EXTENSION COUKSE IN SOILS. 31 
Obtain about 16 quarts of moist soil, mix well, and fill one 2-gallon crock within 
half an inch of the top, leaving the surface smooth and compact. In filling this 
crock tamp the soil gently so as to bring the soil particles in close contact with each 
other. Fill another crock in a similar manner within an inch and a half of the top. 
Cover this surface, which should not be too compact, with an inch and a half of loose 
dry soil. Place both crocks exposed to sun and circulating air. Do not water. After 
a week or 10 days take off the dust mulch in crock No. 2 and compare the moisture 
content of the soil beneath with the soil 1^ inches beneath the surface of crock No. 1. 
What precautions should be observed in frequent cultivation during a dry period? 
Is it possible to keep a heavy soil in good tillable condition if soil mulching is prac- 
ticed? What should be done with the garden during dry seasons to conserve the water 
applied in the evenings? When should this be done? Why? 
Field excursions. — Observations may be made concerning methods of cultivation, 
soil mulching, crop growth on low, wet lands, on gravelly knolls, etc. 
REVIEW QUESTIONS, LESSON IV. 
1 . What is meant by water-holding capacity of a soil? 
2. Distinguish between gravitational or drainage water and capillary water. Draw 
a diagram to illustrate how capillary water is held by the soil. 
3. What is meant by the ground water table? 
4. Explain the relation between texture and capillary water content of soils. 
5. Why is the water-holding capacity of soils affected by the percentage of humus 
they contain? 
6. Compare the quantity of water available for growing crops, a few days after 
heavy rains, in the depth of 4 feet of a silt loam and a very sandy soil. 
7. Explain the cause of capillary rise of water in soils. 
8. Explain fully the way in which evaporation of moisture from the soil may be 
lessened and state the principles underlying this method. 
9. Why is it that a rainfall of 15 inches in the northern part of the United States is 
as effective for the growth of crops as one of 25 inches in the southern portion? 
10. To what extent do you think that the moisture in the subsoil at a depth of 20 
feet may be counted on for support in growing large crops? Discuss fully. 
LESSON V. SOIL TEMPERATURE AND DRAINAGE. 
SOIL TEMPERATURE. 
(Ref. No. 2, pp. 218-238; or No. 3, pp. 289-294; 314-317; 325.) 
It is a well-known fact that the soil must be comparatively warm 
before plants will grow. The limits of temperature for growth vary 
considerably for different farm crops, and there is some variation in the 
temperature necessary for the growth of any one crop in different 
latitudes. It has been found that, with other conditions favorable, 
staple crops will grow when the soil temperatures are as low as from 
40° to 50° Fahrenheit, and as high as from 110° to 120°. The best 
growth ordinarily takes place at temperatures ranging from 65° to 70°. 
In the United States, especially in the northern half, the average soil 
temperatures for the growing season are considerably below these 
figures. Besides being necessary for the performance of the functions 
of growth in plants, certain temperatures are also essential in order 
that the chemical reactions and the microbiological activities furnish- 
ing available plant food may take place in the soil. 
