Roots have power to choose their food; that is, they do not absorb all substances with which they come in contact. They do not take up great quantities of useless or harmful materials, even though these materials may be abundant in the soil; but they may take up a greater quantity of some of the plant-foods than the plant can use to advantage. Plants respond very quickly to liberal feeding,—that is, to the application of plant-food to the soil (Fig 40). The poorer the soil, the more marked are the results, as a rule, of the application of fertilizers. Certain substances, as common salt, will kill the roots.
Fig. 41.—Nodules on Roots of Red Clover.
Roots absorb Substances only in Solution.—Substances cannot be taken in solid particles. These materials are in solution in the soil water, and the roots themselves also have the power to dissolve the soil materials to some extent by means of substances that they excrete. The materials that come into the plant through the roots are water and mostly the mineral substances, as compounds of potassium, iron, phosphorus, calcium, magnesium, sulphur, and chlorine. These mineral substances compose the ash when the plant is burned. The carbon is derived from the air through the green parts. Oxygen is derived from the air and the soil water.
Fig. 42.—Nodules on Vetch.
Nitrogen enters through the Roots.—All plants must have nitrogen; yet, although about four-fifths of the air is nitrogen, plants are not able, so far as we know, to take it in through their leaves. It enters through the roots in combination with other elements, chiefly in the form of nitrates (certain combinations with oxygen and a mineral base). The great family of leguminous plants, however (as peas, beans, cowpea, clover, alfalfa, vetch), use the nitrogen contained in the air in the soil. They are able to utilize it through the agency of nodules on their roots (Figs. [41], [42]). These nodules contain bacteria, which appropriate the free or uncombined nitrogen and pass it on to the plant. The nitrogen becomes incorporated in the plant tissue, so that these crops are high in their nitrogen content. Inasmuch as nitrogen in any form is expensive to purchase in fertilizers, the use of leguminous crops to plough under is a very important agricultural practice in preparing the land for other crops. In order that leguminous crops may acquire atmospheric nitrogen more freely and thereby thrive better, the land is sometimes sown or inoculated with the nodule-forming bacteria.
Fig. 43.—Two Kinds of Soil that have been Wet and then Dried. The loamy soil above remains loose and capable of growing plants; the clay soil below has baked and cracked.
Roots require moisture in order to serve the plant. The soil water that is valuable to the plant is not the free water, but the thin film of moisture which adheres to each little particle of soil. The finer the soil, the greater the number of particles, and therefore the greater is the quantity of film moisture that it can hold. This moisture surrounding the grains may not be perceptible, yet the plant can use it. Root absorption may continue in a soil which seems to be dust dry. Soils that are very hard and “baked” (Fig. [43]) contain very little moisture or air,—not so much as similar soils that are granular or mellow.