THE ROOT 71 
sorption, roots must have access to a suitable soil. To pro- 
duce the best results a soil must contain (1) all the essential 
mineral constituents (62); (2) moisture fer dissolving these 
materials ; and (3) air enough to supply the oxygen which is 
necessary to the life processes of all green plants. 
75. Composition of soils.— Sand, clay, and humus, or 
vegetable mold, with the various substances dissolved in 
them, constitute the basis of cultivated soils. A mixture 
of sand, clay, and humus is called loam. When the propor- 
tion of humus is very large and well decomposed, the mixture 
is called muck. Pure sand contains but little nourishing 
matter and is too porous to retain water well. Pure clay 
is too compact to be easily permeable to either air or water. 
Most soils are composed of a mixture of the two with vege- 
table mold in varying proportions, giving a sandy loam, or 
a clay loam, as the case may be. 
76. Tillage. — The advantages of tillage are: (a) that by 
breaking up the hard lumps it renders the soil more per- 
meable to air and water and more easily penetrable by the 
roots in their search for food; (6) the covering of loose, 
friable earth left by the plow and the harrow acts as a mulch, 
and by shading the soil below, prevents too rapid a loss of 
water by evaporation. Where the essential food ingredients 
are present, good tillage counts for more in making a crop 
than the original quality of the soil. 
77. Light and heavy soils. — These terms are used by 
farmers not in relation to the weight of soils, but in reference 
to the ease or difficulty with which they are worked. Light 
soils contain a preponderance of sand; heavy ones, of clay. 
Practical Questions 
1. Will plants grow better in an earthen pot or a wooden box than 
in a vessel of glass or metal? Why? (Exp. 46.) 
2. Which absorb more from the soil, plants with light roots and abun- 
dant foliage, or those with heavy roots and scant foliage? (Suggestion: 
roots absorb from the soil; leaves, mainly from the air.) 
