Raised Bed Gardening Using the French Biointensive Double Digging Method
The essence of it is that you invest a lot of effort and materials up front, inverting the soil, fragmenting the crumb structure, with associated temporary disruption of soil life and soil structure, oxidation of organic matter and release of co2 through exposure of humus to air in order to improve soil tilth, fertility, drainage, water holding capacity, cation exchange capacity, organic matter content and overall soil quality provifing an improved and healthy medium for hosting beneficial microorganisms and other creatures that make garden soil their home. You remove much of the soil in the bed, add organic matter and mineral based soil amendments to it, homogenize these materials then return the amended, enriched soil mix to the bed. This is done in increments of one (typical short (42" to 48") English D-handled *see below) digging spade blade width wide and one spade blade length deep for the width of the bed (38-50" wide depending on the gardener's height).
You want highly mineralized soil, high in tilth and fertility, producing vegetables and fruits with exceptional flavor. Add rock dust from quarries to your garden soil for trace minerals, microbe food and tilth, permanently eliminating soil compaction. This is called siltation pond fines. It is so fine that if you put some in a glass of water it turns cloudy and stays that way for quite a while before settling into a layer of sludge on the bottom; so fine it would take a microscope to see the individual particles. This is great material to feed plants and soil microorganisms and invertebrates in your garden. I think we are talking 200 mesh or finer; some fines are a little coarse by comparison to my basalt/tuff fines and some are an aggregate with the largest component almost microscopic, like the talcum powder in pyrophylite fines (dredged from the quarry's siltation ponds). To prevent compaction, permanently, fines act mechanically to interfere with clay and silt particles that when compressed when wet become highly compacted when dried out - a serious problem for gardeners needing productive high-tilth soil. I have used this material to enrich soil for the benefit of plants, microorganisms and invertebrates and to permanently prevent any compaction of my soil that would significantly reduce the productivity of the gardens. The materials include "siltation pond fines" from the following types of rock: granite, volcanic tuff (maybe basalt, very hard rock, 2nd hardest in NC) and pyrophyllite screenings. I am glad to hear that granite, high in silica is a sink for lime making my soil acidic, good for most crops, pH 6.1 to 6.6 or so. Tillage is expensive, disruptive of soil life and inconvenient when it needs to be done and the soil is too wet; i.e. weeds can get out of control and go to seed when tillage can't be done, like in Spring, to remove them and create a fine, weed free seedbed for new plantings. Fines are mixed with existing soil on site, sometimes in a 30 or 40 to 50 ratio. The clay or silt particles become permanently separated from each other and cannot be seriously compacted again. This effect is also achieved with the addition of compost or manures to the soil. The fines feed the microorganisms that help break down the fines and make trace minerals available to plants; these microbes also break down any raw or composted organic matter added to soil to become humus, feed released nutrients to crops and improve the soil crumb structure providing added tilth to your garden soil. The extra trace minerals in garden soil makes crops taste much better. The famous tomatoes grown in volcanic soils in places in Italy taste so good for this same reason. That's how fines reelieve compaction and add to soil health and vitality, crop health and disease and predator resistance and crop flavor, appearance and keeping qualities. More about fines dissolved in water. In places in the high country areas in Eastern countries people drink cold river water that is cloudy and high in mineral content; they live long lives.
I would remove the top 8" or more of your raised bed-to-be. Then, "in situ" (without turning or inverting the soil layers) tilth or dig up and pulverize an additional, lower 8-12" of subsoil; add amendments of all sorts to the 8" of soil you removed then put it back in place on top of the subsoil layers you just tilthed - then you will have a "raised bed", the new garden bed surface will now extend 4-8" above existing grade.
This will address any concern about your garden drying out during drought times if you use raised beds. The benefits outweight the disadvantages aand with the following technique you will have no problems whatsoever.
Avoid having paths between the beds that are level with existing grade but ditches or furrows between the beds. These provide much needed drainage and you can fill these ditches with mulch, weeds, hay, etc all the way up to the tops of your raised beds.
You want to to accomplish the objective of increasing depth of tilth and fertility in the soil in your beds. Mulching the top layer is mandatory. Mulch will decompose leaving the seedbed fine and loose while leaching humus and compost tea into lower layers of garden soil including sub soil areas to depths of several feet (partly because you have already tilled this zone). You want a permanent, notill, biointensive raised bed that gets better each year through the action of compost tea, humus and nutrients that leach from the top layers into the lower layers; that's a real "trickle-down bioeco-economy" with big pay back in quality produce for the table and for market. This is real Pay Dirt as J.I. Rodale described it.
After this process is completed you need never again turn your soil, except to a depth no greater than is necessary to control newly emerging weeds, break crust and create a fine, loose seedbed. You now have biointensive no-till, permanent raised beds.
This technology has been around for centuries and it works. There is a very big net gain for soil, its inhabitants and the environment.
See John Jeavons book How To Grow More Vegetables", Alan Chadwick's literature and lectures (links on my website) and Aquatias' "Intensive Culture of Vegetables, French System", All Books by Eliot Coleman and many more outstanding works of timeless value. You can read a few transcribed Alan Chadwick lectures here: http://www.ibiblio.org/ecolandtech/orgfarm/permaculture/Alan.Chadwick/ Archives of the writings of Emelia Hazelip and Marc Bonfils are here: souscayrous' biological farming & permaculture collection http://www.ibiblio.org/ecolandtech/souscayrous/ Emilia Hazelip's Synergistic Agriculture - Collected Papers Marc Bonfils' Agricultural Research souscayrous' albums (the two above) on PicasaWeb Videos of Emelia Hazelip showing raised bed construction: permascience June 08, 2008 http://www.youtube.com/watch?v=ugFd1JdFaE0 "Emilia Hazelip (1938 - February 1, 2003) was an organic Permaculture gardener who was born in Spain and began gardening seriously in the late '60s. A former Merry Prankster and pioneer of the concept of synergistic gardening, her farming methods were inspired by the work of Masanobu Fukuoka. Where Fukuoka focused most of his attention on orchards and the rice/barley crop rotation, Emilia Hazelip focused on creating and maintaining market gardens of vegetables and herbs. Emilia Hazelip, who introduced the concept of permaculture to France over a decade ago, drew on many sources as she continued to develop gardens. The work of Permaculturist Marc Bonfils with self-fertile cereal production and the microbiological research of Alan Smith and Elaine Ingham are frequently mentioned. To see more videos by the maker of this film and for contact information on how to purchase a high quality full length version (SVHS) on DVD please visit: http://www.youtube.com/user/BULLEBOULO For More Information on the Global Permaculture Movement Please Visit: http://www.permacultureplanet.com"
This post talks much about double digging and adding fines to keep clay and silt from compacting. Also talking about it being appropriate anywhere that the climate doesn't dictate otherwise.
What about if the soil dictates otherwise? Is it really appropriate to very very sandy soil? What about very sandy soil in a wet sub-tropical climate?
I usually add my organic matter on top of the existing soil and don't really dig it in. I've found that digging organic matter into my soil means I have plain sand in a few months and the digging it in actually leaves me with garden beds lower than the surrounding grade even after mulching.
By building my beds with huge amounts of organic matter on top of the existing soil, I've had better crops. And the beds I built this way seem to have far better soil over time with only the normal addition of mulch, without having to totally re-build them each planting season.
Any recommendations on getting rock dust for re-mineralizing my soil? Truck loads of heavy dust is costly to ship across the country, Florida doesn't have much in the way of basalt, granite, or volcanic rock dust. I do think my soil is in need of certain minerals and I even see signs of some deficiencies but I have not found a local place to get rock dust.