COMPOSITION OF THE SOIL AS 
COMPOSITION OF THE SOIL 
The ingredients in the soil may be divided into two classes: 
(x) The purely mineral matters. (2) The organic ingredients con- 
stituting the humus. Soil may be considered as a mixture of 
these two ingredients with air and water. The soil ingredients 
may not only be of different chemical composition, but may be 
of various sizes and mixed with the air and water in various 
different ways. 
The size of the particles of the soil ingredients determines the 
texture of the soil. The soils with the smallest particles are the 
clays, those with the largest particles are the sands, while soils 
composed of medium-sized particles are called silts. Loams 
are soils composed of mixtures of sand, silt, and clay particles in 
fairly equal proportions. 
The grouping of these soil particles determines the structure 
of the soil. If they are packed close together like billiard balls in 
a box, the soil has a compact structure. Clay soils, when compact, 
are almost impervious to plant roots. If, on the other hand, the 
particles cling together in masses, which are packed loosely with 
large air spaces in between, the soil is in good tlth, and is well 
adapted to the growth of plants. 
The structure of the soil is important from a bacteriological 
standpoint, because the bacteria live in the tiny film of moisture 
that surrounds the soil particles. When the soil has a loose 
structure, these moisture films are thinner and the bacteria living 
within them have freer access to the soil air than when the struc- 
ture is compact. As nearly all soil bacteria require the presence 
of free oxygen, ready access to the air is an important matter. 
ORIGIN OF SOIL 
The mineral and organic ingredients of the soil have originated 
in distinctly different manners. 
The Mineral Ingredients.—These come primarily from the 
