68 Diseases of Truck Crops 



fited by subirrigation are celery, lettuce, and Irish 

 potatoes. Tomatoes, watermelons, cantaloupes, or 

 sweet potatoes are not benefited by it. 



The advantages claimed for subirrigation are many : 

 (i) The moisture is better controlled in the soil and 

 the roots will have easy access to it. (2) No crust 

 is formed to shut out the air from the soil, or to fa- 

 vor the development of fungous diseases. (3) The 

 soluble salts and fertilizers are not washed down 

 deeply and are not carried beyond the reach of the 

 roots. 



(b) Surface or Spray Irrigation. As this implies, 

 water is applied on the surface overhead, in the form 

 of rain (fig. 10). The many advantages claimed for 

 this system are as f oUows : (i) For the same volume of 

 water a much larger area may be irrigated, or the 

 same area may be watered with a smaller quantity of 

 water. (2) Very little skilled labor is necessary in 

 this system. (3) Large areas for irrigation can be 

 rapidly covered. (4) The rain effect will control 

 frosts. (5) There are no leaky wasteful channels, and 

 no boggy roads. (6) An economy of land in channels 

 and ditches. (7) Spray irrigation is independent 

 of the topography of the field, and may be extended 

 to lands too rolling or rough for subirrigation. 

 Truckers in the arid sections seem in favor of a com- 

 bination of spray and surface irrigation on the same 

 field. The spray is used in preparing the seed bed, 

 germinating the seeds, and for newly set out plants. 

 Later, as the crop advances in age, especially during 

 blossoming and fruiting, irrigation is carried out by 



