PERCOLATION 85 



examples of the efficient supply of water to crops in semi- 

 arid regions by soils destitute of clay, but consisting of fine 

 particles of silt and sand. 



Th. Schloesing junior (Compt. rend., cxxv. 824) has cajled 

 attention to the greater speed with which sulphate of ammo- 

 nium nitrifies in a sandy soil than in one containing much 

 clay, when both contain a similar moderate proportion of 

 water. His experiments were made with artificial mixtures of 

 sand and clay in various proportions, including a small quantity 

 of chalk ; the same percentage of water was added to each 

 mixture. All the soils were in a loose condition, and abundance 

 of air was thus provided. He concludes that the different 

 rate of nitrification is mainly due to the different thickness, 

 and therefore availability, of the water film coating the 

 particles of the various mixtures. By increasing the per- 

 centage of water in the soils containing most clay the rate 

 of nitrification was raised to that observed in sandy soils. 



A soil of coarse sand will show to best advantage in a 

 season of frequent slight showers. These small supplies of 

 water may be as thoroughly retained by the sand as by clay, 

 while they will penetrate the sand to a much greater depth, 

 and more effectually supply the needs of plant roots. 



Percolation. The conditions which affect the passage of 

 water through the soil require some remark. Percolation 

 is of course greatest where the retention of water is least ; 

 the characters of the soil which produce little retention are 

 thus favourable to a large percolation, and vice versa. 



King (The Soil, 171) determined the rate at which water 

 would pass through columns of sand of different degrees 

 of fineness, columns of clay loam, and black marsh soil. The 

 columns were one-tenth of a square foot in cross section, and 



