This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
The quantity of root is always observed to increase with the poverty of the soil in which it is growing. - Duhamel found the roots of some young oaks, in a poor soil, to be nearly four feet long, though the stem was not more than six inches. Every one may have noticed this familiarly instanced in Poa annua, the grass most commonly growing on a gravel walk, its stem minute, its root a mass of widely-extending fibres. The cause of this is evident: the nourishment which is required for the growth of the plant, can only be obtained by an increased, widely-extending surface of root, and, to form this, more sap is often required than the plant, owing to the poverty of the earth, can obtain for itself; in that case a soil is sterile, for the plant must evidently perish.
A root always proceeds in that direction where food is most abundant; and, from knowledge of this fact, we should be circumspect in our mode of applying manures, according to the crop and object we have in view. We know a soil which, being shallow, never produced a carrot, or a parsnip, of any size; but almost every root consisted of numerous forks thickly coated with fibres. Digging two spades deep produced no material advantage, the gardener applying-, as usual, manure to the surface; but, by trenching as before, and turning in a small quantify of manure at the bottom, the root always spindled well, grew clean, and had few lateral fibres. For late crops of peas, which mildew chiefly from a deficiency of moisture to the root, it is an object to keep their radicular near the surface for the sake of the light depositions of moisture incident to their season of growth; hence it will always be found of benefit to cover the earth over the roots with a little well-rotted dung, and to point it in lightly.
If it be desirable to prevent the roots of any plant travelling in a certain direction, the soil on that side should be excavated, and the cavity refilled with sand or some other unfertile earth whilst the soil on those sides of the plant whither the roots are desired to tend should be made as fertile as is permissible with its habits.
To keep the roots of trees near the surface, gardeners make an impervious substratum beneath their borders, either by ramming a bed at the requisite distance from the surface, or by placing there an asphaltic mixture of hot coal tar and lime rubbish. Roots coming in contact with these do not turn aside, but immediately cease extending in length, and produce laterals.