We would expect a healthy soil to have a good level of organic matter in it. This comes about as a result of the decomposition of plant material on the surface of the soil entering the soil, usually by the means of earth worms. It is then further broken down by smaller soil born insect life and then by micro organisms. We would be looking for between 3% and 6% organic matter in productive soils, the breaking down of which creates humic acid which further binds soil particle together, thus improving the structure which allow air in, which then also improves drainage.
When farmers and growers were encouraged to use artificial or inorganic fertilizers instead of organic ones, the soil organism populations reduced as the organic matter was depleted, and with that the structure diminished and the soil deteriorated. The more they used it, the worse it became. Together with the removal of hedgerows around fields to allow for the use of larger machines, and the subsequent loss of localised microclimates, in some instances the topsoil either blew away, or was drained away into the rivers. If soil has no structure, or if through compaction any structure is squashed out of it, it can be clearly be seen why it is that root activity will reduce and water can no longer drain away.