Chap. V] THE SOIL 85 



ncss, which may also occur in calcareous soils and affords conditions un- 

 favourable to plant-life on account of the stagnation of the oxygen within it 1 . 

 In spite of its great importance, the purely physical analysis of soils 

 cannot quite replace a physiological analysis that is based on experimental 

 cultures. Indeed only the combination of both methods can explain the 

 connexion between the physical quality of a soil and the physiological 

 process of the absorption of water. The water-capacity of a given soil 

 does not enable us to judge of the quantity of water that a certain plant 

 is capable of taking from it. For instance, in some researches made by 

 Sachs the ratios of the water-capacities of a sandy beech-humus, of a loam, 

 and of a pure quartz sand, were 46 : 52-1 : 20-8, but the ratios of the 

 amounts available in each to a tobacco-plant were 33-7 : 44-1 : 19-3. In 

 other words, that fraction of the force of attraction of the soil for water 

 which the power of suction exerted by the roots could not overcome 

 differed with the kind of soil and varied as 12-3 : 8 : 1-5. 



These ratios have recently been more closely investigated by Gain in reference to 

 several kinds of soil, and to three species of plants, namely, Phaseolus vulgaris, 

 Erigeron canadense, Lupinus albus, all of which make unequal demands on moisture. 

 We do not go more closely into the question, as its importance for the topographical 

 distribution of vegetation appears doubtful. However this importance is assumed by 

 Gain, who refers, to a greater extent than is usual, chemical influences of the sub- 

 stratum to the inequality of the force of attraction for water. He assumes, for instance, 

 that the amount of water in the soil of a geographical district might sink to 3 % ; then 

 the species of plants with which he experimented could continue to exist in sand or in 

 garden soil, but not in humus, loam, or heath soil. This assumption may be admitted 

 hypothetically, but is without any importance in geographical botany, since in 

 each geographical area numerous qualities of soil occur retaining very different 

 quantities of water. 



2. THE CHEMICAL PROPERTIES OF SOILS. 



The chemistry and physics of the substratum are in many ways inti- 

 mately connected. Besides the size of the particles of soil their chemical 

 properties have place in determining the forces of adhesion and capillarity. 

 Soils with equally fine particles, for instance, act differently according 

 as they are composed of clay, lime, or quartz. Physical action is also 

 affected by the salts that are dissolved by the water in the soil, since 

 the salts influence the osmotic processes and consequently the absorption 

 of water. As has been already explained on a preceding page, roots take 

 up more water when it is offered in a chemically pure condition than they 

 do when it is in solutions, and for every species of plant there is a fixed 

 limit of concentration of solutions, rarely exceeding 5°/ o , beyond which the 



1 See p. 72. 



