Imágenes de páginas
PDF
EPUB

soil .3 of an inch of rain would double the water content of the surface foot and place

FINEST CLAY
SOIL

COARSE SANDY
SOIL

THE THREE CIRCLES REPRESENT THE

EFFECTIVE DIAMETERS OF THE GRAINS OF THE COARSEST
FINEST, AND MEDIUM LOAMY SOILS, EACH MAGNIFIED 518

DIAMETERS.

LOAMY
SOIL

THE SPACE BETWEEN THE TWO LINES OF EACH CIRCLE
REPRESENTS THE THICKNESS OF A FILM OF WATER WHICH WOULD GIVE
TO THE COARSE SANDY SOIL 1%, TO THE LOAMY SOIL 6 % AND
TO THE FINEST CLAY SOIL 38% OF MOISTURE.

[merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

it in good growing condition, while in the clay soil the same rainfall would be largely retained near the surface, above the roots,

and be quickly lost by surface evaporation from the soil itself. On the sandy soil the rain would have dissolved any soluble salts accumulated near the surface and have carried them down about the surfaces of the active root hairs, so that the crop would have been fertilized as well as watered. On the clay soil the water might even have the effect of strengthening the upward capillary rise from below, leaving the deeper soil both drier and less richly charged with soluble plant food than before.

Because of these differences of surface, too, I think of the coarse, sandy soil as less able to retain available plant food upon its grains in the fixed water and, therefore, more liable to lose it by leaching, while at the same time a smaller application of fertilizer will be able to effect a given increase in yield than can be the case with the clay soil with its much larger surface and greater quantity of fixed water. Our experiments have demonstrated that clean, coarse sand may retain in the fixed water films, against ten repeated washings in distilled water, large

quantities of potassium nitrate which, however, was recovered by suitable methods, so that, without doubt, soils do have the power to retain, against leaching, soluble salts in purely physical solution within the fixed water about the soil grain surfaces. But when the root hairs of plants place themselves alongside these fixed water films and the two films join, we have the necessary physical conditions for the transfer by diffusion of the soluble plant food, carried in the fixed films, over to the plant. It is thus easy to see that profound differences, purely physical, may exist between the large simple grains of the coarse, sandy soil and the compound granules made up of multitudes of smaller ones in the well-granulated fine clay soil in good tilth.

When, by bad management, a fine clay soil is injured in texture by puddling, by breaking down the compound grain structure which has given to it a texture more nearly like that of the coarse, sandy soil, every fine particle torn out of contact in the compound grain is now able to appropriate

[graphic]

Fig. 9-Tea bushes with the ground mulched with straw. Japan.

[graphic]

Fig. 10-Rows of eggplants heavily mulched with straw. Near Tokyo, Japan.

[graphic]

Fig. 17-Irrigated lands in the citrus belt of southern California.

« AnteriorContinuar »