76 Physics of the Soil. 



times as much magnesia as do the humid soils with which 

 they have been compared. They also contain some more of 

 each of the other essential plant foods except sulphur, the 

 sulphuric acid being less. 



If, however, a comparison is made between the arid soils 

 and the mean of the 10 clay soils given in the first table, 

 it will be seen that, excepting potash, lime and magnesia, 

 these contain more of the essential ingredients of plant 

 food than do the arid soils, and so, too, there is more solu- 

 ble silica. 



88. Humus. It is this product in the soil which gives to 

 it usually its dark color, but so far as its chemical composi- 

 tion is concerned its nature is not yet well understood. It 

 is a very important ingredient of fertile soils and is the 

 product of decaying organic matter 



In torrid climates where the scil is warm the whole year 

 and in arid regions where the soil is more open on account 

 of deficient moisture as well as on sandy soils wherever 

 found, the rate of complete decay is so rapid that the 

 amount of humus is generally relatively small ; but in tem- 

 perate climates, where the soil is damp, its texture close and 

 rains frequent, the organic matter decays more slowly and 

 the amount of humus in the soil is relatively greater. 



The great importance of humus in agricultural soils is 

 found in the fact that it is relatively insoluble under good 

 field conditions and does not leach away and in this form 

 becomes the food of niter-forming germs which convert it 

 by degrees into nitric acid, as one of their waste products, 

 but the essential form of nitrogen for the food of most 

 higher plants. A soil entirely devoid of humus must neces- 

 sarily be manured or given nitrogen in some other form 

 in order to make it fertile. 



89. Difference Between the Humus of Arid and Humid Cli- 

 mates. Hilgard and Jaffa have made the important dis- 

 covery that the humus of arid soils is relatively richer in 

 nitrogen than is that of humid soils and hence that smaller 



