USE OF THE HUMUS. 121 



quantity. Putrefaction is known to be a most 

 powerful deoxidising process, the influence of 

 which extends to all surrounding bodies, even 

 to the roots and the plants themselves. All sub- 

 stances from which oxygen can be extracted yield 

 it to putrefying bodies ; yellow oxide of iron passes 

 into the state of black oxide, sulphate of iron into 

 sulphuret of iron, &c. 



The frequent renewal of air by ploughing, and 

 the preparation of the soil, especially its contact 

 with alkaline metallic oxides, the ashes of brown 

 coal, burnt lime or limestone, change the putre- 

 faction of its organic constituents into a pure pro- 

 cess of oxidation ; and from the moment- at which 

 all the organic matter existing in a soil enters 

 into a state of oxidation or decay, its fertility is / 

 -'increased. The oxygen is no longer employed for 

 the conversion of the brown soluble matter into 

 the insoluble coal of humus, but serves for the 

 formation of carbonic acid. This change takes 

 place very slowly, and, in some instances, the 

 oxygen is completely excluded by it. And, when- 

 ever this happens, the soil loses its fertility. Thus, 

 in the vicinity of Salzhausen (a village in Hesse 

 Darmstadt, famed for its mineral springs), upon 

 a meadow called Grimschwalheimer, unfruitful 

 spots are seen here and there covered with a 

 yellow grass. If a hole be bored from 20 to 25 

 feet deep in one of these spots, carbonic acid is 

 emitted from it with such violence, that the noise 



