&6 THE ART OF CULTURE. 



stance named " coal of humus." Now if a soil were impreg- 

 nated with this matter, the effect on the roots of plants would be 

 the same as that of entirely depriving the soil of oxygen ; plants 

 would be as little able to grow in such ground as they would if 

 hyd rated protoxide of iron were mixed with the soil. All plants 

 die in soils and water destitute of oxygen ; absence of air acts 

 exactly in the same manner as an excess of carbonic acid. 

 Stagnant water on a marshy soil excludes air, but a renewal of 

 water has the same effect as a renewal of air, because water 

 contains it in solution. When the water is withdrawn from a 

 marsh, free access is given to the air, and the marsh is changed 

 into a fruitful meadow. 



In a soil to which air has no access, or at most but very little, 

 the remains of animals and vegetables do not decay, for they can 

 only do so when freely supplied with oxygen ; but they undergo 

 putrefaction, for the commencement of which air is present in 

 sufficient quantity. Now putrefaction is known to be a most 

 powerful deoxidizing process, the influence of which extends to 

 all surrounding bodies, even to the roots and the plants themselves. 

 All substances 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 prepara- 

 tion of the soil, especially its contact with alkaline metallic ox- 

 ides, the ashes of brown coal, burnt lime, or limestone, change 

 the putrefaction of its organic constituents into a pure process 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 fer- 

 tility 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 whenever this happens, the 

 soil loses its fertility. Thus, in the vicinity of Salzhausen (a 

 village in Hesse Darmstadt, famed for its mineral springs), upon 

 the meadows of Griinschwalheim, unfruitful spots are seen here 

 and there covered with a yellow grass. If a hole be bored from 

 twenty to twenty-five feet deep in one of these spots, carbonic 



