EFFECTS OF COPIOUS 1KRIGATION. 99 



the field may be considerable, of washing the soil, or cut- 

 ting the surface into ruts or gullies. Water may be turn- 

 ed on to the sod without fear of excessive irrigation if it 

 is only kept in motion. The more water that passes oyer 

 the surface the more valuable nutriment is brought with- 

 in the reach of the plant. Every blade of grass acts as 

 a part of a filter which retains matter that may be either 

 in solution or in suspension in the water which slowly 

 finds its way over the surface. The mechanical resistance 

 offered by the myriads of stems and leaves of the grass 



Fig. 43. IRRIGATING A RIVER-BOTTOM. 



to a current of water are such that the combined effect is 

 equal to a loss of head or level of 16 inches in 200 feet. 



This retardation of the flow helps to cause the deposit 

 of any solid matter suspended in the water, from which 

 but few springs or streams are free, and also to bring 

 every particle of the water into contact with the surface 

 of the soil, or the surface roots of the plants. JSTot only, 

 therefore, is the plant supplied with nutriment while the 

 water is in contact with it, but a supply of nutriment is 

 deposited and stored for future use. This freedom of 

 application does not exist when cultivated or plowed 

 lands are irrigated, and in their case more care and greater 

 caution must be exercised to avoid injury. It is there- 

 fore advisable, in localities where only partial irrigation is 



