10 BULLETIN 521, U. S. DEPARTMENT OF AGRICULTURE. 
different conditions so that results may be compared and conclusions 
drawn. A written report should bring out answers to the follow- 
ing questions : What conclusions may be drawn as to the influence of 
color upon the temperature of soils? Are the differences recorded 
in sunlight as marked as when the sun is not shining? Why does 
dark soil warm up more quickly than light soil? What is your con- 
clusion as to the temperature of drained and undrained soils? Is it 
possible to lengthen the growing season by draining wet soils? Give 
reasons for the difference in temperature of the boxes tilted in differ- 
ent directions. What factors would you consider in selecting land 
that will produce early crops? What does this exercise show with 
regard to the value of humus in the soil ? 
Lesson 7. — Chemical Nature of Soils. 
1. The soil as a source of plant food. 
2. Relation to rock-forming minerals. 
3. Relation to water movements. 
Illustrative material: Samples of common rocks and minerals. 
Lesson 8. — Organic Matter in the Soil. 
1. Relation to physical nature of soils — texture, weight, color, tempera- 
ture, ventilation, and water-holding capacity. 
2. Relation to chemical nature of soils — a source of plant food. 
3. Humus. 
Illustrative material : Samples of muck, peat, and leaf mold. 
Exercise 7. — Effects of Organic Matter upon Soils. 
Purpose : To show how organic matter increases the water-holding 
capacity of soils and enhances their production. 
Directions : Repeat Exercise 5 with samples 2, sand, and 4, clay, 
after mixing with each, one-third of its volume of leaf mold or well- 
rotted manure. Compare the weight of each sample before and after 
mixing. Compare the w T ater-holding capacity of the mixtures with 
that of the original samples. 
Fill flower pots, 4 inches or larger (tin cans will serve the pur- 
pose if holes are punched in the bottoms for drainage), with soil 
as follows: (1) Sand; (2) two-thirds sand; one-third leaf mold; 
(3) clay; (4) two-thirds clay, one-third leaf mold; (5) loam; (6) 
two-thirds loam, one-third leaf mold; and (7) one-third loam, one- 
third sand, and one-third leaf mold. Plant the same quantity of 
wheat, peas, or some other quick-growing plant in each pot and 
keep all under equal conditions in the sunlight, giving all an equal 
quantity of water, using sample 7 as a guide for the need of water. 
Record and report: Have each student make a record of the results 
of the two tests and answer the following questions in his report: 
What is the effect of organic matter on the actual weight of soils? 
Why does organic matter increase the water-holding capacity of both 
sand and clay? Why do barnyard and green manures make soils 
