We have found the conditions which are necessary for the growth and development of plant roots, namely: A firm, mellow soil. A moist soil. A soil supplied with available plant food. A warm soil. A ventilated soil. These are the most important facts about plant growth so far as the plant grower is concerned. In other words, these conditions which are necessary for root growth and development are the most important truths of agriculture, or they are the foundation truths or principles upon which all agriculture is based. Having found these conditions, the next most important step is to find out how to bring them about in the soil, or, if they already exist, how to keep them or to improve them. This brings us, then, to a study of soils. [Illustration: FIG. 12. To show where growth in length of the root takes place. Forty hours before the photograph was taken the tip of the root was 1/4 inch from the lowest thread. The glass cover was taken from this in order to get a good picture of the root.] [Illustration: FIG. 13. Radish seeds sprouted on dark cloth. To show root hairs.] [Illustration: FIG. 14. To show how water gets into the roots of plants. Water passed up into the egg through the skin, or membrane, and forced the contents up the glass tube until it began to overflow.] [Illustration: FIG. 15. To show osmose (see page 19).] CHAPTER III SOILS The soil considered agriculturally, is that part of the earth's crust which is occupied by the roots of plants and from which they absorb food and moisture. RELATION OF SOIL TO PLANTS We have learned that plant roots penetrate the soil to hold the plant in a firm and stable position, to absorb moisture and with it plant food. We learned also that for roots to do these things well, the soil in which they grow must be mellow and firm, and must contain moisture and plant food, air must circulate in its pores and it must be warm. How can we bring about these conditions? To answer this question intelligently it will be necessary for us to study the soil to find out something about its structure, its composition, its characteristics; also, how it was made and what forces or agencies were active in making it. Are these forces acting on the soil at the present time? Do they have any influence over the conditions which are favorable or unfavorable to plant growth? If so, can we control them in their action for the benefit or injury of plants? We will begin this soil study with an excursion and a few experiments. Go to the field. Examine the soil in the holes dug for the root lessons, noticing the difference between the upper or surface soil and the under or subsoil. Examine as many kinds of surface soils and subsoils as possible, also decayed leaf mould, the black soil of the woods, etc. If there are in the neighborhood any exposed embankments where a road has been cut through a hill, or where a river or the sea water has cut into a bank of soil, visit them and examine the exposed soils.