MULCHED-BASIN SYSTEM OF IRRIGATED CITRUS CULTURE. 3 
CULTURAL PRACTICE IN THE CITRUS DISTRICTS OF CALIFORNIA. 
Irrigation is necessary in all the citrus districts of California, the 
rainfall being confined almost wholly to the winter months. The 
location of many of the groves on high land near the foothills, in 
order to lessen the danger of frost injury, has necessitated pumping 
water to high levels, with a proportionate increase in the cost. The 
limited water supply and its high cost have in turn led to the nearly 
universal practice of maintaining a deep soil mulch throughout the 
grove during the summer months, in order to conserve the moisture 
supply as much as possible. 
The groves are usually irrigated through furrows about every 30 
days during the summer. Three to five furrows are made between 
the tree rows, extending through the mulch to the uncultivated soil. 
As soon as the soil is dry enough after irrigation to permit working, 
the furrows are closed and the mulch again established. In the 
Riverside district the furrows are not, as a rule, led under the trees or 
between the trees in the tree row, so that the soil in the tree row 
(nearly half the total area) receives no irrigation water directly. 
The lateral movement of water in many of the soils studied is so 
slight that the soil in the tree row receives very little water during 
the summer and often becomes very dry and hard. 
The feeding root system of citrus trees is usually shallow, and deep 
plowing results in a severe root pruning. It is perhaps on this ac- 
count that deep plowing is not more often practiced. Cover crops are 
sometimes grown during the winter, and these are plowed under if 
heavy; but the disk harrow is more commonly used to work manure 
and fertilizer into the soil. 
ACCUMULATION OF PLANT FOOD IN THE SURFACE SOIL UNDER FURROW 
IRRIGATION. 
The bottom of the irrigation furrow is usually below the cultivated 
soil. The water is consequently applied for the most part below the 
soil containing the fertilizer. During irrigation the water moves 
upward by capillary action from the bottom of the furrow into the 
surface soil containing the fertilizer, but there is little or no down- 
ward movement of the water through the fertilized layer of soil into 
the root zone. After irrigation the upward capillary movement of 
the soil solution continues for a time, the soluble salts being carried 
to the surface, where they are deposited as the moisture evaporates. 
The result is that the plant-food elements reach the root zone very 
slowly, the tendency being for the soluble constituents to accumulate 
in the cultivated surface soil, where there is practically no root de- 
velopment. It is only during the rainy season that there is any ap- 
preciable downward movement of the water through this surface soil 
into the root zone. 
