No one will call clay pot irrigation rocket science, but according to Bill Mollison of “Global Gardener” fame, the use of ollas in the agricultural field is “the most efficient irrigation system in the world.” An olla is an unglazed clay pot with a fat belly and a narrow neck. And they were not originally meant for irrigation.
The origin of clay pots
Before the advent of stainless steel and aluminum, which made metal vessels durable as well as cheaper, not everyone could afford copper and brass vessels, let alone the silver and gold ones reserved for royalty. Untreated iron rusts so quickly that it was not a great option for containers that came in contact with water.
People of all stations could, however, afford pottery, which used to be as ubiquitous as plastic is today. Water from wells and streams were carried and stored in mud pots made durable by firing them at high temperatures. Most ancient civilizations used pottery, as evidenced by the archeological finds at different sites. China is credited with the oldest pottery ever known, dating back to 20,000 BC!
Romans had mud pots, or ollas, with wide necks for cooking. They were widely used for storing grains and other seeds until the next season. Those with tall, narrow necks served as containers for liquids for the obvious reason that it prevented spillage while carrying the pots around. It is these narrow–necked clay pots  that that are used for irrigation. Originally a Roman idea, or not, the Spanish are credited with introducing it in America. Farmers in many developing countries with water scarcity successfully use this method to supply water to the plants with minimum loss.
Using ollas for irrigation
Many indigenous agricultural practices are born out of a need for improvisation, rather than scientific research. But the science behind them is just as valid. Unglazed  clay pots are typically porous, and when water is stored in them, some of it sweats through these pores. Evaporation of this water makes the water inside cool, in the same way as sweat on our skin cools our body. When water-filled clay pots are buried in the ground, the surrounding soil becomes damp, thus providing water to the roots of nearby plants.
In many developing countries, plastic pots of similar size and shape, but considerably lighter and practically indestructible, have replaced ollas as a means of carrying water across long distances. Nevertheless, cheap clay pots are still available because they can be locally produced with clayey soil dug up from the fields. They are still favored for water storage, too, because they keep the water cool, a great plus in tropical countries where refrigerators are not common, and electricity to run them is too expensive and unreliable. A simple irrigation system using the ollas is ideal for such situations.
How to install a clay pot watering system:
Dig a hole in the ground big enough to accommodate olla so that only the neck is above the soil surface. To prevent water loss through the neck and the mouth of the pot, the portion above the ground is painted white, and the mouth is covered with a removable clay plate. The lid will keep the water clean, and free from breeding insects like mosquitoes. The pot  should be ¾ full of water at all times for uniform distribution of water. The frequency of topping up may vary according to weather conditions.
In a more elaborate system consisting of several pots, they can be connected to the pipeline with a float attachment which will ensure that the pots are refilled automatically. The area irrigated by a pot may depend on the size of the pot, its porosity, and the permeability of the soil. Plant the seeds or seedlings 2 to 5 inches away from the pot. They may need to be watered in the usual way for a few days until established.
If ollas are not available, flower pots with sealed drain holes may be used to make a reservoir for water. It should have a suitable lid to prevent evaporation through the mouth. A table-top self-watering olla system designed by product designer Joey Roth consists of a large cylindrical pot within which there is a narrower central cylinder with a well-fitting cover for water.
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The many advantages of clay pot watering system:
- Conservation of water. The recent interest in clay pot irrigation in sustainable gardening circles stems mainly from the need to conserve water. When gardens are watered in the conventional way, evaporation from the surface of the soil is very high. Even where drip irrigation supplying water directly to the root zone is installed, some of it is lost through percolation into the deeper layers of soil. In clay pot irrigation water slowly spreads into the subsoil as ink on a blotting paper, minimizing loss.
- Cultivation of arid land. Brazil is a country where clay pot irrigation has been popularized since 1978. It has greatly increased their agricultural output as more land could be brought into cultivation and crops could be cultivated throughout the year. Clay pot irrigation is ideal for areas with desert-like climate with limited seasonal rainfall.
- Auto-regulation of water supply. No complex equipments are required to monitor and regulate the amount of water supplied to the plants through the clay pot. The release of water by the pot into the soil is regulated in much the same way as sweating by our body is controlled by the humidity in the atmosphere. When the water content in the soil decreases due to absorption by the roots, as well as evaporation into the air, more water passes into the soil. It keeps the dampness of the soil more or less constant at all times. That’s why we need to refill the clay pots more frequently in hot and windy weather.
- Weed Control. Seeds often thrive much better than carefully tended plants in our gardens and vegetable patches. One secret of their success is that they self-seed readily. But since the clay pot supplies water to the lower layers of the soil only, while most of the seeds scattered by the weeds remain on the surface, they do not get a chance to germinate, unlike in top watering.
A few disadvantages:
- Labor-intensive. Burying a few pots in a closely planted area of your garden may not be considered labor-intensive, but translated to larger areas, all this digging and covering up of the pots may involve a lot of time and labor. Farmers typically use it for watering trees, and the clay pot is “planted” in the same hole into which the sapling goes, saving additional labor.
- Loss of porosity. If you use liquid fertilizers in the water used for irrigation, salt build-up may clog the minute pores. Over time, it may result in the pots losing some, if not all, of the porosity. Hard water also causes the same problem. With the passage of time, the amount of water dispersed by the pot will come down drastically. Re-baking the pot can open up clogged pores. Replacing the pot with a new one is another option.
- Losses due to breakage. Clay pots , by nature, are prone to breakage; you can lose some, if you are in the habit of digging around the plants. But turning the soil is rarely necessary when ollas are used because soil does not get compacted unlike in surface irrigated areas.
Whether it is for convenience, or for conserving water, this low-tech irrigation system can be a part of sustainable farming. Once installed, it may function without additional inputs for several years until replacement of the pot becomes necessary.