150 CALIFORNIA FRUITS: HOW TO GROW THEM 



We now leave the orange orchard till spring, when we plow under the 

 weeds, manure and lime. We thus aim to supply our soil with nitrate of lime, 

 potash and magnesia. Carbonic acid gas is absorbed by the water and attacks 

 the inert plant food in the soil; hard-pan is prevented both by the mechanical 

 effects and the vegetable matter and the lime. 



The basin method of irrigation, to which allusion is made, will be 

 more fully described in the following chapter. 



FERTILIZING MATTERS IN IRRIGATION WATER 



Water used for irrigation may carry in solution injurious sub- 

 stances, as, for example, alkali, as will be noted in the following 

 chapter; or it may carry very valuable fertilizing properties. These 

 facts can only be determined by analysis. Professor Hilgard has 

 found that the water of one creek in Alameda county carries to the 

 land it irrigates about half a grain of potash in each gallon, which 

 means that if twelve inches of such water were used on the ground 

 during the season, each acre would receive therefrom about twenty 

 pounds of fully available potash. At Riverside a crop of oranges 

 requires about forty-two pounds of potash per acre, of which the 

 amount the irrigation water used in that case contained thirty-five 

 pounds, beside other matters required by plants. These things have 

 a definite cash value in the market ; and this value the irrigator gets 

 as a free gift in addition to the water. Even in the case of the Nile, the 

 sediment is only part of the sum of fertility conveyed by the river. 



GREEN MANURING OR COVER CROPS 



Green manuring consists in plowing under a growth of weeds or a 

 sown crop to secure by its decay a contribution of humus to the soil. 

 Plants grown for this purpose are currently called "cover crops" 

 because they cover the soil instead of allowing it to remain bare in 

 "clean culture" of orchard or "bare fallow" of grain fields. 



All plants by their decay in the soil add organic matter to it, and 

 this matter is of nitrogenous character, but leguminous plants do this 

 and a great deal more, through their exclusive ability to use atmos- 

 pheric nitrogen gathered by the bacteria which cause nodules upon 

 their roots. There is also special value in deep-rooting legume in soil 

 amelioration. As has already been stated, where moisture is ample 

 for both alfalfa and trees this plant is being used for a permanent 

 cover of orchard ground as a substitute for clean culture. This is 

 being successfully done to some extent with almonds and walnuts, as 

 with other fruits also. In the San Jose districts apricot trees have 

 been grown for several years in irrigated alfalfa, with a gain in the 

 product of the trees. Alfalfa can be used for a certain time even when 

 its permanent stand is not desirable, for it is not difficult to destroy 

 alfalfa with a well-sharpened plow, although the roots may have 

 attained considerable thickness. Of course this practice depends upon 

 moisture supply ; where that is not abundant, clean culture for moisture 

 conservation is unavoidable. But where moisture in excess of the 



