MTJLCHED-BASIN SYSTEM OP IRRIGATED CITRUS CUI/MIM.. 3 



CULTURAL PRACTICE IN THE CITRUS DLSTRICTS 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 us dally 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. 



