268 PRINCIPLES OF AGRICULTURAL, CHEMISTRY 



with their roots just above an accumulation of alkali, but if the 

 alkali rises to the roots, or if deeper rooting crops are grown, 

 injury will result. 



In order to prevent the rise and accumulation of alkali, the 

 irrigator must keep the main movement of the water downward 

 through the soil and out in the ground-water. If the water 

 movement is maintained in this direction, the alkali will not 

 accumulate in the soil. The movement need not be at all times 

 in this direction, but the main movement must be this way, or 

 alkali will accumulate. 



If the main movement of the water is up through the soil, 

 alkali will accumulate. In a sub-irrigated soil, the movement of 

 water is up, due to evaporation of water, and to transpiration 

 from the leaves of plants. Hence, sub-irrigation means an 

 accumulation of alkali. 



Furrow-irrigation is a kind of sub-irrigation. The soil at the 

 top of the furrow is irrigated from below, and, in this particular 

 part of the soil, the movement of the water and dissolved salts 

 is up ; so that it will accumulate alkali. If furrow-irrigation is 

 used in places liable to alkali, the soil should occasionally bq 

 leveled and flooded sufficiently often to keep the main move- 

 ment of the water down through the soil. 



If flooding is practiced, and hillocks occur which are not 

 covered by the water, such spots are sub-irrigated, and alkali 

 will accumulate in them. Within these hillocks, the movement of 

 water is upwards, due to evaporation from the surface. Such 

 hillocks should be leveled before flooding begins. 



Anything which will counteract surface evaporation will aid in 

 keeping the main movement of the water down and through the 

 soil. The greater the extent that evaporation is prevented, the 

 less water is needed for the irrigation. Evaporation from the 

 surface, therefore, should be prevented as much as possible. 

 Careful and frequent cultivation will produce a mulch which will 

 do much towards checking evaporation. Trees or crops on the 

 land will also shade the surface and check evaporation. 



If the water-table is too near the surface, flooding must be 



