276 SOIL PHYSICS AND MANAGEMENT 



30 inches or more gives sufficient moisture for producing fair crops 

 of nearly all kinds if the rainfall is distributed properly. Drouthy 

 periods are quite common. At Columbia, 8. C., 02 fifteen-day 

 periods with less than one 'inch of rainfall during the growing 

 season, April to October, occurred from 1900-11)09. At Vineland, 

 N. J., 40 periods, at Oshkosh, Wis., 27 periods, and at Ames, Iowa, 

 23 similar periods occurred during the same time. At the Illinois 

 Station from 1906-1915 there were 49 periods of drouth 15 days 

 long, while 16 were more than 25 days and six more than 30 days 

 in length. 



While this uneven distribution indicates that irrigation might 

 be practiced during some years with profit, it is very doubtful, how- 

 ever, whether it will ever be profitable for the ordinary cereals. A 

 four-year rotation 7 of corn, oats, and clover was followed on brown 

 silt loam, the common prairie soil of the corn belt, for 10 years. 

 Without irrigation a ten-year average yield was 43.5 bushels, while 

 adjoining plots, irrigated when necessary, gave a yield of 49.9 

 bushels per acre, an increase of 6.4 bushels. During the dry seasons 

 of 1911, 1913 and 1914 the yield of corn averaged 32.3 bushels 

 without and 50.8 bushels with irrigation, an iiicrease of 18.5 bushels. 

 Even with this large increase for dry seasons the average increase 

 is insufficient to pay for irrigation. 



Irrigation of truck and some fruit crops, without doubt, could 

 be practiced profitably, and in general the more valuable the crop 

 the more profitable irrigation becomes. Strawberries and bush 

 fruits respond well to irrigation, both with a finer quality of fruit 

 and a longer fruiting period. 



i 

 QUESTIONS 



1. Upon what factors does the profit from irrigation depend? 



2. Why is the irrigable area so limited? 



3. Look up some of the projects given in the table on page 257. 



4. What are the sources of irrigation water? 



5. What preparation is necessary before the land can be irrigated? 



6. Why sliould not saline water be used for irrigation ? 



7. Is the sediment carried in ^suspension detrimental or not? If bene- 



ficial, why? 



8. What are the advantages and disadvantages of irrigation when the 



crop is not growing? 



9. May too much water be used in irrigation ? 



10. What is meant by the " duty of water"? 



11. What is a second-foot of water? 



12. What is the absolute duty of water? How is it expressed? 



13. How much will a second-foot irrigate? 



14. What causes this variation? 



