MOVEMENTS OF SALTS IN SOIL. 25 



The loss of nitrates from highly manured land during a 

 wet season is very considerable, and will frequently be 

 equal to several hundred pounds of nitrate of sodium per 

 acre. When dry weather sets in evaporation takes place 

 at the surface of the soil, the water of the subsoil is slowly 

 brought again to the surface by capillary attraction, and 

 the salts it contains are concentrated once more in the 

 upper soil, forming in some rare instances a white crust 

 of salt upon the surface. Capillary attraction has little 

 influence in the case of sandy soils. 



Besides the rapid movements of salts due to a movement 

 of the water in the soil, they have also a slow movement 

 due to their molecular diffusive power, by which their par- 

 ticles continually pass from a stronger solution to one 

 weaker. This movement is always in action in moist soil, 

 and tends to the equal distribution of all soluble matter. 

 If a dressing of nitrate or chloride of sodium is applied to 

 moist soil, the manure will dissolve and slowly spread 

 downwards, even before rain falls. Again, when a heavy 

 rain has washed all soluble salts out of the surface soil, 

 they will slowly rise again by diffusion as soon as rain 

 ceases. 



Of the soluble and diffusible salts occurring in soil the 

 nitrates are of the greatest importance as plant food. The 

 quantity of nitrates in a surface soil will vary greatly, 

 depending on the richness of the soil in nitrogen, the 

 previous conditions as to temperature and moisture, the 

 extent of recent washing by rain, and on whether the soil 

 is or is not under crop. Where a crop is growing the 

 nitrates will be kept nearer the surface, the evaporation 

 of water from a growing crop being far greater than from 

 a bare soil. The nitrates will also be constantly taken up 



