37 



It was not expected that chemical absorption would take place. 

 The experiment was suggested by the statements of many 

 writers to the effect that humus is active in retaining plant food 

 by virtue of its physical structure. The data given above indicate 

 that insoluble free humic acid exerts no influence in rendering 

 phosphoric acid insoluble in water. 



The organic acids present in the soil do not, however, exist 

 to any great extent in the free state except in bogs and morasses. 

 Soils suitable for agricultural purposes contain little or no free 

 acid except carbonic acid. In most cases they give an alkaline 

 reaction. Humic acid exists as humates of the bases predomi- 

 nating in the soil. These compounds may be divided into two 

 classes, namely, (i) those insoluble or but slightly affected by 

 water, including humates of calcium, magnesium, iron, and 

 aluminum, and (2) those soluble in water including humates of 

 the alkalies. Representative of the first class, calcium humate 

 was selected for study. 



This compound was prepared by mixing 10 grams of the 

 humic acid already described with one litre of lime water con- 

 taining 2.42 grams of CaO, and allowing the mixture to stand 

 for five days with occasional shaking. The insoluble residue 

 was then filtered off and washed with cold water until the wash- 

 ings were neutral. On being subjected to combustion analysis 

 the product showed the following composition, calculated on the 

 basis of dry matter. 



Carbon 55. 



Hydrogen 4. 1 1 '; 



Oxygen 34. 



Calcium 5. 



This preparation was treated with a solution of superphos- 

 phate in the manner already described for humic acid. The solu- 

 tion contained, 



P 2 5 o. 1550 grs. 



Ca 0.0185 grs. 



The quantities of phosphoric acid remaining unabsorbed by i 

 gram of the calcium humate are given below. 



