Planting 'digger' plants which grow roots and leave air cavities behind them when the roots die is part of how we achieve a stable and constant balance of soil water and air. This kind of idea gives us really rich and living soil in the end, because the longer you leave soil organisms undisturbed in a perfectly aerated soil with enough water and organic material, the richer and more fertile your soil becomes. Just keep feeding from the top and if you planned well enough for drainage, and your original soil make up of clay like and sandy particles is well balanced, the soil in your beds will become a living cornucopia of every kind of nutrient and useful molecule that a whole range of plants will need. A properly living soil, as opposed to the periodically reactivated soil on an industrialised monoculture farm, is the ultimate goal of a biodynamic farming system. All products of the growing and harvesting of plants and animals of all kinds can be used to feed the soil. Using Bokashi bins (anaerobic fermentation) and compost heaps and worm farms being fed bokashi processed solid waste we can turn any organic material back into soil. Easily. So it is easy to see that the creation of good soil is ultimately a cycle. We like to think ofit as a Positive Cycle. Feeding soil that has been allowed to develop naturally. Reaping the bumper harvests that result from growing crops in living soil. Returning what we don't use to the soil to be used again as a new harvest. That's how we survived all these millions of years.