The Soil. 57 



age water united with calcium. This is illustrated in the fol- 

 lowing equation: 



Calcium carbonate -f- potassium chloride = calcium 



chloride + potassium carbonate. 



The addition of marl or limestone may thus greatly increase the 

 .retentive power of a soil for bases. The bases absorbed by the 

 soil may be slowly removed by the action of water. This of 

 course occurs to the least degree in a soil that has absorbed little 

 or has been already washed, and is greatest in a soil that has been 

 .heavily manured. 



The permanent fertility of a soil is closely connected with its 

 power of retaining plant food. In soils containing clay, only 

 traces of phosphoric acid, ammonia or potash are ever found in 

 the drainage water. Sandy soils, from their smaller chemical 

 retentive power and free drainage, are of less natural fertility 

 and much more dependent on immediate supplies of plant food. 



There can be little doubt that the active plant food contained 

 in a soil, which is capable of being taken up by roots, exists 

 either in solution or in the states of combination just referred 

 to that is, in union with ferric oxide, hydrous silicates or hu- 

 mus. Different crops have very different powers of attacking 

 these various forms of plant food. 



Nitrification. Perhaps the most important reactions going on 

 in a soil are those connected with the decay of organic matter 

 and the changes in the state of combination of the nitrogen. The 

 organic matter is continually being oxidized, the carbon being 

 mainly converted into carbon dioxide. The material from which 

 the nitrogenous matter of soils is derived contains always a large 

 proportion of carbon. In the roots and stubble of cereal crops 

 the relation of nitrogen to carbon is about 1:43; in those of 

 leguminous crops 1 :23 ; in moderately rotted farm manure 1 :18. 

 In an aerated soil these materials are oxidized by the action of 

 various organisms (worms, fungi, and bacteria) and large quan- 

 tities of carbon dioxide produced. As a result of this loss of 

 Carbon, we find that the surface soil of a pasture (roots removed) 



