698 SOILS: PROPERTIES AND MANAGEMENT 



uniform distribution of water. They operate under 

 little or no pressure. The system has a number of ad- 

 vantages, but in practice these are usually more than 

 offset by its disadvantages. The advantages are : 



1. The system is permanent. 2. It is economical of 

 water. 3. There is no injury to the physical properties 

 of the soil. 4. There are no obstructions at the surface. 

 5. The deep rooting of crops is encouraged. 6. There is 

 very little expense for supervision of the distribution of 

 water. 7. The accumulation of soluble salts on the surface 

 of the soil by evaporation is reduced. 8. The system may 

 sometimes be used as a means of drainage also. 



The disadvantages are : 



1 . There is a strong tendency for the pipes to be clogged 

 by the entrance of roots, especially where perennial crops 

 are grown. The porous-plug method of discharging water 

 is designed to reduce this difficulty. 2. The slow lateral 

 capillary diffusion of water in dry soil makes it necessary 

 to install the lines of pipe near together, which entails 

 heavy expense. 



The method is best adapted to shallow-rooted annual 

 crops, and least adapted to orchards. The seepage of 

 water from the pipes attracts the growing roots, which 

 are likely to enter the pipes, break up into many small 

 fibers, and clog the system. 



There are soil conditions under which this method is 

 especially useful. Where the soil is a porous sand or 

 gravel underlaid at a depth of four feet or less by a rather 

 impervious stratum, the water may be distributed rapidly 

 from the pipes so that it accumulates on the hard sub- 

 stratum and saturates the soil, the pipes being quickly 

 emptied. There is then no tendency for the roots to 

 enter the pipes, and the porous nature of the soil permits 



