Olla Irrigation

An olla is an unglazed clay pot buried up to its neck in a bed and filled with water. Clay fired without a glaze stays porous, so water passes slowly through the wall into the soil, reaching roots directly instead of sitting on the surface to evaporate or run off. It needs no pump, no pressure and no power.

It meters itself. Dry soil pulls water through the wall faster and wet soil slows it, so the pot delivers at roughly the rate the plants are using. What it cannot do is reach far. Moisture spreads only about 20 cm (8 in) from the wall, and that figure decides everything else: plants are sited around the pot rather than the pot dropped into an existing bed, a sizeable bed needs many pots, and direct-sown seed or shallow-rooted crops like lettuce are not served. Soil texture matters as much. In coarse sand the water runs down past the roots; in finer or layered soil it spreads sideways, which is what the method needs - so it works least well on the dry sandy ground where saving water is worth most.

The water has to be clean. Silt, organic matter and dissolved salts seal the pores, after which the pot quietly stops seeping while still looking full; brushing the wall or flushing it with something acidic between seasons clears it. A garden-size pot of about 2 litres (half a gallon) is refilled daily through the growing season, which is the real cost of a device with no moving parts. A hose or a line with an emitter over each pot fills a row at once. It pairs with any gravity water source, with mulch on the ground around it, since the pot controls delivery loss but not evaporation, and with transplants of thirsty deep-rooted crops such as tomatoes and squash. Where rain is too light to wash salts down through the soil, watering from below draws them up. A full pot left in freezing ground cracks, so where the ground freezes the pots are lifted, emptied and stored dry.