222 THE PRINCIPLES OF SOIL MANAGEMENT 



condition where the natural rainfall is deficient at any 

 period of the growing season. Consequently, irrigation 

 is practised even under the very humid climate of 

 Florida, with sixty inches of rainfall, around New 

 York City and Boston, with forty inches of rainfall, and 

 at many other places in the United States and Europe, 

 where a so-called humid climate prevails. In these latter 

 places it is identified with special crops of high value 

 which will justify the expense involved. In France, Ger- 

 many and other European countries, there are extensive 

 areas of grass land which are artificially watered, often 

 with sewage, which adds the element of food supply as 

 well as water. Of course, all greenhouse management in- 

 volves the practice of irrigation. 



Many engineering problems are involved in the prac- 

 tice of irrigation, and have to do with the collection, 

 storage and application of water to the land. But the 

 principles which govern the application the method, 

 time and amounts of water suitable for each crop and 

 soil are purely agricultural considerations, to be han- 

 dled in each case as the local conditions may indicate. 



The amount of water necessary to be added to pro- 

 duce a full crop constitutes the "duty," or efficiency, 

 of water. It is the least amount of water which will 

 produce a given yield under a given set of conditions. 

 The "duty of water" depends upon a great many 

 factors; in fact, is limited by as many things as affect 

 the moisture supply of soils in humid regions. The dis- 

 cussion of irrigation which follows presupposes an ade- 

 quate supply of water, a condition often not fulfilled. 

 For example, the area of the Western States containing 



