I am attempting to start a small urban farm of around 7 acres this year and because of dry conditions I have been doing some serious research and thinking about how to manage drought conditions. I am in Sacramento, Ca which is already a Mediterranean climate so the current drought increases the challenge of growing without irrigation (which is the long term goal of my system). So I have been thinking and a few things have helped me come up with a possible solution to this problem/puzzle:
1) Jack Spirko's take on Hugelkulture (or wood core beds as he likes to call them)
2) The free PDF book Gardening Without Irrigation by Steve Solomon (do a Google search for easy download)
Hypothesis: If a Hugelkulture bed is built with poles or branches deeply embedded into the soil the bed will be able to access subsurface moisture that is always present but cannot be accessed by many plants especially annuals typically cultivated for food.
This potentially means a vegetable bed that can support lush growth with no irrigation even in very dry climates (6+ months of dry weather), basically I believe that this will be a functional improvement on the typical Hugelculture bed.
Explanation: When I observe soil moisture levels in my area I have noticed that if I dig deep enough to reach a layer of clay (1 to 2 feet down usually) there is always moisture in the clay, it can be rolled into a ball or a cylinder without becoming brittle or dusty like clay exposed to the sun would -- clay only a few feet down contains moisture year around even in the driest summers! The problem seems to be that many plants cannot access this moisture or struggle to do so. Most annual vegetables will grow to a layer of clay and then the roots will form a mat running horizontal, parallel to the clay surface. Unable to penetrate this layer the roots must rely on the moisture above this clay layer and the scant 2-dimensional contact they make with the layer for moisture. If the upper soil layer dries out the plants ability to extract moisture from the clay layer will not be able to keep up with normal respiration, growth then slows and if severe enough difference exists between respiration rate and moisture uptake the plant dies either from cellular dysfunction or predation. Drought tolerant species are able to penetrate deeper into the soil, access moisture, and thus survive where other plants die. This is obvious the question then becomes can we replicate this for the benefit of plants which cannot reach greater soil depth; can we create an artificial root system to supply moisture to annual vegetables so they can survive and thrive like a desert acacia?
This idea really started to come together for me when I watch one of Jack Spirko's video on 6 core Permaculture techniques. In it he presents an argument for why the view that hugelkulture facilitates moisture balance by simply holding onto moisture in biomass is slightly flawed. While this does account for some of the benefit he sites wicking action and mycelium as other reasons for the positive effect of burying wood. Furthermore, the blog post on vertical Hugelkulture seems to highlight the beneficial effect of wicking moisture from low to high thus creating better plant growth, apparently because of greater moisture balance.
Building the Bed: In a dry climate tall Hugelkulture beds must be used more sparingly as they dry out rapidly when exposed to dry desiccating winds. They do however show promise as windbreaks for this very reason, perhaps planted with drought tolerant herbs like rosemary, lavender, and thyme. Most of the beds should be under 2 feet in height to minimize dessication. A trench is dug 1 to 2 feet deep and holes are bored throughout the base either with a post hole digger or an auger an additional 2 to 3 feet deep. The holes are filled with vertical branches stems and sticks; the trench is filled with stumps; and branches and stems are laid horizontally over the stumps (if no stumps are available the trench can simple be filled with horizontally oriented branches and limbs); finally, cover with soil, a deep mulch (6"+), and plant.
In theory all of those bore holes filled with branches, limbs and stems will act like an artificial deep root system. This effect will increase as time progresses and mycelium networks develop. It is superior to simply laying biomass horizontally because the increase surface area in contact with the moist clay increase potential respiration rates for veggies planted on top. The deeper you bore and the more "fingers" of biomass that reach into the soil the greater the effect.
Lazy farmer permaculture alternative: Plant a bunch of fast growing, deep rooted coppice trees (preferably nitrogen fixing) amongst your vegetable beds and then coppice them to allow enough light in for healthy veggie growth. The trees will penetrate the clay subsoil and when coppiced or when one eventually dies the roots will form channels of biomass deep into the soil which can wick moisture vertically and distribute it horizontally. As soil builds mycelium will function as a nutrient and moisture highway. Making you and your plants happy.
What do you guys/gals think... am I crazy or what?
Not crazy at all. If you want to use an auger or a posthole digger to dig a 4" diameter hole, that's up to you, but I would point you to my post on hoserkultur if you think you can get by on holes that are only about 1-1/2" in diameter.
You're right that growing on compacted clay is just about as difficult as growing on top of concrete -- when the top layer dries out, neither one is giving up any of the water trapped below. At a molecular level, clay has quite a bit of porosity in the plane; perpendicular to it, not nearly as much. That means that if you are on the downside of a big hill of clay, water will keep replenishing your property, much to the chagrin of someone who is trying to grow crops on top of it.
That is true my thinking is by tapping into this moisture reserve artificially we can create systems that allow drought suitable species to thrive even in dry conditions. They are like giant wicking beds. Perhaps some of the benefit you saw with your hoserkulture was simple due to improved moisture balance.
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