Desert hugels or Zai pits. These are the beds we use in our semi-arid region of the desert SW, on our specific soil type. We also used this method in the desert near Joshua Tree in California. Soils there were different, decomposing granite sand, but the method still worked well because the organisms are similar and the soil was deep. This method would not work as well on land with minimal topsoil and a ton of rocks - in those desert areas we'd use different approaches. It is awful digging up loads and loads of rock, so when we come to patches of the garden with heavy rock content (50% or more), we use those areas for growing cactus.
Soils in our current location are very hard when dry - essentially like an adobe brick. Deep, dry, hard, compact sandy clay. You can't poke a piece of rebar in the ground without a hammer, that is how hard the soil is here. A primary goal in a desert garden is to have no water leave the garden from runoff - all will hopefully soak in and be stored in the ground. To make this happen on hard, compacted, dry desert soils, you have to dramatically increase water infiltration into the soil in some way. These beds serve that purpose and others.
This soil is alkaline, very low in phosphorus and nitrogen, low in iron, but has decent amounts of other minerals. There are no worms in the soil here. The main plant composting organisms are termites,
ants, crickets, cockroaches, and fungi. We are giving them the food they need to create humus and soil that can retain and infiltrate moisture.
To achieve this, we use free, (typically low-nitrogen) compostable material buried in the ground. These could be referred to as a desert hugel or as a zai pit. If I had manure I would use it - but it's not necessary for the bed to work and produce ample food. Since the primary deficit we have is phosphorus, we decided in the beginning to approach this issue by using plant matter. Also, we don't like using off-farm inputs for both cost and contamination problems (like accidentally bringing pesticide containing straw, compost, or manure into your organic garden).
We use as many long-tap-rooted plants and iron concentrating plants as we can collect here, like pigweed amaranth and also
local common wild plants with deep taproots, like Ericameria nauseosa (chamissa, rabbitbrush) and Isocama spp. Plants with long taproots are typically dynamic accumulators that draw minerals from deep in the soil. Pigweed amaranth is great example of an iron and phosphorus accumulator. We also use any wood we can scrounge.
The keys to these beds are:
1. Loosen the soil to a good depth that will increase moisture infiltration and allow garden plants to
root deeply. I aim for my knee height, about 20 inches.
2. Get compostable high-carbon material into the ground where termites, ants and fungi can break it down. We use whatever plant material we can collect on our property - branches, garden trimmings, grass and vigorous wild plants (pigweed amaranth is a great one), live and dead material. I particularly look for any plant with a deep taproot. We also throw our kitchen waste in the pits, including bones, but in the end that makes up barely 5% of what we toss in there. Leave room for approximately 1 foot of soil on top when the plant matter is compressed.
3. When refilling the beds, leave the end result sunken. This way the bed collects rainwater, could eventually be flood irrigated if that's your method, and no water is lost from the bed when watering using other methods. You can leave excess soil on the pathways, raising them slightly. These beds will sink more as the plant matter breaks down but that can take awhile. Our first beds we filled in a little too much and they are taking a long time to sink.
Tips to consider:
We've used materials in these pit beds that some people would be wary to use - such as plants perceived as allelopathic - and they worked fine. For example, we put eucalyptus logs that were a couple years old and well-aged eucalyptus leaf mulch in our first sunken beds and grew excellent tomatoes, basil, peas, and peppers.
To dig the beds requires a pick-axe and adequate moisture to first loosen the soil, then shovel it up. This morning I was digging a new bed after a couple days of monsoons rains softened the ground - but otherwise we have to wet the ground with a hose.
I don't have pictures of myself using a pick or shovel or hoe. Those seem self explanatory - except maybe the pick. My husband happily uses a 5 lb pick and I can barely move that thing. I bought myself a 2.5 lb pick and I don't swing it over my head, instead I use it much like an eye hoe/farmers hoe. The easiest way on my body is lifting and dropping it and mostly allowing the weight of the tool to do the work.
Pictures to demonstrate the rest:
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