SOILS 149 
clay or silt loam is considered the best kind of soil. Humus 
is an essential constituent of all loams. 
Fertile Soil. — Let us now learn what are the factors that 
make a soil fertile. By this we mean a productive soil, one 
in which plants thrive. Many people have the idea that the 
only requirement of a fertile soil is the presence of enough 
plant food, but we must understand that several other factors 
are fully as essential. 
(a) Physical Properties. — The growing roots of plants not 
only require food, but water, air, and warmth. Moreover, 
the soil must be so mellow that the roots can push their way 
through it without difficulty, and yet so compact that they 
shall press firmly against the particles and come in close con- 
tact with them. The degree of compactness of the soil, its 
power to absorb rain water from the surface, to bring up 
water from below, and to retain water depend upon the relative 
amount of sand, clay, and humus, and to some extent these’ 
qualities are the result of tillage. To a certain extent, there- 
fore, we may control the physical properties of the soil and 
therefore its fertility by keeping it supplied with a sufficient 
amount of humus and by proper cultivation. 
(b) Soil Bacteria. — Bacteria are among the smallest and 
simplest of all plants. Perhaps we hear of them most fre- 
quently in connection with epidemic diseases like diphtheria 
and typhoid fever, when they are often called germs or mi- 
crobes, and hence we may easily get the idea that all bacteria 
are injurious. But while it is true that some of them cause 
diseases in animals and others produce them in plants, it 
should be understood. that the harmful kinds are only a very 
small proportion of these lower plants, just as among the 
higher plants only a few are poisonous. 
Bacteria are so small that they must be enlarged with a 
