invention admixed with an aqueous solution to form a colloidal dispersion solution or suspension, to a soil surface. The suspension or colloidal dispersion solution facilitates penetration of the polymer composition through the soil surface and into the soil, migrating typically to lower water potential areas and, more typically, along drainage channels located within the soil. After penetration of the soil surface, the polymer network is then exposed to a stimulus, typically present in the soil, whereby the non-permanent cross-links are removed and the polymer composition undergoes a volumetric change. This results in the prevention, arrest or decelerated loss of water from the targeted area, for example the plant root zone, which allows for improved water usage efficiency by plants, grasses, vegetation, etc.