This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
As already observed above, the root hairs of plants apply themselves very closely to the particles of soil, in order to absorb the thin film of water adhering to them. This film contains plant food in a state of solution, and in greater quantity than in the root hairs themselves, but at the same time the solution is very dilute and the root hairs have to absorb a much greater quantity of water than is actually required by the plant in order to get a sufficiency of food. The nature of a soil bears a definite relation to its fertility. A sandy soil, being made up of relatively large particles, can hold only a very limited quantity of water, because the spaces between the particles are large and filled with air. If manure is applied it rapidly decays and much of the plant food in it is washed away into the drainage by rain. If liquid manure is applied, most of it runs away. On the other hand, the particles of a clay soil are much finer, hold more water and plant food, either in solid or liquid form. If some of the latter is poured on a clay soil, it can abstract ammonia, free potash, phosphoric acid, and various salts containing plant food and hold them till they are absorbed by plants. All clay soils, if not originally fertile, can readily be made so by artificial means, and it only remains for the cultivator to make them sufficiently porous by good tilth to enable the roots of cultivated plants to penetrate freely and collect the food stored.
Fig. 16. - Meadow Grass -Fibrous Root.