Note the sandy loam soil and sheer amount of compost double dug in. Sand has very low cation exchange capacity (CEC). Compost has very high CEC, or in simpler terms, it holds onto nutrients.
Double digging can be a great way to distribute soil organic matter and particularly carbon into the soil profile that otherwise decreases linearly the deeper you go and the less air there is. Initially this can help soil microbes deeper in the soil profile form soil aggregates and build soil structure for roots to penetrate. Soils are typically aerated to some extent down to about 60cm, exactly what the double dig doctor ordered.
Ideally double digging is amended as you go with a nitrogen and carbon source as the deeper in the profile you go the less air and hence nitrogen there is for microbes and plant roots to build soil structure, so adding a nitrogen and carbon source will speed the process. Australian subsoil manuring trials recommend a C:N amendment of less than 25:1 for our heavy acidic clays, clays that are not something you can easily double dig and so special machinery has been developed.
Another option is keyline plowing, or ripping. However this approach tends to only be beneficial on some soils, especially those with excessive run-off, and when nitrogen and carbon rich organic matter is injected into the subsoil during the process.
It’s important to note that depending on the soil type, double digging alone can destroy soil aggregates and fungal hyphae, especially when left fallow and to dry out or not amended with a food source for microbes that create these aggregates and soil structure, so it’s best to always amend when you dig.
It only takes two weeks of drying and UV exposure for the glues that microbes create to dry out and start to leach away, leaving unstructured soil that compacts easily. Keep those microbes fed and watered!
If you watch John Jeavons videos you’ll see on their farm they use nitrogen amendments like alfalfa meal, also kelp meal for potassium which encourages colonisation by mycorrhizal fungi and can reduce sodicity in soils, also some compost. Mixing the topsoil will release soil carbon to dissolve and the nitrogen they add quite easily leaches into the lower profile of soil with watering.
Note the soil type… the Willits Area of California where I understand their Mini-Farm is located, is dominated by loam, fine sandy loam, fine sandy loam,gravelly clay loam, fine sandy loam, loam, silt loam, silt clay. They appear to have a good, but not ideal amount of soil carbon in that area too with between 5 and 10% over 1 meter. Adding more in the form of compost will bring it pretty quickly up to the 2.5% point throughout the soil profile that produces highest yield.