148 NATURE STUDY AND AGRICULTURE 
determines whether or not growing plants may obtain from 
the soil an abundance of food. (2) The mineral matter may 
differ greatly in respect to the size of the pieces or particles. 
They are smallest in clay; in sand most of the particles are 
a little larger; and in gravel they are larger still. 
Humus is the part that makes the soil dark in color. Only 
a small proportion of ordinary soil is humus, but that is a 
very important part. It improves the physical condition 
of the soil, making it loose and mellow so that the roots of 
plants can live and grow in it. It increases its capacity to 
absorb water and to hold it. It is also the source of some 
elements which plants use for food. When land is farmed the 
humus is easily exhausted, and the supply must be replenished 
from time to time. Worn out lands usually suffer more from 
lack of humus than from lack of actual plant food. 
Kinds of Soil. — As the materials that enter into the com- 
position of soils vary greatly, there are many kinds of soil, 
the chief types being clay soils, sandy soils, and loams. Soils’ 
that are composed almost exclusively of fine clay are very 
compact. They do not admit water easily, and when they 
dry out they often “ bake ” and become hard. On the other 
hand, sandy soils allow the rain waters to run through and 
carry off the plant food that they may contain. 
The best soils are loams, of which there are many grades. 
In all of them the mineral matter is intermediate in texture, 
being neither composed exclusively of coarse sand nor of a 
fine grade of clay. A sandy loam has sand for its basis, but 
it is comparatively fine sand, and a large portion of the 
particles are so fine that they rank as silt or clay. On the 
other hand, clay loams have clay for their basis, but the 
particles are not of the smallest size, and a good portion of 
them are large enough to be called grains of fine sand. A 
