10 BULLETIN 499, U. S. DEPARTMENT OF AGRICULTURE. 



Table II gives the percentage of total moisture found in the soil 

 under the various treatments. The trees in the trenching experi- 

 ments were irrigated by running a furrow over the trenches, the 

 assumption being that the water would follow the trench encircling 

 the tree base. The basins Avere irrigated separately from a furroAA 7 

 along the sides, and the coA r er crops and disk-cultivated tree rows 

 (including treatments 1, 2, and 8) were irrigated by furroAvs in the 

 usual manner. 



The trenches did not take up much water after the first tAVO irriga- 

 tions. The soil samples Avere taken directly in the trench, but 90° 

 from the point of contact of the trench Avith the irrigation furrow, 

 so that to reach the point of sampling the AA'ater AA T ould have had to 

 move about 7 feet in the trench. These trenched trees were later 

 basined, as they AA'ere not being properly irrigated, oAA'ing to the 

 packing and decomposition of the organic matter in the trenches. 



The barley did not make a strong growth and when nearly mature 

 was from 18 to 20 inches high. It was disked into the soil in the 

 latter part of July. The sweet clover was at first sIoav in growth, 

 but by the middle of August it was 2 to 3 feet high. It was then 

 disked in and the trees basined. 



The results in Table II show that the use of the summer cover 

 crops, especially sweet clover, resulted in a marked reduction of the 

 Avater content of the soil compared with the other treatments. With 

 the limited quantity of water aA^ailable for irrigation in this dis- 

 trict it is not practicable to maintain a suitable supply of organic 

 matter in the soil by growing summer crops. The trees grown in 

 conjunction with a cover crop in these trials also showed less neAv 

 growth during the season than the trees under the other treatments, 

 and the leaves were lighter in color. The cover crop, with its fine 

 root system, secures so much of the water that the orange trees, 

 which have a relatively coarse root system unprovided with root 

 hairs, are bound to suffer. 



The effect of the different treatments in conserving the groAvth 

 water 1 is shoAA'n in Table III, which giA^es the percentage of available 

 moisture to a depth of 3 feet before irrigation, this being the critical 

 moisture period. The basin treatment AA 7 as most effective in the con- 

 servation of moisture, the cultivated rows were next best, and culti- 



1 It is well known that a crop can not remove all the moisture from the soil in which it 

 is growing. The percentage remaining at the time the crop wilts varies greatly witli the 

 soil type. The wilting coefficient of a soil is defined as the moisture content of the soil 

 (expressed as a percentage of the dry weight) at the time when the leaves of the plant 

 growing in that soil first undergo a permanent reduction in their moisture content as the 

 result of a deficiency in the soil-moisture supply. By a permanent reduction is meant a 

 condition from which the leaves can not recover in an approximately saturated atmos- 

 phere without the addition of water to the soil. The water in a soil in excess of its 

 willing coefficient is called the "growth water; " that is, it is the portion available for 

 the growth of plants. 



