2 BULLETII^ 355^ U. S. DEPAETMENT OF AGEICULTTniE. 



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 m.any 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 aU 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. 



Rocks and minerals as soil factors (Ref. 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 famihar minerals are gypsum 

 or land plaster, and calcite, which occurs in marble and Hmestone. 

 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 



