Simple Olla Irrigation

Clay vessels or pots made from low fire terra cotta clay are porous enough to allow water to pass through them when buried in soil. The rate of water flow varies depending on the soil water potential - the soils affinity for water - and by the plants uptake of water through their roots. This all occurs at atmospheric pressure and requires no timers or pressure regulators to maintain soil moisture at near field capacity. That is the amount of water soil will hold in its pore spaces without gravity pulling it downward to deeper levels. This irrigation system is ancient and widespread in arid and semi-arid regions where water is scarce or intermittent. The ollas need frequent filling with clean water to work well for plant irrigation.

What are the advantages?

Water is delivered as needed (when the olla has water in it) directly to the soil where plants are rooted. Little water is lost from the soil or lost from runoff or evaporation into the air – as happens with sprinklers. There is also no water loss into the soil below the root horizon. The soil is never under or over saturated because the optimum soil water level for plant growth is controlled by demand from the soil and plant roots. Because of these factors, olla irrigation is 70-90% more efficient than flood or sprinkler irrigation and even more efficient than modern low-pressure drip systems.

How much water is delivered?

Watering capacity of an olla is determined by clay porosity, soil type, organic matter content and planting density. In an idea garden soil the wetted zone around an olla is 6–8 inches from the olla surface. Root growth will be concentrated where soil moisture is optimum. When the olla is not delivering adequate water to the soil, root growth will be concentrated around the olla and may form a dense root mat that takes up all available water near the olla surface.

What is the Difference between Ollas and Clay Capsules?

Not much, but it turns out that the small differences matter. Modern clay capsules can be made from ordinary terra cotta flower pots found at nurseries and big box stores. The same principles apply because the pots are unglazed and porous, but an olla is open to the air and a clay capsule is closed or sealed and fed by tubing that keeps it full of water at all times. Because the capsules are sealed they can be plumbed together to work as a unit and they can also be used as a low pressure drip system.

To make a one quart capacity capsule, take two 4 inch flower pots plug the bottom hole of one with epoxy putty (or any other permanent material), insert drip fittings into the other bottom hole (which will become the top) and then glue the pots together (Guerilla Glue makes a permanent bond that hold up when buried in soil). You now have a porous clay capsule.

The same process will work with larger pots. Eight inch flower pots hold about three quarts with glued together and are a little harder to seal up.