Ollas: Unglazed Clay Pots for Garden Irrigation

ollaI first encountered the concept of using unglazed clay vessels for sub-surface irrigation in Bill Mollison’s “The Global Gardener” film series.  Mollison comments that the technique might be the (paraphrase) “the most efficient irrigation system in the world.”  More recently I noted with interest that the fine folks at Path to Freedom were employing these clay pots for some of their raised beds, which led me to wonder about how I might experiment with these clay pots as a potential sub-surface irrigation system. Here’s what I found…

Ollas (pronounced “oy-yahs”) are unglazed clay/terra-cotta pots with a bottle or tapered shape (see image) that are buried in the ground with the top/neck exposed above the soil surface and filled with water for sub-surface irrigation of plants.  This irrigation technology is an ancient method, thought to have originated in Northern Africa with evidence of use in China for over 4000 years and still practiced today in several countries, notably India, Iran, Brazil (Bulten, 2006; Power, 1985; Yadav, 1974; Anon, 1978 and 1983) and Burkina Faso (Laker, 2000; AE Daka, 2001).

ollarootsOllas may be the most efficient method of local plant irrigation in drylands known to humanity due to the microporous (unglazed) walls that do “not allow water to flow freely from the pot, but guides water seepage from it in the direction where suction develops. When buried neck deep into the ground, filled with water and crops planted adjacent to it, the clay pot effects sub-surface irrigation as water oozes out of it due to the suction force which attracts water molecules to the plant roots. The suction force is created by soil moisture tension and/or plant roots themselves.” (AE Daka – 2001)  The plant roots grow around the pots and only “pull” moisture when needed, never wasting a single drop.  “Ollas virtually eliminate the runoff and evaporation common in modern irrigation systems, allowing the plant to absorb nearly 100 percent of water.” (City of Austin Water Conservation, 2006)

To use ollas in a garden or farm, one buries the olla in the soil leaving the top slightly protruding from the soil (ideally the neck of the olla is glazed to prevent evaporation or it should be reasonable to apply a surface mulch that covers the neck of the olla without spilling into the opening). The olla is filled with water and the opening is then capped (with a rock, clay plate or other available material to prevent mosquito breeding, soil intrusion and evaporation).

“Depending on factors such as the plant’s water needs, soil type, time of year, and   environment ollas may need filling weekly or daily.  Water usually takes between 24 and 72 hours to flow through an olla.” (Bulten, 2006)   Water should be added to an olla whenever the water level in the olla falls below 50% in order to avoid build up of salt residues along surfaces of the olla that may prevent desired seepage. 

When assessed in the context of a movement towards local self reliance, the advantages of ollas seem to be astounding (the following list is provided by AE Daka’s research):

1.    Since clay pots are [can be] made by rural women [and/or men] they create employment and opportunities for small-scale home industries to manufacture them in rural areas. This will help generate rural income for household food security.
2.    They are affordable [when locally produced in rural locations] A 5 liter capacity clay pot costs US$0.25.
3.    Clay pot irrigation allows a farmer to raise seedlings in situ instead of transporting them from nurseries. Clay pots are installed directly where seedlings are to be planted and this allows a farmer to plant the seed next to the clay pot where it germinates and gets established
4.    The system is suitable for vegetables as well as perennial horticultural orchard or plantation crops and woodlots [(it has been noted that plants with woody perennial plant root growth can and likely will break the pots, but they can still be used for system establishment)].
5.    Water savings of 50-70 % are realized, particularly for vegetable crops.  Loss of water due to deep percolation beyond the rootzone is reduced if not avoided.
6.    Soil moisture is always available almost at field capacity giving the crop full security against water stress.
7.    The system inherently checks against over-irrigation.
8.    The much smaller quantities of water and less frequent watering required, reduce the amount of labour required for irrigation tremendously.
9.    Much less labour is required for weeding since weeds do not prosper, as the soil surface remains dry throughout the growing season.
10.    Domestic water effluent [graywater]from kitchens can easily be recycled and used in clay pot irrigation in backyards. The water used for cleaning utensils in the kitchen can be used to refill the pots in a backyard garden. This saves on scarce water and reduces the need to use fresh water.
11.    It saves on the amount of fertilizer to be applied [some studies suggesting up to 50% less] per unit area of land if the fertilizer is applied in clay pots and is later absorbed as solute via water movement to the plants.
12.    The soil of the seedbed under the clay pot system does not get sealed due to water impact but remains loose and well aerated.
13.    The clay pots can be installed on undulating ground.

Some of the disadvantages of ollas include the potential for winter breakage if left in the ground in areas with a winter freeze (“our research has shown damage to some Ollas (out of hundreds) when left buried in the ground over winter.” Bulten, 2006)  Of course, for kitchen garden beds in temperate climates, digging up ollas at freezing could be standard garden maintenance.  Prolonged use is likely to decrease porosity, some heavy soils may be inappropriate to site ollas and the longevity of ollas (without frost) is unknown but estimated in one study as 5 years or more.  Also, despite the purported efficiencies, long history of use and simple manufacturing requirements (more below), ollas are difficult to find locally, and may be, especially in the affluent, “over-regulated world,” prohibitively expensive to deploy.  Finally, there seems to be somewhat conflicting and insufficient research as to the optimal shape, volume and materials for ollas. 

ollas_shapeThe consensus from available research is that the optimal size and shape of the olla is dependent on the plants being irrigated.  No research seems to be available on the consequences of using ollas in a dense polyculture.  One “should match olla porosity, size and shape to plants’ water needs, root size and root distribution.” (City of Austin Water Conservation, 2006)  “As a general guide, smaller ollas are good for container gardening.  The larger ollas are good for larger containers or outside ground applications.” (Bulten, 2006).  Intuitively, a more tapered, flat-bottomed vessel with a narrow neck (to reduce evaporation and contamination) should be more efficient due to an increased surface area and, theoretically increased water spread, allowing for less ollas to be used to sufficiently irrigate a greater space.  Capacities of 5 liters to 12 liters have been described with 10-12 liter volumes being used to irrigate vine crops (tomatoes, curcurbits, etc.).  More empirical research would be beneficial to the world community.

Similarly, available research is not clear as to the optimal spacing of plants around the ollas. Clearly spacing will be dependent on the shape and size of the ollas so this does not seem surprising.  Based on available research the following tables can be created to describe potential spacing of ollas based on a rough estimate of water spread.
Additionally, John Bulten provides the following notes and diagram:
“Plant seeds or plants within 2” – 5” radius based on olla size.”
ollagardenIn another study, “clay pots with a capacity of 5 liters each and made by rural women wereinstalled at 0.5 m intervals in the study plots by burying them neck deep in the prepared seed beds.” (AE Daka – 2001)

There appears to be similar, but distinct approaches to making ollas, mostly defined by the local availability of materials and technology.  I’ve included descriptions on making ollas verbatim with the intent of assembling a loose set of guidelines to inform local artisans to invent an appropriate approach for the SF Bay Area (or wherever else olla manufacture is being attempted)

“Maria created her highly prized black pots by using the bottom of an old plate (puki)…Beginning by patting a tortilla shaped piece of clay in the puki, Maria then rolled a lump of clay between her palms, creating a long clay rope of uniform thickness.  Pinching and pressing the this coil onto a clay tortilla while turning her puki with her other hand, Maria formed the base of the olla. Successive layers of coils were added until the vessel was completed.”(Hoxie)

“To make the urns, the ministry created plaster of Paris molds from pumpkins, squash and gourds of various sizes. Workers pour liquid clay into the molds to shape the urns and then fire them in the kiln to solidify the clay. The urns retail for $12 to $15 depending on size.” (City of Austin Water Conservation, 2006)

“The clay pots are made from a mixture of clay and sand in the ratio of 4:1 and with an effective porosity ranging from 10-15%. The clay pots are made by rural women using their hands to mould them into different shapes, i.e. cylindrical/round with somewhat flat bottom.  After they are made, glazing is not done so as to retain their natural porosity i.e. the walls remain micro-porous. The pots are then tempered by burning them in a pit fire from firewood at undetermined temperature. Small-scale earthen-ware manufacturers use kilns to temper such ceramic pots at 1200oC. This is done in order to eliminate the swelling and shrinking properties of clay, which would cause cracking of the pots. Women believe that the type of clay used to make the pots is very important and it requires an experienced old woman to identify clay that would not crack unduly during the tempering process and indeed when installed under field conditions.” (AE Daka – 2001)

“If suitable pots are not available, they can be easily made by hand or on a pottery wheel. Depending on the clay, sand, rice hulls, or sawdust may be added at a ratio of up to 1:4 to increase the porosity of the pots. Although closed-oven firing at temperatures exceeding 450 degrees Celsius is ideal, pots can be fired in open pits at temperatures of 200 to 300 degrees Celsius. Opening: narrow neck (reduce opening size to reduce evaporation and contamination) (Barak, 2006)
Composition: unglazed porous clay – you can either use a crude clay which has larger/mixed particulate sizes a is not quite pure which will result in larger pores during the firing process. Or you can mix 20% sand with 20% quality clays (the best option) or the same % of sifted rice hulls or sawdust. The firing process will of course burn out the filler leaving uniform pores and a high-quality pot.  (Barak, 2006)
The pots I use are low-quality clay w/ a low firing temperature so they are prone to breakage and/or having pores that transmit water very rapidly. As best as I can tell they use course red clay with sand impurities and some straw mixed in (probably less than 20%) and are fired are probably 800 F which is what you normally achieve in open firing pits.” (Barak, 2006)

This video shows a potter’s wheel technique:

This video shows a different technique:


Some links and references:ollaberries