A 
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 were irrigated separately from a furrow 
along the sides, and the cover crops and disk-cultivated tree rows 
(including treatments 1, 2, and 8) were irrigated by furrows in the 
usual manner. 
The trenches did not take up much water after the first two irriga- 
tions. The soil samples were taken directly in the trench, but 90° 
from the point of contact of the trench with the irrigation furrow, 
so that to reach the point of sampling the water would have had to 
move about 7 feet in the trench. These trenched trees were later 
basined, as they were not being properly irrigated, owing 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 slow 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 
water content of the soil compared with the other treatments. With 
the limited quantity of water available 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 new 
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 growth 
water ‘is shown in Table I11, which gives the percentage of available 
moisture to a depth of 3 feet before irrigation, this being the critical 
moisture period. The basin treatment was most effective in the con- 
servation of moisture, the cultivated rows were next best, and culti- 
1Tt 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 with 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 
wilting coefficient is called the ‘“‘ growth water; ” that is, it is the portion available for 
the growth of plants. ; 
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