66 Principles of Plant Culture. 
for this purpose, when wisely used. Not only does organic 
matter in the soil furnish plant food, but while in a partially 
decomposed state (humus), it renders the soil porous and 
greatly increases its water-holding power. 
94. The Soil Needs Ventilation. The roots of grow- 
ing plants, and the decomposition of organic matter in the 
soil, tend constantly to exhaust the soil of its free oxygen, 
and to replace this with carbonic acid, which is not used by 
roots. Hence, without some interchange between the con- 
tents of the soil cavities and the atmosphere above, the roots 
sooner or later become smothered and perish. In sufficiently 
porous soil, changes in temperature and atmospheric pres- 
sure, aided by wind and rain, furnish the needed soil venti- 
lation, but in poorly drained soils, and soils not thoroughly 
tilled, the roots of plants often suffer from insufficient oxy- 
gen. A puddled crust on the surface of clayey soil, due to 
the compacting influence of rain, is a great hindrance to soil 
ventilation. 
95. Hotbeds (365) Require Especial Care in Venti- 
lation, since they usually contain large quantities of decom- 
posing organic matter (manure), which rapidly absorbs oxy- 
gen from the soil, replacing it with carbonic acid. 
96. Drainage Promotes Soil Aeration by forming an 
outlet for the surplus water that would otherwise fill the 
cavities. Although moisture is essential to root growth, 
land plants do not prosper with their roots immersed in 
water. True, most plants may be grown in “ water culture,” 
i. e., with their roots, from germination, grown in water that 
is freely exposed to the air; but the roots of land plants 
soon smother for want of free oxygen, when the soil cavities 
