Chemical Nature of Soils. 73 



85. Comparison Between Clay Soils and Swamp Soils. If 

 a comparison is made between the clayey soils, which are 

 generally productive naturally, and the humus soils it will 

 be seen that the latter contain about twice as much potash, 

 magnesia, sulphuric acid and organic matter, six times as 

 much lime and a little more phosphoric acid, and yet for 

 some reason the humus soils, when well drained, may not 

 naturally be as productive as the clay soils are and here is 

 where the present methods of soil analysis fail to tell the 

 whole truth. 



86. Comparison Between Clayey Soils and loess Soils. 

 The loess soils do not show a much larger percentage 

 amount of the essential ingredients of plant food than do 

 the clayey ones. Indeed there is less of organic matter and 

 only a little more of potash, phosphoric and sulphuric acids. 

 The chief and great difference lies in the large amount of 

 lime and magnesia which they contain, the first being more 

 than 9, and the latter more than 8 times as large. If it is 

 true that these soils are largely wind-formed it is to be ex- 

 pected that these ^wo substances would appear at the sur- 

 face to be taken up by the winds more than any other of the 

 essential ingredients, first, because they are comparatively 

 soluble and hence likely to be brought up by the capillary 

 waters and left after evaporation where the wind has free 

 access to them ; and second, because they are not so soluble 

 as to be completely dissolved by the heavy rains and car- 

 ried back into the ground again. 



87. Difference Between Arid and Humid Soils. The soils 

 which have accumulated in the arid climates of the world 

 are quite markedly different from those of the more humid 

 portions, both in physical and chemical properties. The 

 per cents, given in the table of arid and humid soils are 

 those of Hilgard and are averages of 466 analyses from hu- 

 mid climates and 313 from arid. 



It will be seen that the arid soils contain more than 3 

 times as much potash, nearly 13 times aa much lime and 6 



