THE SOIL, HOW MADE, AND FROM WHAT 25 



hold moisture, to loosen the soil, and to supply one of 

 the most important elements of plant food; namely, 

 nitrogen. 



In countries where much rain falls, the fallen leaves 

 and twigs, as well as the dead roots, supply the humus 

 in the soil. Where rains are scanty, the soil humus 

 comes mainly from the roots. For this reason the 

 soils of dry climates contain little humus and are 

 not usually dark. They are mostly whitish or gray 

 (page 32). This is seen in the larger valleys and 

 mesas of California, and in the plains of eastern Oregon 

 and Washington, in contrast to the dark-colored soils of 

 the lands west of the Cascade Mountains. 



What is the outcome of rock weathering ? The 

 decay of the rock particles continues in the soils formed 

 from them. This goes on most rapidly in soils contain- 

 ing much humus, because in its decay, humus gives 

 off a good deal of the gas which also largely causes 

 rock decay in the air. This is carbonic gas (carbon 

 dioxid), the same gas that is produced in the burning 

 of fuel, in the breathing of animals, and in many other 

 ways. It is also the gas that causes the "fizzing" of 

 soda water, and gives it the agreeable, slightly acid 

 taste. This is because it forms with water an acid 

 called carbonic acid. In most soils containing humus 

 carbonic acid is always present, and helps to weather 

 the powdered rock. It extracts from the rock powder 

 the mineral food that plants require for their growth, 



