IRRIGATION 249 



water, the plant suffers, its growth is stunted, seed sets too 

 early, vitality is weakened, it becomes less able to resist the 

 attack of fungi and insect enemies, and in extreme drouth 

 it dies. 



Value of irrigation. This method of supplying plants 

 with water should be regarded as an important phase of 

 agriculture. It is an effective means of soil improvement, 

 and without it the use of fertilizers and cultivation in a dry 

 country are of little value. The practice is not limited 

 exclusively to any one part of the country, and, while it 

 has been used more extensively on the Pacific than on the 

 Atlantic coast, it has been found profitable in the East and 

 Southeast. Even in a country of abundant rainfall not a 

 season passes without a period in which additional yields 

 could be secured if there were some means at hand to supply 

 water to the growing crop at the critical time. As intimated 

 in the chapter on climatology, there may be need of irrigation 

 in regions of heavy rainfall if it is largely concentrated in 

 certain periods. 



Need of irrigation. The need of irrigation depends upon 

 the character of the soil, nature of the crop, exposure to 

 winds at certain seasons, period of time at which the rain 

 falls, temperatures, and method of cultivation. Coarse 

 sand or gravel soils dry out much more rapidly than heavy 

 clay soil. A reasonably deep soil underlaid with a retentive 

 subsoil does not dry so quickly as a shallow one underlaid 

 with hardpan or coarse open strata. Shallow-rooted crops 

 are affected by lack of rain in a shorter period than deep- 

 rooted crops, as the deeper the roots extend into the soil, the 

 more opportunity they have for reaching moisture. Areas 

 that are subject to dry warm winds lose surface moisture 

 by evaporation more rapidly than unexposed areas. The 

 proper cultivation of the surface of the soil, which forms a 

 dust mulch, preserves the water content of the soil by lessen- 

 ing evaporation. The time of year at which the fruit is 

 formed or ripened or the main growth of the plant occurs 



17 



