Posted: 11/04/2013 at 11:25
One other benefit of rotavation is to improve soil 'conditioning'. Heavy clay soils are rich in nutrients, but the fine particles stick together (a complex process involving chemical ions that I don't fully understand), which makes the soil claggy and saturated in winter (plant roots hate this) and it dries out like concrete in summer, with cracks that allow moisture to run through into the sub-soil so the upper leyer dries out. And walking on this type of soil compacts it much more readily than sandy or loamy soil. So plants are more likely to drown in winter and die of thirst in summer, despite all those nutrients. And planting is harder work, and in summer the garden hoe just skims off the surface.
Digging and turning the soil so it becomes 'friable' - i.e. small particles - helps aerate the soil. If organic matter such as manure or garden compost, or lime or gypsum are added at this stage, they have special properties that reduce the ionic effect that attract the fine soil particles to one another, so that the soil behaves a lot better in retaining moisture without going 'claggy'.
Digging heavy clay soil is hard work at any time of the year, but a rotavator makes light work of it. And if you chuck your conditioning material onto the soil while rotavating, it becomes permanently improved.