1930] Peace CurnTure IN CALIFORNIA 
35 
IRRIGATION 
The majority of the feeding roots of peach trees are probably 
within the upper six feet of soil. The moisture in this soil area is 
rapidly depleted during the growing season, but it appears possible 
for roots to take some moisture from greater depths. 
In most localities irrigation is necessary during the summer. It is 
impossible to store moisture in the upper six feet of soil in mature 
orchards for any long period of time during the growing season as 
the trees exhaust the supply rapidly. Ilence, it is customary in 
many peach orchards to begin irrigation between April 15 and May 15, 
and follow with one or more irrigations before the crop is harvested. 
For normal growth of the tree and fruit it seems desirable for the 
soil to be adequately supplied during the growing season. 
There is little evidence that winter irrigation is necessary where 
winter rains are ample. In some districts, however, where irrigation 
water is available only during the winter months, and the rains are 
insufficient to fill the soil to the water holding capacity, winter 
irrigations may be given (fig. 9). Trees, however, should be ade- 
quately irrigated in the fall, and the soil should not be permitted to 
become dry after harvest and during the weeks preceding the winter 
rains. 
It is estimated that the peach should have an average annual 
minimum of 16 inches of rainfall, or its equivalent on soil having a 
high water holding capacity. Twelve to sixteen inches of rainfall in 
the winter insures enough water to fill six feet of average soil to the 
maximum field capacity. The excess water percolates to lower depths 
and may help to maintain the tree over dry summer months through 
its extraction by the deeper roots. Tt is necessary though to supple- 
ment the rainfall with irrigation. Where the soil depth is limited 
by bedrock and hardpan, water in excess of a limited amount which 
can be held in the soil or on the bedrock is lost as runoff. Experience 
shows that, even with abundant winter rainfall, summer irrigation 
will usually pay. 
A decision as to whether there is enough moisture in the soil is 
dependent upon the particular orchard. Young trees use less water 
than older trees, although it is usually advisable to irrigate to promote 
the growth of young trees. 
A soil sample taken to the depth of six feet by means of a soi 
auger or other tool is the best way to determine when to irrigate. . A 
simple method is to take some of the soil sample and squeeze it in the 
hand: if the soil crumbles easily upon releasing, irrigation is needed.