=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 plow 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_. [Illustration: FIG. 42.--NODULES ON VETCH.] [Illustration: 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. =Proper Temperature for Root Action.=--_The root must be warm in order to perform its functions._ Should the soil of fields or greenhouses be much colder than the air, the plant suffers. When in a warm atmosphere, or in a dry atmosphere, plants need to absorb much water from the soil, and the roots must be warm if the root-hairs are to supply the water as rapidly as it is needed. _If the roots are chilled, the plant may wilt or die._