Posted by & filed under Irrigation, Water Conservation.


A Sri Lankan villager fills his olla
Photo copyright © Craig Mackintosh

I 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, to 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 them 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 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).

Ollas 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.


Pottery production is an age-old skill and low-tech cottage industry
that should see a rebirth. Photo copyright © Craig Mackintosh

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 root zone 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 the onset of winter 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, and the 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. 


Clay pots, like these in Sri Lanka, can also be used as water coolers
in dry climates. See A Refrigerator that Runs Without Electricity
to see how this works. Photo copyright © Craig Mackintosh

The 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 on 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.”

In another study, “clay pots with a capacity of 5 liters each and made by rural women were installed 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 San Francisco Bay Area (or wherever else olla manufacture is being attempted):


Ollas in Sri Lanka
Photo copyright © Craig Mackintosh

“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 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 and 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 with 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:

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This video shows a different technique:

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Some links and references:

33 Responses to “Ollas: Unglazed Clay Pots for Garden Irrigation”

  1. Daniel

    Wow, never heard this before. I don’t have my own land, but i can see it useful when going on vacation and leaving all my plants alone for a week :)

    Reply
  2. PDS

    Kevin is a spectacular teacher and permaculture practitioner in the San Francisco Bay Area. He teaches part-time urban PDC’s quarterly. I couldn’t recommend anyone more highly. Take his course(s) with co-teacher David Cody and re-imagine the urban environment through the permaculture lens.

    http://www.upisf.com

    Nice work Kevin,

    P. David Stockhausen

    Reply
  3. Zainil Zainuddin

    Thank you so much for this. Like you, I first heard of this concept when I did my PDC with Bill and Geoff but it remained as a concept to look into at some point in the future. Great to see that it is being applied. Very useful and informative.

    Reply
  4. Adam T

    Fantastic! I’m going to buy cheap terracotta urns to bury all over the garden. If the plants don’t use that water, then the soil lifeforms will surely appreciate it.

    Reply
  5. Kim

    We are experimenting with the two flowerpot stuck together version of this method.

    We have found that the results for reduced water use may well be bourne out by our experience.

    This method we are considering for whanau enterprise development.

    Regrds

    Reply
  6. Lisa

    Anyone know of any pottery places that do custom orders? I really want these for the garden. Surely getting a supply made of these shouldn’t be too hard?

    Reply
  7. Fadhol M. Dahir

    Clay pot irrigation is one of the a very effective system espishally in high porous soil.

    Reply
  8. John

    OK now I am curious, I wonder if anyone has tried using unglazed terracotta pipe, my thought is to make a single reach row down a fence line, bury the pipe with a slight down slope cap one end and have it 90 degree to the surface on the other end. Any thoughts??

    Reply
  9. Jennifer Hyatt

    I found a place in Australia that sells a “wet pot” system. It is basically a bunch of ollas connected by irrigation tubing and a rain barrel. The rain barrel is placed up higher than the ground and gravity feeds the water down the larger irrigation pipe. From the larger pipe you feed the smaller irrigation tubes to the ollas. The ollas have t junctions and the tops are covered and buried completely. The water feeds into the ollas as needed and all you need to do is fill the rain barrel. If you have a way to fill the rain barrel with rain..great…if not…you can just fill it with regular water as needed. Here is the link to the site I saw: http://www.productivegardens.com.au/products/wetpot-watering-system I haven’t done this yet, but I can’t wait. I wanted to plant some container trees and I think I will try making my own ollas out of terra cotta pots and silicone and then linking them to irrigation piping and a barrel.

    Reply
  10. Graham K

    An alternative to fragile pots are above-ground water containers feeding ‘rope’ wicks.
    I have experimented with this with some success.

    Reply
  11. Jane K

    Clay ollas – purpose made for irrigation are available in New Zealand from Selao Garden Art. They come in 4 sizes with lids and the necks are sealed with water proof clear sealer.

    Reply
  12. Lori H

    I have started a company to supply clay pots for olla irrigation in the US. It is primarily a wholesale company but I can sell direct for large orders. See my website DrippingSpringsOLLAS.com if you are interested.

    Reply
  13. Bobbi Hightower

    I am looking for a porous clay pot made in the US to be used as a water filter much like the ones used in other countrys to gain drinkable water. These pots will fit into a 5 gallon bucket and the lip of the clay porous pot will slightly overlap over the top edge of the bucket. Water is filtered through the pot into the bucket. Does anyone know who makes these type of pots or know of anybody in the US that does make them? If so, a web address or phone number would be appreciated.

    Reply
  14. Mark Hilliard

    Hi Bobby,
    Are they made from stoneware or earthenware? I’d like to see a photo or a drawing of the pots you’re talking about. I can see how they would work and perhaps I can make one for you as a demo piece to see if it works.

    Reply
  15. Andrea

    Greetings from Spain
    well we are doing an olla experiement here on a Spanish Island. We are working with local potters, but till now, ollas don´t work well. We are tryng different clay types and firing temperatures of the ollas, but till now, we do not have good results. I was doing a lot of internet research, but i cannot find exact details about how to achieve a certain porosity, the one you like. Actually, at the moment we are trying to make some ollas mixing clay wth a certain portion of sawdust to gain more porosity, and see how it works…there are some cientific studies you can find in internet which deal about porosoity and hydraulic conductivity of clay pots, but i have the feeling that knowledge about this technology needs to be socialized so that the aplicaton of this ancient technology can really make a difference on a big scale..if anyone has more details it would be great to hear from you and of course i will share further results of the experiement
    P.D We also tried the version with making ollas by flower pots but water seeps out to quickly..

    Reply
  16. Mark Hilliard

    Hi Andrea,
    I’m on my third year of making ollas. This time, I’ve reduced my firing temperature by letting the kiln get to an even red heat and soaking at that temperature for no more than an hour. I’m using a Cone 10 Stoneware. Theoretically it will remain porous until I fire it to maturity when it vitrifies at Cone 10 or appx. 2300 degrees Fahrenheit. I’m estimating that I hit about 1400 degrees fahrenheit or Cone 015.
    There is a retired professor, Curtis Smith, that has done alot of olla research here in the American Southwest. He retired from New Mexico State University and is now, part time, at Wayland Baptist University Albuquerque. They did some porosity testing of ollas in Socorro, New Mexico at the college, there. I can’t recall the name of the college. It’s a Science and Technology school.
    One thing that I discovered was that ollas do not perform very well in a new bed. They need a mature bed. I also plant right next to the olla instead of 18″ away. So, I think all olla users will find that they need to adjust to their local environment rather than depend too much on what is found on the Internet. We are in our third year of drought here in Texas. 2011 was an all time record setter for us, hence, I don’t have any ‘benchmark’ of olla performance compared to a so-called ‘normal’ year.
    I recommend that you figure out at what temperature your clay matures and becomes vitreous and then step-down the firing temperatures, incrementally, until you arrive at a useful porosity.
    Like you, I’ve scoured the Internet and I have not found very much information that is useful. Ollas work by the principle of ‘Osmotic Potential’ where water moves from areas of high pressure (wet) to areas of low pressure (dry). Eventually, all water moves vertical through the soil profile. So, I use the olla as a miniature aquifer from which a plant can drink. That’s why I plant really close to the olla rather than what some people on the Internet recommend such as 18″.
    Mark Hilliard Associate Professor of Art, Wayland Baptist University, Plainview, Texas

    Reply
  17. Graham Knight

    Hi Andrea,

    As you may know I gave some time last year researching ollas for people in Nigeria. See photo
    Apart from porosity there can be the problem of cost as their potential is not as obvious as it may seem at first glance.

    If there are local potters who can make the ollas very cheaply and this is done near where they are to be used than it can be viable
    But the labour required is considerable – and the breakages awful!

    As you say, one has to add sawdust or similar to the clay thus making a weaker, more fragile pot.
    If they have to be transported you can easily find half of them get broken. There are other problems that come later!

    I suspect that you really have a choice between drip and wick irrigation. There is plenty of info about drip lines etc and I can help with wick irrigation if you
    want to try it out!

    Yours

    Graham Knight
    BioDesign
    Will send photos later if required

    Reply
  18. Andrea

    Thank you for your answers

    yes, indeed, i know we have to experiement gradually with the firing temperature and adding materials like grog or sawdust…since every clay has different performance at different firing temperatures, i see now that we have to experiement more with our local clays..i got also some helpful advice now from different people which appoint in that direction and we realized that we have to spend more time in adjusting temperature and mixing grog, sawdust etc..indeed complicated because it needs time and money but we will go on..thank you for answering. Drip lines are very common here, too..maybe we will think about it again, but at the moment we will go on with the olla projekt..but thanks for offering help!

    Reply
  19. Mary

    Something’s not right with the olla water saturation charts.. First chart says “11.8″ diameter olla has a soil saturation diameter of 39″, the second chart says a 12″ olla has a saturation diameter of 24″?

    Reply
    • Lori

      The charts are a bit confusing. The first chart gives a diameter of 36″ for the 11.8″ pot. The second chart indicates that the ollas should be spaced with walls of 12″ pot 24″ apart. From center to center of each pot would be 36″. I find with planting around 12″ pots that variables – plants, soil, temperatures -effect spacing. Ex. I plant tomatoes approx 18″ from center (4 plants around olla) and lettuces 10″ from center (4-6 around olla).

      Reply
      • Mary

        Thanks Lori for the clarification. So the first chart still reads 39″ for the 11.8″ pot to me?, but 36″ would make allot more sense. I’ll start with that & see what it does with my soil :) I’ve not yet planted anything, I’m just trying to get size appropriate ollas with the right spacing for all my raised beds so that every square inch (or close to it) is covered/reachable by water from an olla.

        Reply
  20. Mark Hilliard

    I plant close to the olla-12 inches. My soil is alkaline here in Northwest Texas. I had Texas A&M do a soil analysis and my pH was 8.3! So, my water does not move horizontally like the chart may imply. Still, an olla works by means of the Osmotic Potential. Meaning water moves from areas of high pressure (wet) to areas of low pressure (dry). For water to move long distances horizontally, it must have a significant ‘flow rate/volume’ at its head. Even if an olla could produce this rate, it would saturate the soil close to the olla and water would be lost due to deep percolation below the root zone much in the same way furrow irrigation does. I’m going to recommend that you plan on having a dry year and plant close to the olla, just to be safe. Water tends to move vertically through the soil profile and less, horizontally.

    Reply
    • Mary

      I am happy to report that in my preliminary trials my 2 gal 11″ diameter ollas have a saturation diameter of 37+ inches horizontally! Perhaps it is due to the soil being heavily amended with compost, with a good 4-6″ topper of composted horse manure & wood shaving mulch, but 16 hrs after filling the ollas to the 1/2 way mark of the neck the soil was beautifully & evenly moist in all directions.:)

      Reply
      • Mark Hilliard

        Hi Mary!
        That sounds great. I wish my soil would do that well. Can you do something for us? If you have a thin metal probe like a length of 3/8″ round bar/rod, push it down vertically close to the olla and measure how deep it can penetrate. Then do the same at the outer part of the saturated diameter. It might be useful to know some more about how deep the saturation is along with its horizontal spread.

        Reply
  21. Steven

    We put ollas in the ground every four feet through our 600 sqft garden last summer and watered only using our aquaponic fish emulsified water on a weekly basis and had terrific results. There was a time in our Central Texas July heat that the water was not moving quite well enough horizontally to keep everything decently wet. In those soils that didn’t move the water well we tore up some polyester fabric and put one end in the olla all the way to the bottom. The other end was buried under the soil two feet in both directions. The result was everything we needed to keep our garden moist (mostly for arugula) in July and August.

    Reply
    • Mark Hilliard

      That makes me wonder if I could make an olla specifically to ‘wick’! That sure might help with my situation. It’s getting close to planting time in my area and I’ve been making ollas for the University garden and the Master Gardeners program in town. I might just have to design a ‘wicking’ olla! Thanks for the idea Steven!

      Reply

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