10 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
6. What are residual soils, and what kinds of residual soils are formed from sand- 
stone, limestone, and granite? 
7. What is loess, and how is it produced? 
8. Why are alluvial soils often found to be very coarse in the subsoil? 
9. What are the characteristics of glacial soils, and how are they related to the 
rocks from which they were derived? 
LESSON II. THE SOIL AND PLANT GROWTH. 
Under favorable conditions of sunshine and heat, aeration and 
moisture, plants grow from materials furnished to them from the air 
and from the soil. Since plant-food materials are constantly being 
removed from the soil in the growth and harvest of crops it is impor- 
tant to understand to what extent the different farm crops draw upon 
the soil for plant food, what amounts of these materials are contained 
in the different soils, and by what means the soil replenishes the essen- 
tial food materials for the needs of crops. 
What the air and the soil furnish to plants (Ref. Nos. 1, pp. 16-20, 
31-34; 3, pp. 477-482). — The atmosphere is one of the sources 
from which plant food is derived. The air is made up almost entirely 
of gases, nearly four-fifths being nitrogen, about one-fifth oxygen, and 
only four one-hundredths of 1 per cent, or about 4 parts in 10,000, 
carbon dioxid. Oxygen is used directly by plants as by animals. 
The air passes into the leaves, where a small amount of oxygen is 
taken up and combines with other materials in the cells. Carbon- 
dioxid gas is a compound of the elements carbon and oxygen. (See 
p. 4). In sunlight the green leaves of plants decompose this gas, 
fixing the carbon and returning the oxygen to the air. The carbon 
thus used comprises about 50 per cent of the dry weight of plants. 
Nitrogen is not taken directly from the air by plants, although it is a 
most important plant food. 
When a quantity of any green farm crop is cut and allowed to wilt 
and cure in the sun it loses a large part of its weight by the evapora- 
tion of the water which it contains. If the cured material is heated 
in an oven at 212° F., the temperature of boiling water, it again loses 
weight for a time from evaporation. What remains is called dry 
matter. If this be burned, the organic matter passes away as gases 
while the mineral matter remains as ash. The water from evapora- 
tion contains the elements hydrogen and oyxgen; the escaping gases 
include the elements carbon, hydrogen, oxygen, and nitrogen, and the 
ash contains compounds of the elements potassium, phosphorus, cal- 
cium, magnesium, iron, sulphur, chlorin, sodium, and silicon. All of 
these elements except carbon and a small quantity of oxygen were 
secured by the growing plants from the soil. 
It has been found by chemical analysis that the 13 elements men- 
tioned above are present in all growing crops, but they vary in 
