LAND AND OTHER AGENTS OF PRODUCTION 143 



distributed throughout the year the arable land can usually be 

 farmed. This measure of aridity divides the United States into two 

 nearly equal parts, east and west, marked by a belt of semiarid coun- 

 try located near the ninety-ninth meridian. Although this division 

 is based upon the moisture supply of the climate, it is not a true index 

 of the need of irrigation, as many sections having an annual rainfall 

 of more than 20 inches do not during the growing season receive a 

 dependable precipitation sufficient for farming purposes. Rains must 

 come at such times and in such amounts as will properly moisten the 

 soil for the preparation of the seedbed and will furnish a reasonably 

 constant supply of moisture to germinate the seed and develop the 

 plant until it reaches maturity. A check in this supply of soil moisture 

 at any stage of the growth affects the quality and quantity of the 

 yield and may greatly reduce the profits of the grower. The real test 

 of what is a humid section is therefore not the total annual rainfall, 

 but the monthly, and, in the case of many plants, the weekly amount 

 during the growing season. Viewed in this light, irrigation becomes 

 a national need rather than merely a western practice. 



One of the main advantages of farming under irrigation is that 

 the water supply needed for the growth of crops, which is one of the 

 most, if not the most, uncertain factors in other farming, is very 

 largely under the control of the grower. This advantage, however, 

 has hitherto scarcely been grasped by the farmers in the humid sec- 

 tion, and few realize that with a small outlay an irrigation plant can 

 be installed which will insure them against complete or partial crop 

 failures during droughts. Although the annual precipitation in the 

 citrus regions of Florida is 55 inches, while that at Riverside, Cali- 

 fornia, is only 10 . 74 inches, one should not conclude that irrigation is 

 not necessary in the former, as there are periods when less than i inch 

 of rain falls in 30 days, and at such times the application of a small 

 amount of water may be followed by as good results as at Riverside. 

 Under average conditions it is safe to say a drought occurs whenever 

 the precipitation in any 1 5-day period falls below i inch. It has been 

 the writer's observation that crops will usually suffer if they do not 

 receive considerably more than this amount of rain, especially during 

 the spring and early summer months. Later in the season this 

 quantity may not be needed excepting for late garden truck and some 

 fruits. The accompanying table, compiled from rainfall records of 

 the Weather Bureau taken at representative points in the humid 

 region during 10 growing seasons, 1900-1909, inclusive, shows the 



