204 INTRODUCTION TO BOTANY 
of securing this nutriment result in partial or complete break- 
ing down of the food substance. This is known as decay. 
While a body is undergoing decay, usually several kinds of 
bacteria and other organisms live in turn upon it. In com- 
plete decay all of the nutrient organism is used as food, or 
passes into the air as gases, or is dissolved in water and 
carried into the earth or into streams. The materials that re- 
sult from decay are not only directly the remnants of the orig- 
inal plant or animal body, but may also contain excretions 
from decay-producing organisms. Furthermore, many of these 
organisms of decay have themselves died and decayed. 
Processes of decay are of great biological importance. It is 
necessary to have the dead bodies and the waste products of 
living bodies reduced to a form that makes their removal possi- 
ble. The materials that are broken down are thus made usable 
for the future growth of plants and animals. Without decay 
all usable food material would eventually be rendered unavail- 
able for the future growth of plants and animals, so that life 
on the earth would cease. The earth’s supply of food materials 
would be locked up in organized plant and animal bodies. 
It has long been known that the introduction of decayed and 
decaying organic matter into soils enables them to sustain a 
more luxuriant vegetation. Undecayed organic matter is not 
available for agricultural or horticultural plants. Such material 
must await more complete disorganization before it can be use- 
ful. It is desirable to regulate this decomposition so that the 
largest possible amount of its products may be retained in the 
soil. This is one of the problems of scientific agriculture (see 
Chapter XIX). For example, if stable manure in large masses 
is allowed to heat under the rapid destructive action of the bac- 
teria and other living things which flourish in it, much valuable 
ammonia is given off into the atmosphere and lost. Slower 
decay, especially if underground, wastes but little ammonia. 
191. Bacteria and soils. In an earlier part of this book we 
learned that nitrogen compourids are necessary for the con- 
struction of part of the food which plants must have. In the 
