260 SOILS 



these small furrows as into larger furrows. 

 Grain fields are more commonly irrigated by wild 

 flooding. Furrow irrigation is used almost ex- 

 clusively for fruits and farm and garden crops that 

 require inter-tillage. In many places it has sup- 

 planted flooding for watering grains and grasses. 

 It is the dominant system of irrigation in America 

 to-day. 



SUB-IRRIGATION 



The great loss of water by evaporation under 

 surface irrigation, and the inconvenience of having 

 the surface broken by ditches and furrows, has led 

 to many experiments in applying water below the 

 surface through tiles, perforated iron pipes or per- 

 forated cement pipes. Theoretically sub-irrigation 

 is vastly superior to surface watering; the surface 

 is undisturbed, the soil is not puddled as it some- 

 times is in surface watering, no water is lost and 

 it is all applied just where it is needed most be- 

 neath the surface mulch of dry soil. But sub- 

 irrigation has been found impracticable in most 

 cases where it has been tried. The one great 

 difficulty with it is the cost, which is usually out of 

 all proportion to the benefits. It costs from $60 

 to $90 an acre to equip an average field for sub- 

 irrigation with drain tile if the lines of tiles are 

 placed from four to seven feet apart, as is usually 

 necessary. This outlay cannot be justified ex- 

 cept, perhaps, on high-priced land, and especially 

 land used for market gardening. 



Another great difficulty with sub-irrigation, in 

 some cases, is in being unable to supply suffi- 

 cient water to wet the surface soil thoroughly, 

 owing to the poor water-moving power of some 



