514 



Journal of Agricultural Research 



Vol. XV, No. 9 



bility of the soil minerals (5). The nature of the compounds thus pro- 

 duced has not been investigated, nor has the effect of the by-products 

 of the decomposition on the inorganic colloid matter of the soil. 



The addition of i per cent of organic matter represents approximately 

 10 tons dry matter per acre 6 inches. This is probably more than is 

 usually applied per acre, at any one time in commercial citriculture in 

 southern California. The improvement in the physical condition of the 

 soil by the addition of organic matter in the customary amounts would 

 thus hardly seem to be due to any marked changes in the state of the 

 inorganic colloid matter of the soil but rather to the mechanical separa- 

 tion of the soil aggregates. 



Soil from the same groves represented in Table VII was treated with 

 certain organic and inorganic substances, put into pots in the laboratory 

 The soils were irrigated intermittently for over a year. The soils were 

 then rolled, passed through a 2-mm. sieve, and subjected to inorganic 

 colloid determinations. Table VIII gives the results obtained. 



Table VIII. — Effect of the addition of certain organic and inorganic substances to soils 

 on the amount of inorganic colloid suspension. Substances in contact with the soils for 

 more than a year 



All the clay-loam soils are from the same large composite sample, but 

 owing to the impossibility of selecting samples entirely comparable, each 

 soil number should be considered as a separate experiment. It would 

 seem evident, however, that the organic matter tended to counteract 

 the flocculating effect of the lime in the clay-loam soil. The organic 



