46 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
acre, 25 acres of oats averaging 40 bushels per acre, and 50 acres of corn yielding 52 
bushels of shelled corn per acre? 
2. A clay or silt loam soil -weighs in round numbers 2,000,000 pounds per acre, 8 
inches deep. How many pounds of nitrogen are contained in an acre 8 inches deep 
of a fertile clay loam that analyzes 0.25 per cent of nitrogen? (a) How many 65- 
bushel corn crops will the nitrogen contained in an acre of this soil supply? 
3. An acre of sand 8 inches deep weighs, in round numbers, 2,500,000 pounds. 
What is the nitrogen content of an acre of poor sand that analyzes 0.04 per cent nitrogen? 
4. An acre of peat soil 8 inches deep weighs, in round numbers, 350,000 pounds. 
How many pounds of nitrogen are contained in an acre 8 inches deep of a soil of this 
kind that analyzes 2\ per cent nitrogen? 
5. A certain silt loam contains 0.2 per cent nitrogen and a peat 3 per cent. In 
comparing these percentages, how may times more nitrogen are contained in the peat 
than in the silt loam? 
(a) In comparing the actual number of pounds per acre 8 inches, how many times 
more nitrogen does the peat contain than the silt loam? Why this difference? 
6. One ton of red-clover hay contains about 40 pounds of nitrogen, and 1 ton of alfalfa 
hay contains about 50 pounds. How many pounds of nitrogen are contained in 30 
acres of clover yielding 2 tons per acre and 20 acres of alfalfa averaging 5^ tons per 
acre from three cuttings? 
(a) How many pounds of nitrogen can reasonably be assumed to have been fixed 
from the air by these two crops? 
(b) At 15 cents per pound what is the value of the nitrogen contained in 5 tons of 
alfalfa hay? 
(c) Wheat bran contains 2.5 per cent nitrogen. How much bran is equivalent to 
1 ton of alfalfa iu nitrogen content? 
7. How many square inches of air over 1 acre? 
8. Atmospheric pressure averages about 15 pounds per square inch. How many 
tons of air over 1 acre? 
9. About four-fifths of the atmosphere consists of nitrogen. How many tons of 
nitrogen over 1 acre? Do you think legumes will ever run short of this element in 
their work of nitrogen fixation? 
REVIEW QUESTIONS, LESSON VI. 
1. Discuss fully the fixation of nitrogen in the soil by nodule bacteria. 
2. Name some leguminous plants. In what particulars, from the standpoint of 
soil fertility, do they differ from nonleguminous plants? 
3. Explain what is meant by inoculation of soils. 
4. What conditions affect the amount of nitrogen fixed by legumes? 
5. About how much nitrogen is fixed by a 2-ton clover crop? 
6. Explain fully how legumes may be made of most use in increasing the amount of 
nitrogen in the soil of a farm. 
7. What is meant by nitrification, and how does it differ from nitrogen fixation? 
8. Name some of the commercial materials used to increase the nitrogen content of 
the soil. 
9. Compare the value of these commercial materials with the products of vegetable 
decay in general farm practice. 
10. Is all vegetable matter in soils helpful in supplying fertility? Explain. 
11. May soils be considerd inexhaustible in fertility? 
12. Explain fully why a given soil may produce a large growth of native vegetation 
while the same soil after being brought under cultivation may fail to produce a large 
yield if the crops are removed from the land each year. 
