178 



PLANT SOCIOLOGY 



Wai erfall 

 Rapids 



Ponded siream 



nitrifying bacteria causes a material change in the reaction constancy of 

 many soils. 



Soil Types and pH. — Both organic content and H ion concentration 

 can be used for the characterization of soil types. The pH value 

 should not be taken at random but should be restricted to the ecto- 

 dynamomorphic climax soils. In the upper soil layer of warm arid 

 regions values of more than pH 7 {i.e., alkaline soils) occur almost 

 exclusively. In warm semiarid regions (red-earth region) more or less 

 neutral climax soils are the rule. In beech- wood regions the climax soils 

 maintain a pH 6.5 to 4.5. In spruce forest it is around pH 6.0 to 3.6. 



In the moist, cool climax regions of the Curvuletum in the Central 

 Alps the pH values of the climax soils lie between pH 5.0 and 4.0. The 



PH 

 7.5 



7.H 

 7.3 

 7.2 

 7.1 

 7.0 

 6S 

 6.8 

 6.7 

 6.6 



as 



6.¥ 

 6.3 

 62 

 6.1 



2A.M. 6 8 10 12 JP.M. 6 9 12 



Fig. 99. — Daily fluctuation of pH values in quiet and moving waters. {After Cowles 



and Schwitalia, 1923.) 



extreme pH values of soil solutions that have been ascertained are 

 pH 2.9 and 11 (the latter in Egypt). 



pH Values of Aquatic Communities. — The H ion concentration of 

 the water shows a distinct relation to the amount of H2CO3 present. 

 The same stream may show greatly varying pH values in quiet spots 

 and in places of considerable movement. While no pH changes may 

 be shown in quiet spots during the course of the day, such changes may 

 be very considerable in waterfalls (Fig. 99). The photosynthesis of 

 floating species has a decided influence on the pH value of the water. 

 Thus Iversen (1929) has stated that in a pond in Denmark, at a depth 

 of 50 cm., the reaction of the water was pH 6.8; at 40 cm., pH 7.0; at 

 30 cm., pH 7.6; at 20 cm., pH 8.2; and at 10 cm., pH 8.8. The vegeta- 

 tion was very dense {Elodea, etc.), and in the upper stratum photo- 

 synthesis was very active but decreased with depth on account of 

 diminishing light. 



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