Earthworks are meant to create conditions conducive for growth and a beneficial interaction with water. Although raised beds seem to be intertwined somehow with Permaculture, sunken beds are often a better choice in some respects depending on your context and climatic factors. Although most often employed in drylands areas, they can be applied to any climate really, especially in terms of seasonal growing during either drier or hotter seasons. With the advent of climate extremes, these beds could prove to be a good precautionary choice for when drought strikes.
Why Sunken Beds?
Simply put, sunken beds act as if they were a valley instead of a mountain (raised bed). Valley landscapes tend to have more accumulated water, be richer in organic matter, and boast more biodiversity. While raised beds do have their advantages, there is the obvious pattern recognition that raised beds dry out more quickly than sunken beds. The sun is not able to hit the south facing part of the bed as easily while wind passes over the growing space instead of interacting with it in a drying fashion. Also with raised beds, organic matter and mulch tends to slide down to the pathway over time and requires scooping it back up to the raised bed to truly harvest this carbon resource. However when working with sunken beds, the valley space accumulates organic material and does not allow it to wash away as it does with raised beds. This accumulation of organic matter reduces water consumption and boosts soil fertility, which cascades into more disease and pest resilient plants. It also brings the height of the bed quickly back up to almost a level ground surface after just a short period of intensive mulching. Furthermore, when irrigating the water doesnt shed quickly from the sunken bed making this precious resource even more efficient. This is very important in drylands areas and beyond as water becomes an increasingly scarce resource. In some cases I have even seen how sunken beds can easily be flood irrigated to create a large and even distribution of water where heavier clay soils are present. One main disadvantage of sunken beds is the ability to reach into the bed with a comfortable feeling. However by adding raised keyhole paths you can gain comfortable access without sacrificing growing space as the beds can be expanded from normal dimensions. Also sunken beds don’t warm as easily as raised beds but what I am finding in Portugal is that by April 15-20, you can still get your tomatoes in the ground early with no problem. That is a month ahead of my hometown in Cincinnati, Ohio, USA where we plant in mid May normally.
To create sunken beds, simply invert the process for raised beds. Their layout according to aspect, contour, and proximity to water resources is part of your design process. What zone they will be in, what sectors are they influenced by, and how they interact with other elements of the farm are all part of the process of design. For the part below I am referring more to the actual earthwork. However, size wise, they do often follow similar dimensions as a raised bed in terms of width three to four feet (1-1.2 m). To compensate for reaching down, you may want to reduce their width or better yet add a couple of stepping stones or keyhole paths to maintain access and space efficiency.
The first step once you have pegged out more or less where they will go is to remove the top soil. Usually you will be dealing with zero to four inches inches worth (0-10 cm). Any more and I would ask if it is even necessary for the earthwork as to not disturb richer soils. However even in deeper soils if water is scarce, if dry seasons are extended, this may indeed be necessary. Once you have removed the top soil layer store it aside where it will not be trampled on to resume later. From there dig down one to one and half feet down (30 cm-45 cm). With the subsoil at the bottom of the sunken beds bring that up to the pathway areas and begin to raise them. With raising the pathways your overall depth may go down to two feet (60 cm) in all. On slopes your bottom wall will be higher essentially making a terrace wall. From there continue the earthwork to loosen and level the sunken bed and if you have a broadfork loosen the bottom once more with depth. Once the earthwork feels more or less complete, put the top soil back in. This will mean there is some microbial life in tact and some organic matter already accumulated. While it does take more time to preserve the top soil, I do recommend this step as seeing it go into the raised pathway is a bit heartbreaking. From there put as much organic matter in as possible to fill the bed to a certain degree. I usually aim for about one foot (30 cm) in the first mulching implementation. I water this thoroughly and then apply a compost extract to reseed the microbes and speed up the healing process after this soil disturbance. The mulch should be a chop and drop of green material and be a diverse mix. The diverse approach is also true of the brown material that goes in layers with the green, which most often it is raked leaves or straw. If one has compost one can also insert in layers of this or just after the top soil. Often buying in a mix of horse manure and some bedding material is possible but do watch out for manures from animals that have been treated with chemicals. No matter the mix, in general, I leave this for six months to break down before planting. In Mediterranean climates one would want to do this earthwork in the fall when the autumn rains hit and organic matter is more plentiful. The humid winter aids in breakdown as the fungal strands have the right microclimate to really expand and start with this energy cycling. By the next summer planting season you will have built organic material and the bed in which to grow tomatoes or zucchini for example. In this waiting period I do water in dry periods and also continue to add compost extract. The more frequent the application of this microbial seeding, the quicker things will breakdown. After a growing season I do recommend either growing a cover crop or going again with a deep mulch for the winter period instead of growing a crop of say cabbage. After this second application you will be virtually at the top of the original soil level with your mulch and well on your way to growing a good crop the next spring or summer.
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