44 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



cularly in the deeper layers, because of the respiration of subterranean 

 organs, plants (bacteria) and animals, and because of the decomposition 

 of organic bodies. The amount of carbon dioxide varies with the quantity 

 of organic matter in the soil, the vegetation, the contour and humidity 

 of the land, the size of the soil-particles, the depth of stratum (the upper- 

 most layers of soil have less carbonic acid than have the lower ones), and 

 the temperature (season). 



The internal structure of the plant is correlated with the amount of air 

 contained in the soil ; in very wet soil, as a rule (with the special excep- 

 tion of bacteria), only such plants can thrive as have large internal air- 

 spaces, which are in communication with one another throughout the whole 

 plant, and can convey air from the atmosphere itself to the most distant 

 root-tips and parts of the rhizome (aquatic and paludal plants ; horse- 

 tails in firm clay contrasting with plants in heath-moors, which contain 

 much more air).^ 



CHAPTER XII. WATER IN THE SOIL 



Water is the third component of soil. It is attracted by the solid 

 particles of soil, and surrounds them as a thinner or thicker film.^ 



The amount of water varies greatly in different places and at different 

 times in the same place. After Norlin we may distinguish the following 

 grades which, as a rule, are only approximately estimated : i = very 

 dry, 2 = moderately dry, 3 = moderately fresh, 4 = fresh, 5 = some- 

 what moist, 6 = moist, 7 = very moist, 8 = moderately wet, 9 = wet, 

 10 = very wet.^ In more detailed scientific research, the amount of 

 water must be expressed in percentages of the weight or volume of soil. 

 The quantity of water in soil is practically indicated best of all by the 

 plants growing on it ; for no factor has such an influence upon the disposi- 

 tion of species as the amount of water in the soil. 



The amount of water in the soil is one of the most important direct 

 factors operating on plant life : this follows from the statements in Chap. 

 VII respecting the fundamental significance of water in plant-economy. 

 Water must be present in certain proportions, which are definite for each 

 species (in cultivated plants usually not more than sixty per cent, for 

 any prolonged period) ; too much or too Uttle is injurious in this as in 

 other cases. The significance to plant life of the quantity of water in 

 soil is demonstrated, for example, by Fittbogen's investigations on oats : 

 on soil, the humidity of which varied between forty and eighty per cent., 

 there was no great difference to the resulting crops ; but with a humidity 

 of twenty per cent, the crop was halved, and with one of ten per cent, 

 the crop was reduced to an eighth. Lack of water in the soil causes the 

 plants to be ill-nourished, because roots can obtain nutriment from such 

 a soil only with difficulty. 



Water is also of indirect significance, as it affects animals and 

 bacteria living in the soil ; a certain degree of humidity is essential to 

 the production of humus. 



' See Sections III, IV, V. ^ See Sachs, J. von, 1865, p. 171 ; Hedgcock, 1902. 



3 See Hult, 1881. 



