. Soil physics and management. ts. — The mechanicalaction of plants is shown by thegrowth of roots in crevices or fissuresof rocks and the prying apart of greatmasses, thus giving other agenciesan opportunity for effective force exerted by mushrooms ortoadstools is sometimes sufficient toraise blocks of stone, while cementwalks are frequently ruined by thelifting action of roots of trees grow-ing adjacent (Fig. 12). Hilgard*makes this statement, Actual meas-urement has shown the force with which the root, , of the gardenpea penetrates, to be equal to from seven to ten atmospheres
. Soil physics and management. ts. — The mechanicalaction of plants is shown by thegrowth of roots in crevices or fissuresof rocks and the prying apart of greatmasses, thus giving other agenciesan opportunity for effective force exerted by mushrooms ortoadstools is sometimes sufficient toraise blocks of stone, while cementwalks are frequently ruined by thelifting action of roots of trees grow-ing adjacent (Fig. 12). Hilgard*makes this statement, Actual meas-urement has shown the force with which the root, , of the gardenpea penetrates, to be equal to from seven to ten atmospheres persquare inch. II. CHEMICAL AGENCIES. (a) Acids.—The atmosphere in all localities contains more orless acid gases, which in combination with the moisture of the airform acids that are brought down with the rain. These acids aremuch more abundant in the vicinity of manufacturing plants, smel-ters, and large cities where they are produced, largely by the burningof coal. Sulfuric acid is probably the most common of these and. Fig. 12.—The roots of trees formwedges for prying rocks apart. (Gil-bert, U. S. Geol. Survey.) 20 SOIL PHYSICS AND MANAGEMENT contributes much toward the breaking do\\Ti of rocks. Nitric acid isformed under certain conditions in the atmosphere, and, althoughthe amount reaching the surface of the earth per acre per annum issmall, amounting at Eothamsted, England, to from pounds pounds, yet the long-continued action of this acid during geo-logical time has done a great deal toward breaking down rocks intosoil material. In some localities hydrochloric acid forms a veryactive agent, especially upon limestone and marble. (b) Carbon Dioxide.—The most effective acid in decomposingrocks is that produced by the union of carbon dioxide and water, orcarbonic acid. Carbon dioxide is found in the atmosphere in alllocalities, but, of course, in slightly greater quantities near citiesand factories than at other jDlaces. It is considered a weak acid, yet
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