18 PLANT PHYSIOLOGY AND ECOLOGY 



irregular capillary spaces between them grow smaller, and the 

 upward or capillary movement is increased. On the contrary, 

 the downward movement of water, i.e., percolation, which is 

 caused by gravity, is retarded by a decreasee in the size of the 

 soil grains, and hastened by an increase. The properties of the 

 soil which regulate the upward and downward movement of 

 water are respectively capillarity and porosity. Both are de- 

 pendent upon the structure or fineness of the soil, though in a 

 manner directly opposite to each other. Capillarity increases 

 with the fineness of the soil, porosity with its coarseness. Capil- 

 larity augments the water content of the upper layers, while 

 porosity decreases it. Upon this basis alone, soils fall into two 

 groups, capillary soils and porous soils, the former fine-grained 

 and of high water content, the latter coarse-grained and with 

 relatively little water. However, a third factor of great impor- 

 tance must be taken into account. This is the pull exerted upon 

 each water film by the soil particle itself. The pull seems to in- 

 crease in strength as the film grows thinner, and this explains 

 why it finally becomes impossible for the root-hairs to draw mois- 

 ture from the soil. This property, like capillarity, is most pro- 

 nounced in fine-grained soils, such as clays, and is least evident 

 in the coarser sands and gravels. It furnishes the explanation 

 of non-available water, and indicates that the chresard is directly 

 connected with soil texture. 



23. Chemical nature of soils. Apart from the effect of exces- 

 sive amounts of acids or salts, the chemical nature of the soil 

 is of slight importance, except in the case of soils exhausted by 

 intensive cultivation. In nature the necessary nutrient salts 

 are so uniformly distributed that the chemical composition of 

 the original rock is immaterial. A soil can modify the plants 

 upon it only through its water content, or the soluble salts, or 

 solutes, that it contains. Hence, when differences of structure 

 or distribution occur between habitats with different soils, the 

 cause is not to be sought in the fact that the soil is silicious, cal- 

 careous, or argillaceous, but in the effect of the texture upon water 

 content. It now appears entirely incorrect to ascribe the pres- 

 ence or absence of certain species on limestone soils to the chem- 

 ical nature of the latter. The most important chemical elements 

 in the s'oil appearing in the form of salts and connected ^\dth the 

 growth of green plants are nitrogen, sulphur, phosphorus, iron, 



