68 CALIFORNIA VEGETABLES 



rule the best combination of moisture, heat, aeration and rapid root- 

 extension, which pushes the plant to its utmost in rapid and satis- 

 factory growth. 



WINTER IRRIGATION. 



Winter irrigation is increasing in California as a surety that 

 the year's water supply will be above a certain minimum. Decidu- 

 ous trees and vines, on soil that is fairly retentive, can be carried 

 through a satisfactory year's growth and fruiting with good culti- 

 vation, by artificially soaking the soil in winter. In this way injury 

 to the trees or vines by a year of scant rainfall is avoided. The 

 practice has not the same value in garden practice because there 

 still will remain the demand for summer irrigation if succession of 

 fresh vegetables is to be secured. But for ample crops of staple 

 field vegetables which are usually grown without summer irriga- 

 tion, the winter soaking method is of the same importance that it 

 is with fruit-trees it insures ample moisture every year. 



Fall and winter irrigation are very important in gardening in 

 regions of uncertain rainfall because they bring the soil into condi- 

 t;on for the early planting which is often the secret of satisfaction 

 and success. In southern California and the interior of the central 

 regions of the state as well, he who waits for rainfall to start his 

 gardening often loses half the season's producing capacity. In 

 parts of the state the rain at its maximum is seldom excessive, con- 

 sequently one incurs no danger, but invites every benefit by wetting 

 the soil well and going to work at least with the hardier vegetables 

 while the autumn sunshine still imparts warmth to the soil. 



The use of winter storm water often results in a considerable 

 contribution to the fertility of the soil in the form of silt and other 

 materials rich in plant food. 



TEMPERATURE OF IRRIGATION WATER 



It is a fact of common observation that a considerable amount 

 of water either from cold rains or from cold irrigation water may 

 cause a "shock" to a growing plant and interfere with its normal 

 development. This fact is accounted for by Professor Hilgard in 

 this way: 



ince the capacity of water for heat is approximately five times greater 

 than that of* the average soil, equal weights being considered, it follows that 

 the temperature of the soil water must exert a controlling influence over that 

 of the soil. It is not surprising then that the occurrence of cold or warm 



