From Ecology Action
Thank you for your letter asking for clarification on the uses and benefits of double-digging!
Good soil structure is always the goal and double-digging may assist that process, particularly on newly cultivated land. Unfortunately, much of the world’s soil is compacted, which prevents adequate air from getting into the soil, and soil organic matter is needed to hold water. The amount of soil organic matter (humus) required for creating a good sponge for water is 4% to 6% in temperate zones and 3% in tropical zones. Additionally, if soils are demineralized the plant biomass both above and below ground will be reduced, negatively affecting microbes and humus.
There are illustrations of root depths on pages 13-16 in How to Grow More Vegetables, 7th Edition (Ten Speed Press, Berkeley, CA) that shows that roots actually reach further down than 2 feet. The further down the roots are able to reach, the more nutrients will be available to them and more nutrient cycling can occur. Well loosened soil increases air and water availability to plant roots and helps access nutrients stored deeper in the soil.
When done properly, double-digging can actually enhance the soil environment, and even support microbes and mycorrhizal threads that feed root systems by providing the air they need. The soil is never turned in the double-digging process, only loosened and slid forward off the shovel to insure that the soil strata are mixed as little as possible. That is only in the upper 12 inches. The 12 inches below that are just loosened with a garden fork. The digging board used in the process distributes any weight so as to avoid creating compaction. This is all designed to be very low impact for the soil environment. In addition, having the right tools that are proportional to your height and using the proper body posture makes the process easy and even enjoyable.
The Royal Horticulture Society in England has stated that from their experience double-digging does make a significant difference in crop growth, yields and health. Our research has shown that the GROW BIOINTENSIVE method actually rebuilds critical, fertile topsoil 60 times faster than would occur unassisted in nature. To achieve soil organic matter levels of 3% to 6% and to have those levels be stable takes much longer. Our goal is to assist the regeneration of soil organic matter as quickly as possible to offset the rapid loss of topsoil that resulted from compaction due to heavily mechanized and chemical dependent agricultural practices. We must also address the rapidly increasing need to teach people how to efficiently feed themselves.
Sheet mulching can tie up nutrients for long periods while it is decomposing, depending on the materials used. It may take from 30 to 90 days for microbes to breakdown the mulch, during which time the soil is generally unavailable for either food or compost crops. If the mulch is high in nitrogen it will take carbon from the soil and breakdown humus that is beneficial to plant growth. If the sheet mulch material is too high in carbon, additionally, it will take nitrogen from the soil and/or it will not break down very fast. If the C:N ratio is just right is still takes 2 to 3 months for the breakdown process
Once good soil structure has been developed through any or all of the following: double-digging, composting, cover crops, and/or a seven-year special cropping approach1, only surface cultivation will be needed. After good soil structure is achieved, you will probably only need to loosen the top two inches of the soil using a cultivating tool such as a hula hoe. This helps preserve the developed structure and soil organic matter. It will also cut weed roots deeply enough so that the weeds don’t return. For many, the question is: How do I know when I have good soil structure? One simple way to determine whether your soil has good structure is to squeeze a sample of reasonably moist soil firmly in your hand. Then open your hand. If the soil falls apart easily, it does not have good structure. If it holds the shape of your hand even when you press it gently with the fingers of your other hand, it does not have good soil structure. But, if the soil breaks apart into small clumps when you press it with your fingers, it probably has good soil structure.
When cultivation of perennials is done the soil is just loosened, as much as is appropriate, with a border fork and compost made without soil will be used, because soil will not have been removed from the bed during the soil preparation process.
Whenever the lower soil becomes compacted in a previously double-dug bed, the bed may be double-dug again to encourage reestablishment of good soil structure, which is well-aerated.
For more in-depth answers on this and other topics you may want to consider attending one of our 3-Day Workshops. Details on workshops at The Jeavons Center in Willits, California, can be found on Events Calendar. Additionally, there are other 3-Day Workshops held at various locations and you can find information on those at: johnjeavons.info.
1No-Digging Report No. 1. Essex, England: Henry Doubleday Research Association, 1972. Reprint. 40 pp.