126 SOILS. 



action of caustic alkalies upon sugar, gums or cellulose, combines with 

 ammonia as with other bases, and at first the ammonia can be readily 

 expelled from this as from other ammonia salts. But after the lapse of 

 some time it seems that the amidic condition is assumed, so that 

 caustic lye acts but very slowly and cannot expel the whole of the nitro- 

 gen present. This is very important in connection with the practice 

 of fertilization, as any ammonia taken up by or generated in the soil is 

 thus in the course of time rendered comparatively inert, and unavailable 

 to vegetation until nitrified. 



Progressive Changes. The natural neutral humin and 

 ulmin, as found, e. g., in the lower portions of peat beds, are 

 in the course of time by oxidation converted into ulmic and 

 humic acids, capable of combining with bases; by still farther 

 oxidation they form apocrenic and crenic acids, readily soluble 

 in water and in part forming soluble salts with lime, magnesia 

 and other bases. These acids act strongly upon the more 

 readily decomposable silicates of the soil, and in the course of 

 time may dissolve out, and aid in the removal by leaching, of 

 most of the plant-food ingredients as well as the ferric hydrate 

 of a soil. Thus red or rust-colored soils may be rendered al- 

 most white by continued " swamping " with stagnant water, 

 and be greatly impoverished ; and it is doubtless largely through 

 this agency that the underclays of coal beds and the lower por- 

 tions of peat beds, as well as peat and coal ashes, are almost 

 wholly destitute of mineral plant food. 



The Phases of Humification. The progressive changes in- 

 volved in the process of humification of vegetable matter are 

 illustrated in the table below, 1 together with the farther changes 

 by which such matter may ultimately be transformed into the 

 several varieties of coal, and finally into anthracite, which al- 

 ready represents nearly pure carbon, but in nature has some- 

 times been still farther transformed into graphite (black-lead) 

 and diamond. 



1 Data recalculated, omitting ash. 



