150 NATURE STUDY AND AGRICULTURE 
microscope 500 or 1000 times before they can be seen, and an 
ounce of good garden soil may contain several billions of 
them and include many different kinds. What their real office 
in the soil is it is difficult to understand without some knowl- 
edge of chemistry. We must content ourselves here with 
stating that they make an important contribution to its 
fertility, are always much more numerous in good soils than 
in poor soils, and their number depends upon the amount of 
humus, for in that they make their home. - 
(c) Plant Foods. — Chemists tell us that growing plants 
make use of about a dozen substances for building up their 
tissues. Only one of these, carbon, the leaves take from 
the air; all the rest are taken from the soil by the roots. The 
most important of all substances furnished by the soil is 
water, as it is not only used in making plant tissues, but it 
dissolves and takes along with it all other plant foods that 
come from the soil, none of them being used in a solid con- 
dition. An insufficient supply of soil water during some part 
of the growing season is the commonest of all the factors that 
limit the yield ; in other words, we rarely have a year when the 
crops have all the moisture they need throughout the season. 
The other plant foods that sometimes become too scarce 
in the soil are nitrogen, phosphoric acid, and potash. The 
first of these is mostly derived from the roots of legumes and 
from decaying humus, and the others from the mineral matter. 
Nitrogen is more liable to run short than the other substances, 
and we are therefore more frequently compelled to adopt 
some method of renewing the supply. 
Cultivation. — When we explained the essential conditions 
of a fertile soil in the preceding paragraphs, we stated that 
good physical condition or texture is the first requirement, 
and that this is in a measure controlled by proper tillage. 
