Ollas

Ollas (pronounced “oy-yahs”) are unglazed clay/terra-cotta pots with a bottle or tapered shape that are buried in the ground with the top/neck exposed above the soil surface and filled with water for sub-surface irrigation of plants. This irrigation technology is an ancient method, thought to have originated in Northern Africa with evidence of use in China for over 4000 years and still practiced today in several countries, notably India, Iran, Brazil (Bulten, 2006; Power, 1985; Yadav, 1974; Anon, 1978 and 1983) and Burkina Faso (Laker, 2000; AE Daka, 2001).

Ollas may be the most efficient method of local plant irrigation in drylands known to humanity due to the microporous (unglazed) walls that do “not allow water to flow freely from the pot, but guides water seepage from it in the direction where suction develops. When buried neck deep into the ground, filled with water, and crops planted adjacent to it, the clay pot effects sub-surface irrigation as water oozes out of it due to the suction force which attracts water molecules to the plant roots. The suction force is created by soil moisture tension and/or plant roots themselves.” (AE Daka – 2001.) The plant roots grow around the pots and only “pull” moisture when needed, never wasting a single drop. “Ollas virtually eliminate the runoff and evaporation common in modern irrigation systems, allowing the plant to absorb nearly 100 percent of water.” (City of Austin Water Conservation, 2006.)

To use ollas in a garden or farm, one buries the olla in the soil leaving the top slightly protruding from the soil (ideally the neck of the olla is glazed to prevent evaporation or it should be reasonable to apply a surface mulch that covers the neck of the olla without spilling into the opening). The olla is filled with water and the opening is then capped (with a rock, clay plate or other available material to prevent mosquito breeding, soil intrusion and evaporation).

this is picked up from:

http://permaculture.org.au/2010/09/16/ollas-unglazed-clay-pots-for-garden-irrigation/