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Porous Clay Capsule Irrigation

Burying unglazed clay pots filled with water is an ancient method of plant irrigation. The first writing about the technique dates back about 2,000 years to China. It has since been practiced in many arid regions of the world including New Mexico. Introduced here by the Spanish and utilized in local scale agriculture since the Columbian Exchange, the use of clay vessels has been largely replaced by modern drip irrigation technologies.

Clay capsule irrigation is a combination of the original concept of burying a clay olla filled with water and the principles of modern drip irrigation. The capsules are unglazed terra cotta clay - modern slip cast clay flower pots - that are glued together and fitted with connectors and tubing that connects them all together to a continuous source of water at atmospheric pressure. Proper design of the system enables irrigation that takes advantage of the principle of both "olla irrigation" and "low pressure drip irrigation". I'll describe how the system is designed, but first, let’s explore the underlying principles of soil water content and olla irrigation.

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.

Ollas & Plants

Olla and plants demonstrating the relationship between the water source and proper root growth.

Soil Moisture

Three forces determine soil water content. They are gravitational, capillary, and hygroscopic forces. Gravitational force pulls on water molecules drawing them downward against the capillary force that holds water between soil particles. Hygroscopic force binds water molecules within and tightly against soil particles.

When a soil is at field capacity, the capillary attraction of soil particles holds water in place and makes it easily available to plant roots. When the soil is saturated, gravity can pull water down below the root growth level. Excess water also drives out oxygen normally held in the pore spaces between soil particles.

The wilting point for plants comes when they cannot extract water that is tightly held in the soil by hygroscopic force. Most non-arid plants cannot thrive or survive at or near the wilting point.

Olla and porous clay capsule irrigation is the most efficient method for maintaining soil moisture near field capacity and avoiding conditions of saturation or wilting point. Edible garden plants that have optimum soil moisture exert less energy drawing water and nutrients from the soil, grow faster and are less stressed.

Water Delivery in Clay Capsule Systems

Two 4½” clay flowerpots glued together create a porous capsule that holds one quart of water (.946 liters). When connected to a continuous source of water at atmospheric pressure the capsule can deliver water volume as needed by the soil and plants from 0 to about 2 quarts in a 24-hour period. If the series of capsules are installed with an elevated storage reservoir (diagram below) and control valves so that it can function as a low-pressure drip system each capsule can deliver two quarts of water in about 6 hrs. The amount of “head pressure” in the water reservoir (height above level of the capsules) will determine the rate of flow along with system leakage and volume of air captured in the capsules and feeder lines. Switching water pressure from atmospheric to low pressure with in-line valves enables the system to deliver more water at times of summer heat stress when normal atmospheric pressure delivery may not be adequate to prevent wilting.

clay capsule parts
Clay Capsule parts and assembled

In a single clay olla system maxmium water delivery rate depends on the ratio of subterranean water volume to cubic area of soil, clay wall porosity and olla surface area. In a continuously filled porous capsule system maximum water delivery depends on clay wall porosity and clay capsule surface area. Since the capsules are continuously filled the volume of subterranean water is less important. The capacity to switch the system between atmospheric and low pressure makes it possible to maintain adequate water delivery at times of peak need with a lower ratio of subterranean reservoir to cubic feet of soil space. For example, a volume to soil cubic foot ratio of 3½ gallons (14 quarts) to 30 cubic feet delivers adequate water in a switchable system. A porous capsule system that only delivers at one atmosphere would require a ratio of up to 10 gallons (40 quarts) per 30 cubic feet of soil to meet maximum water needs for edible plants. Note: these ratios will vary greatly depending on climate, season, soil type, plant variety and planting density.

Porous Clay Capsule System Diagram

Bed Layouts for Clay Capsule Irrigation

The diagram below provides a layout for intergrating clay capsules, lettuce and peas in a 30 sq ft bed. The capsules are spaced about 16" apart on center with 14" between the two rows.

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Good Gardening and Good Eating

Darrol Shillingburg
Doña Ana Extension Master Gardener

December 2010