86 



6 weeks after irrigation was reduced by soil evaporation and plant trans- 

 piration to about the same or slightly higher than just before irrigation. 

 Based on these figures with irrigation at six week intervals, the percentage 

 of free moisture to add at each irrigation would be about 5 per cent, for 

 sandy loam and 4.5 for clay loam or heavy loam, or about 5 pounds of 

 water to 100 pounds of soil. For a soil air dried weighing from about 100- 

 to 110 pounds, this is equivalent to about 1 inch of water to 12 inches 

 depth of soil. This gives for the correct quantity of water to put in the 

 upper six feet of soil in order to maintain the proper quantity of soil 

 moisture, a depth of about 6 inches of irrigation, applied at six week inter- 

 vals during the growing season. 



These figures are based on results obtained for full grown citrus 

 orchards which in California usually require about twice as much as apple 

 trees or other deciduous trees. Therefore about 3 inches of irrigation 

 water at each irrigation separated by intervals of 45 days should be ample. 

 That this amount may be sufficient for a young orchard at least is shown 

 by the following case in Idaho described by Don H. Bark, in charge of Irri- 

 gation Investigations for the U. S. Department of Agriculture. The orch- 

 ard is a thrifty 5 year old orchard located at Twin Falls where the annual 

 precipitation varies from about 8 to 18 inches. During the growing season 

 of 1910 the rainfall was 1% inches and the depth of irrigation water ap- 

 plied was about 7 *4 inches. The soil was kept well mulched by frequent 

 cultivations and the moisture at the end of the season in the fall was fully 

 as high as at the beginning of the season. This one caee is not conclusive 

 but it shows the possibilities of small quantities of water carefully used. 

 The examples of net duty of water on orchards in eastern Washington and 

 Montana given on page 61 indicate the quantities used with ordinary care 

 in irrigation and cultivation. It is interesting to notice that the measure- 

 ments made on citrus orchards showed that the amount of soil moisture 

 supplied to sandy loam soil was practically the same as for heavy loam soil 

 and clay loam. This is probably true of any soil which is sufficiently deep 

 and not underlaid with a very open sandy or gravelly soil, in which case 

 the soil should be irrigated more frequently and less depth of water applied 

 at each irrigation. 



2. Number of Irrigations Per Season. 



The number of irrigations will depend on the capacity of soil to retain 

 water. The orchardist should be guided mainly by the examination of his 

 soil and should try to keep the moisture content within the limits stated 

 above; that is, the free moisture should be between 5 and 10 per cent, of 

 the weight of the soil. For an open soil, well drained, or for a shallow soil, 

 light irrigations applied frequently are best; for a deep retentive soil three 

 to four irrigations 4 or fi inches deep are ample. The practice in some of 

 the fruit growing districts of Washington, Idaho, Montana and Colorado is 

 described by Professor Fortier in U. S. Department of Agriculture, Farmers' 

 Bulletin 404, on Irrigation of Orchards, as follows: 



"In the Yakima and Wenatchee fruit-growing districts of Washington, 

 the first irrigation is usually given in April or early in May. Then follow 

 three to four waterings at intervals of 20 to 30 days. At Montrose, Colo., 

 water is used three to five times in a season. At Payette, Idaho, the same 

 number of irrigations is applied, beginning about June 1 in ordinary sea- 

 sons and repeating the operation at the end of 30 day intervals. As a rule, 

 the orchards at Lewiston, Idaho, are watered three times, beginning about 



