THE IRRIGATION OF THE CANE. 



when irrigation water containing 200 grains of salt per gallon was used ini 

 excess, thus permitting good drainage from the porous soil employed in the 

 tests. He also found that gypsum and coral sand mitigated the harmful effect 

 of saline irrigation waters. 12 



The nature of the salt in the water has a profound effect ; sulphates or 

 carbonates of lime and magnesia are not harmful ; it is in the chlorides of the 

 alkalies that danger lies. The danger of such water lies in their abuse rather 

 than in their use ; if the soils to which they are applied are ill-drained so that 

 the salt can accumulate, the quantity soon becomes toxic ; the natural rainfall, 

 applications of a purer supply or heavy applications of the saline water, com- 

 bined with good drainage, so as to wash out the accumulated salt, permits 

 their safe use. 



FIG. 23. 



Conservation of Soil Water. After the water has arrived in the 

 soil a great part is always lost by evaporation, and this is capable of control 

 within certain limits. A protective layer of soil in fine tilth prevents the upward 

 movement of the water by capillary attraction to the surface, and is highly 

 efficient in retaining water in the soil. Not less important is the nature of soil ; 

 soils containing much humus are especially water retentive, and this is capable 

 of control in the burying of the trash of the cane and in the plowing in of 

 green manure ; to a certain extent the benefits of these practices may be 

 attributed to the increased water holding capacity of soils treated in this way. 



The velocity and flow of the wind are also of importance in determining 

 the evaporation from the soil, and loss in this way may be controlled by 

 planting wind breaks or belts of trees. 



Another factor of very great importance is the humidity ; Eckart 11 has 

 shown that this entirely masks the effect of temperature, so much so that a 



99 



