The Root and the Soil. 65 
moist air that is in communication with the air above the 
soil. The root-hairs (101) apply themselves intimately to 
the wet surfaces of the soil particles, or extend themselves 
into cavities filled with saturated air, and are thus able to 
draw in the well-aerated soil water, with its dissolved food 
constituents, in sufficient quantity to restore the loss from 
transpiration (75), and to distend the newly-formed cells (63). 
92. The Soil is a Scene of Constant Changes. The 
part of the soil in which the roots of plants grow is far from 
being a dead and changeless medium. On the contrary, it 
is the scene of most potent vital and chemical activities. 
The dead remains of plants and animals it chances to con- 
tain are undergoing decomposition during the warm season, 
by serving as the feeding ground of countless millions of 
microscopic plants (bacteria). Through their agency, nitric 
acid, which supplies the higher plants with their most valu- 
able food element—nitrogen (255), is formed in the soil. The 
carbonic acid these remains took from the air during growth 
is also set free to slowly disintegrate the mineral soil con- 
stituents, rendering these soluble, and thus available as 
plant food. In winter, the frost separates the compacted 
particles of clods, making the latter permeable to air and 
rootlets, or flakes off new fragments of rock, thus unlocking 
new supplies of mineral fertility. 
93. The Importance of Organic Matter in the Soil. 
Crops secure a large part of their nitrogen, as well as other 
food materials, from dead organic matter, i. e., animal or 
vegetable materials. The application of such matter to the 
soil is, therefore, of great importance, where large crops are 
expected. Stable and barn-yard manure, the offal from 
slaughter-houses, tanneries, breweries etc., are all valuable 
