14 THE SCOTTISH BOTANICAL REVIEW 
PART III. THE RELATION OF SOILS TO CLIMATE 
AND PHYSIOGRAPHY. 
Generally speaking, soil characters depend more on 
geography and climate than on the local nature of the 
underlying rocks. The soils of high altitudes, high latitudes, 
and deserts are usually mechanically formed and porous, and 
such as Thurmann designated by the name “ dysgeogenous ” 
(17), while those of low levels in humid, temperate, and tropical 
climates have generally suffered marked chemical decomposi- 
tion, and, according to Thurmann’s classification, would be 
eugeogenous. 
Desert soils are stated by Hilgard to contain a high 
percentage of lime, irrespective of the nature of the district 
rocks (18). The abundance of lime and alkaline salts in 
deserts depends almost entirely on climatic and physiographic 
factors. 
The soils of warm, humid climates undergo profound 
chemical decomposition and leaching. According to Russell, 
deep residual red soils are formed under conditions of con- 
siderable warmth and great atmospheric precipitations from 
rocks of various composition and origin (19). 
In arctic regions the ground is permanently frozen, except 
during a few weeks in summer, when the surface may thaw 
to the depth of a few inches. Wherever humus accumulates, 
the soil must be for the most part acid. The production of 
neutral humus and nitrates, being dependent on an absence of 
acidity and a certain degree of warmth (20), is no doubt chiefly 
confined to slopes facing the sun, and to the alluvial flood plains 
of rivers which have their sources in more southern zones. 
In the cold-temperate regions of the earth, deep residual 
soils are of rare occurrence owing to the slowness in the 
chemical operations of weathering. Large areas have been, 
moreover, recently glaciated, and the soils in consequence 
are comparatively shallow even at low levels, and often based 
upon peculiar sterilised deposits of glacial origin. Porous or 
elevated surfaces undergo leaching in cold-temperate, humid 
climates, since they lead to the formation of acid humus from 
the inactivity of the bacteria of decay, and the want of the 
neutralising effect of lime. These form the centres of distri- 
