134 SOILS. 



gions have been shown by Tollens and others to be due not to 

 ferric hydrate, as had been supposed, but to calcic, magnesic 

 and aluminic humates. None of the mineral bases or acids 

 present can be detected in the humic solution by the usual 

 reagents. 



Mineral Ingredients in Humus. That the mineral plant- 

 food ingredients present in the humus extracted by the Gran- 

 deau process, and which remain as ash when the matiere noire 

 is burned, are capable of nourishing plant growth, was directly 

 shown by Grandeau, Snyder and others. The former was in- 

 clined to consider that those substances were mainly thus taken 

 up by plants, under natural conditions. This theory, however, 

 has not been sustained by subsequent investigations; the min- 

 eral plant-food thus extracted is not a measure of the immediate 

 productiveness of the soils, as demonstrated by Snyder, and the 

 residual soils are not sterile. It is also still doubtful to what 

 extent the mineral bases and acids are naturally combined with 

 the humus-substances, it being contended by some that they are 

 brought into organic combination by the acid and ammonia 

 extraction. The investigations of Snyder and Ladd, above re- 

 ferred to, prove however to some extent at least that the humus- 

 substances are naturally combined with them, and that prob- 

 ably they are largely made available to plants through the 

 direct and indirect action of the humus compounds. This sub- 

 ject is farther considered in chapter 19. 



The nature and amounts of these mineral substances are 

 well exemplified in the subjoined full analysis by Snyder, of 

 the ash of the humus and humates extracted from a compound 

 sample of prairie soils of Minnesota, which had been thrown 

 down from the ammonia solution by simply neutralizing the 

 liquid : * 



ASH OF HUMUS FROM MINNESOTA PRAIRIE SOILS. 



Insoluble matter 2 61.97 



Potash (K 3 O) 7.50 



Soda(Na 5 O) 8.13 



Lime (CaO) 0.09 



Magnesia (MgO) 0.36 



Peroxid of I ron ( Fe,O 3 ) 3.12 



Alumina ( Al 3 Oj) 3.48 



Phosphoric acid T,O 6 ) 12.37 



Sulfuric acid (SO,) 98 



Carbonic acid COj) 1.64 



1 Precipitation with an excess of acid does not greatly change the results. 

 a In California soils this is mostly silica soluble in carbonate of soda. 



