65 



The minimum amount of free soil moisture in four feet of soil necessary 

 to keep trees in good growing condition has been found to be from 2.5 to S 

 per cent, for apples and prunes, 3 to^4 per cent, for pears, and 1 per cent. 

 or even less for apricots and peaches. The irrigator may use these above 

 relations to determine by experiment if his soil has sufficient moisture. The 

 procedure would be to take samples from the soil around the roots and 

 determine the percentage of moisture. This is done as follows: By means 

 of a soil auger take soil samples of each foot of soil beginning with the top 

 soil where the roots begin, which may be 6 inches or more from the surface, 

 down to the depth to which they extend, which may be four or five feet or 

 more. The samples should be immediately placed in a jar to prevent 

 evaporation. A very suitable jar is a glass fruit jar which can be closed air 

 tight. The percentage of moisture is determined by weighing out 100 

 ounces of the sample before losing any moisture; then spread the sample 

 thinly over a tin plate and expose it to the sun for the greater part of a 

 day or until thoroughly air dried. If the weight is now 90 ounces there 

 was 1 ounce of free water for every 10 ounces of moist soil, which is suffi- 

 cient for good growing condition. If the weight air-dried had been 97 

 per cent, it would show that there was only 3 per cent, free moisture and 

 that irrigation was necessary. 



Professor Loughridge found from determination of soil moisture in 

 southern California citrus orchards (Bulletin 203, Office of Experiment Sta- 

 tions, U. S. Department of Agriculture) that the percentage of soil mo'st- 

 ure for a sandy loam averaged about 4.68 per cent, for the upper 4 feet and 

 5.76 per cent, for the upper 13 feet before irrigation and immediately after 

 irrigation the percentage in the upper four feet averaged about 9.64 per 

 cent. Six weeks after irrigation the amount of moisture was a little greater 

 than previous to the application of water. About one-fifth of the water ap- 

 plied was retained, the other four-fifths had been taken up by soil evapora- 

 tion and plant transpiration. For a clay loam soil the percentage of 

 moisture averaged for 5 feet depth of soil, 6.81 before irrigation, 11.27 

 immediately after irrigation, and one month after irrigation the moisture 

 percentage almost returned to its normal amount before irrigation. On a 

 heavy loam the percentage of free moisture was 5.47 before irrigation and 

 10.86 immediately after irrigation. 



3. Evaporation of Soil Water and Methods of Checking It. 



The effects of the various factors influencing soil water evaporation have 

 been investigated through very interesting experiments made in the arid 

 region by the Irrigation Investigations Office of the U. S. Department of 

 Agriculture, the Agricultural College of Utah, and others. The results 

 were obtained by means of tanks or pots filled w T ith soil and placed as nearly 

 as possible under actual field conditions for irrigation and cultivation. The 

 water applied was measured in each case and loss from evaporation was 

 obtained by weighing the tanks at desired intervals. The results obtained 

 have appeared in the following bulletins: U. S. Department of Agricul- 

 ture, Office of Experiment Stations, Bulletin 177; Utah Agricultural Col- 

 lege, Bulletins 80, 86, 104, 105. While these investigations have been 

 carried on for some time, the results so far published are not sufficient to 

 justify definite conclusions, but the work is being continued and with the 

 additional information very valuable and practical results will be obtained. 



