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
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
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.