Dec. 2, 1918 Relation of Inorganic Soil Colloids to Plowsole 



513 



waters used in the area here under consideration carry more bicarbonates 

 than sulphates (j). 



As a whole the deflocculating salts predominate over the flocculating 

 salts in the cases examined by the Office of Biophysical Investigations 

 in Citrus groves in southern California. Hence, so far as the readily 

 soluble soil salts are concerned, the tendency would be for the inorganic 

 colloid matter to be carried down into the soil by irrigation water and 

 rains. As a surface soil mulch is established in most cases as soon as 

 possible, the colloid matter would naturally tend to remain below this 

 layer. 



After the winter rains it was found that the amount of electrolytes in 

 the plowsole layer exceeded that in the soil surface by about 60 per 

 cent, as determined by conductivity measurements. The accumula- 

 tion of soluble salts in the plowsole layer would probably tend to pre- 

 cipitate the colloids, and the denser physical structure of this layer 

 would act as a filter. 



No relation could be established between the percentage of colloid 

 suspension and the percentage of humus or organic carbon in the soil. 

 The addition of organic matter to soil, however, was found to influence 

 the inorganic colloid state in the soil. Various organic substances were 

 added to soils from orange groves, and the treated soils put into flower- 

 pots in the laboratory. These were irrigated intermittently for a period 

 of over one year. The soils were then taken out, rolled, sifted through 

 a 2-mm. sieve, and subjected to inorganic colloid-suspension determina- 

 tions. Table VII gives the results obtained. 



Table VII.— Effect of the addition of organic matter to soils on the amount of inorganic 

 colloid suspension. Organic matter in contact with the soils for more than a year 



In most cases the addition of 3 per cent of organic matter decreased 

 the amount of inorganic colloid matter, while i per cent had less effect. 

 Apparently the state of the colloid matter was more influenced by 

 organic treatment in the sandy-loam soil than in the clay-loam soil. 



It is possible that the effect of the organic matter on the amount of 

 colloid suspension in the above experiment is due in part to the indirect 

 effect of the change in content of soluble salts. It has been found that 

 the addition of organic matter to soils very appreciably affects the solu- 



