2 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
ment as to which will be most profitable for the class to read. No 
attempt should be made to read tables of record data, but many of 
these can be carefully studied by the class and conclusions called for by 
the leader. If in any lesson the references should be too short, it 
will be easy to select others from the reference library; if, on the 
other hand, they should prove to be too long, the leader can cause 
certain parts of least importance to be omitted. 
The exercise equipment and supplies should be put away, and only 
such parts of them as are needed for the exercise in hand should be 
handled or used during the period. The leader should make him- 
self responsible for this practice by the class. 
The queries at the end of each exercise are intended to aid in fixing 
the leading points of the lesson in the minds of the members and 
should be conducted at the close of the practicum work. The 
majority of the questions have to do with facts brought out in the 
lessons, but some of them refer to matters which the class is expected 
to have gathered from experience and thought. 
LESSON I. ORIGIN, FORMATION, AND COMPOSITION OF SOILS. 
The intelligent use and management of the soil is based on an 
understanding of its structure and composition. A good soil consists 
largely of two parts: (1) The organic matter derived mainly from 
plants which have previously grown on the land and have decomposed 
more or less, but also to some extent from the remains of animal life ; 
(2) inorganic or mineral matter, derived originally from rocks. If 
soil is burned at a red heat, the organic matter is burned off, leaving 
the rock material. The organic part is the principal factor con- 
tributing to the dark color of soils. The inorganic is that derived 
from the rock and is made up of particles of all sizes from coarse 
sand or gravel down to those so minute that they can not be seen by 
the naked eye. Both the organic and the inorganic matter play 
important parts in determining soil fertility. 
ORIGIN OF SOIL. 
Bocks and minerals as soil factors (Kef. No. 3, pp. 1-3, 7-12). — 
Minerals are the substances of which rocks are composed and con- 
stitute the inorganic part of soils. Some familiar minerals are gypsum 
or land plaster, and calcite, which occurs in marble and limestone. 
Some of the most common rock-forming minerals are quartz, feld- 
spar, hornblende, and mica. White sand is nearly pure quartz. The 
fertility of the soil is closely related to the minerals which it contains. 
Rocks are masses of minerals, physically united, and form a con- 
siderable portion of the earth's crust. Geologically rocks are grouped 
with regard to their origin and structure. The most important 
group, agriculturally, are the aqueous rocks, so-called because they 
