Dec. a, 1918 Relation of Inorganic Soil Colloids to Plowsole 517 



adobe soil. The Riverside soil is very prone to form plowsole, more so 

 than the Whittier soil. This may be due to the higher content of iron 

 and aluminium in the inorganic colloid suspension. 



Stewart (7) suggests that the plasticity of clay is due to the presence of 

 organic aluminium compounds. The chemical association of aluminium 

 with organic substances in the colloid suspensions here studied has not 

 been examined. 



' SUMMARY 



A hard soil layer, here termed "plowsole," usually forms immediately 

 under the soil mulch in cultivated Citrus groves in southern California. 

 It often seriously limits the root system of the shallow-rooted Citrus 

 trees, and seriously interferes with penetration of irrigation water. After 

 being broken up with a subsoiler, it reforms when cultivation is resumed. 



It does not form in mulched basins, nor seriously in groves surface 

 mulched with organic matter and seldom cultivated. 



Mechanical packing is not necessary for its formation, and it forms in 

 all soil types examined in the area studied. 



No greater accumulation of water-soluble iron, calcium, magnesium, 

 or silica was found in the plowsole than in the soil mulch or subsoil. 



No greater accumulation of total ammonia-soluble silica, iron, alu- 

 minium, or phosphoric acid was found in the plowsole than in the soil 

 mulch or subsoil, after the calcium had been removed with hydrochloric 

 acid. 



The humus extract of plowsole contained more colloidal silica, iron, 

 and aluminium than either the soil mulch or subsoil, but no more colloidal 

 phosphoric acid. 



Both the organic and inorganic colloidal material in the humus extract 

 moved toward the positive pole in an electric current. 



A conventional method for the determination of inorganic "colloid 

 suspension" is given. 



Plowsole contained a markedly higher percentage of inorganic colloid 

 suspension than the soil mulch, and usually a higher percentage than the 

 subsoil. 



When soils were placed in pots in the laboratory, irrigated, and allowed 

 to dry, the percentage of colloid suspension was found to be appreciably 

 greater in the surface soil layer than in the subsurface layer, indicating 

 that the colloids moved with the capillary soil moisture. 



No relation could be observed between the percentage of inoragnic 

 colloid suspension and the percentage of organic carbon or humus in 

 the soil. 



Native uncultivated soils contained appreciably less colloid suspension 

 than did similar soils which had been under cultivation for a number of 

 years. 



