130 THE ART OF CULTURE. 



barren soils have been found to owe their sterility 

 to this very cause. The sulphate of protoxide of 

 iron (copperas), which forms a constituent of these 

 soils, possesses a powerful affinity for oxygen, and 

 consequently prevents the absorption of that gas by 

 the roots of plants in its vicinity.^ All plants die 

 in soils and water which contain no 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. If the w^ater is withdrawn from a marsh, 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 the air has no access, or at most 

 but very little, the remains of animals and vegeta- 

 bles do not decay, for they can only do so when 

 freely supplied with oxygen ; but they undergo putre- 

 faction, for which air is present in sufficient quan- 

 tity. 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 oxy- 

 gen 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 putrefaction of 

 its organic constituents into a pure process of oxi- 

 dation; and from the moment at which all the or- 

 ganic 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 most obvious method of removing this salt from soils in which 

 it may be contained is to manure the land with lime. The lime unites 

 with the sulphuric acid and liberates the protoxide of iron, which ab- 

 sorbs oxyg^en with much rapidity, and is converted into the peroxide 

 of iron. This conversion is accelerated by giving free access to the 

 air, that is, by loosening the soil. 



