The Ollas Project: Part II
by David Troxell, Ecology Action Communications Director

Not all experiments end in success. Most great ideas have several iterations which fail: kinks, unforeseen challenges, and unintended consequences arise that need to be addressed before success can be achieved.

Over the summer of 2019, Ecology Action partnered with local potters to determine best practices for successful use of ollas in a garden (see The Garden Companion’s Fall 2019 issue). Ollas are unfired clay pots with long necks and bulbous bases which are buried up to their necks in the garden soil and then filled with water. The soil around the olla is then irrigated by the water wicking through the semi-porous wall of the pot.

Rain and Maya’s Ceramics here in Willits made us several beautiful ollas to use in this experiment. Each pot had a total volume of around three liters. Two of the ollas had walls 1/4” (6.4mm) thick, and one was thicker at 3/8” (9.5mm), and one was thinner at 1/8” (3.2mm). The thinnest olla was fired longer, almost glazing, and therefore the terra-cotta had a less porous porcelain-like consistency.

We wanted to find out if water would wick out of a thinner pot faster than a thicker one. We found that all thicknesses empty at approximately the same rate: water- wicking speed is determined more by the moisture content of the surrounding soil than vessel wall thickness. However, the olla which was fired longer and had a more porcelain-like consistency (which happened to be the thinnest one) emptied at a slightly slower rate than the others, simply because it was less porus than the thicker vessels. This was expected.

We also wanted to determine whether the ollas should be placed singly, at the base of the plants, or in a regular grid throughout the bed. Since GROW BIOINTENSIVE beds are long and relatively narrow, and since we were using zucchini squash as a test crop, we decided to run the first tests with the ollas at the bases of the plants.

Our experience with zucchini in this garden in 2018 told us to plant on three-foot centers, staggered, and also what to expect yield-wise: that we would have more zucchini than we could feed everybody living on our road by September if we planted just five plants. 2018 was the first year for this section of double-dug beds, and there was quite a bit of organic fertilizer added throughout the growing season, as well as copious overhead watering.

At the beginning of the 2019 growing season we performed a soil test, which showed proper levels of all the nutrients were present. We planted the second-year beds with similar crops to those in the previous season: corn, potatoes, squash, zinnias, sunflowers, zucchini, and a new addition, tomatoes. Only the zucchini, in one half of one bed, were irrigated using the olla system. The rest of the garden continued receiving overhead hand-watering.

Without exception, everything in the garden was stunted compared to the previous year. There was no noticeable deficiency, and soil tests showed no potential cause for the reduced yield and size of everything in the garden, from the sunflowers to the squash. There was almost no corn. The tomatoes were delicious, decent-sized and beautifully colored fruit hanging off of perfectly green, bonsai-sized plants.

This garden is at 3,000 feet elevation, atop a mountain above Laytonville, CA, about 25 miles north of Willits. We had a very late start to our growing season in 2019, with nighttime temps of 30°– 40°F (-1°– 4°C) through July. Still, this seems unlikely to have caused squash to not set fruit, for example. It is possible that watering was less frequent than the previous season, and there was definitely a reduction in organic fertilizers added. Soil compaction seems a likely culprit. Perhaps it was a combination of factors, but overall, something was amiss with this garden.

Having said that, the half of the bed which was irrigated via the ollas system was a complete disaster! Out of the five zucchini plants (which in 2018 drowned us in zucchinis) this year we harvested zero zucchinis from our garden. The plants all looked magnesium deficient.

The only two that set any fruit wound up with blossom end rot, typically caused by a calcium deficiency. How do you add liquid nutrients to a plant that you’re only watering via a clay pot? Good question. Do you pour a little bit of cal-mag into the olla at the next watering? We went with a foliar feed of a light calcium application.

A couple of strange things about the supposed calcium deficiency: 1) the soil test indicated Ca++ was in the desirable range, and 2) none of the other crops, including the tomatoes in the very next bed, displayed signs of calcium or magnesium deficiency. In our experience, if you have a calcium deficiency, all your tomatoes show it in their fruit. None of our tomatoes had blossom-end rot; only the zucchini, a few feet away, did.

Calcium and magnesium deficiencies are usually caused by over-watering. The excess water ionizes with the calcium molecules and drags them down out of the root zone where they become unavailable. We were actually hoping that these kinds of deficiencies could be prevented with this watering system, since it is impossible to overwater using the ollas.

However, it seems the opposite happened. We think the most likely explanation for our results is: the garden was fertilized for the 2019 season using only a topdressing of compost; lack of overhead watering caused the humic acid and nutrients in the compost—which would normally make their way with the water down to the root zone – to stay in the top few inches of the soil. Ollas don’t really begin to water effectively until about six inches down, because the thin neck dries out first. This dry zone also causes a lot of the microbial life in the top layer of soil, which is critical for plant health and success, to dry out.

This year, we will be using the ollas in a different garden, one which did much better in 2019. As a control, we will plant two beds with the same crop from the same source. One bed will be watered using traditional hand-watering, and the other using ollas buried on three-foot centers. This time we will not bury them quite as deep, with the aim of keeping that critical top layer of soil moist.

We feel that mulch would be helpful in this application and in most gardens in arid zones. We understand that this is not always feasible for people coping with arid conditions, but suggest that some of the plants grown for compost could be used as a thin layer of mulch. We will attempt to add a thin layer of mulch to the surface of the soil in 2020 and see how our olla gardens fare! Stay tuned for the results!

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