ECOLOGICAL RELATIONS OF SOIL 



221 



colates. There is not space here to discuss 

 many of the highly important ecological re- 

 sults of this pertinent relationship; a few 

 significant cases must suffice. In regions 

 with underlying limestone, soils are com- 

 monly rich in calcium, and this affects both 

 soil characteristics and the plants and ani- 

 mals associated with such soil. The soil re- 

 action becomes less acid; clays are made 

 more porous; leaching of magnesium and 

 potassium is retarded; bacterial action is 

 increased to the benefit of both the carbon 

 and the nitrogen content. Well-limed soils 

 accumulate heat more readily than do 

 heavier, unlimed clays. 



in New Zealand suffer from a cobalt defi- 

 ciency disease when feeding on natural 

 vegetation from soils with less than 2 or 3 

 ppm. of cobalt; normal growth occurs with 

 as little as 5 to 10 ppm. 



Fluorine is found in soils from practically 

 none up to 8 ppm. or more. Fluorine in 

 drinking water in concentrations over 1 to 

 3 ppm. produces more or less unsightly, 

 mottled human teeth, although such teeth 

 are resistant to decay (Arnold, 1943) 

 Selenium is present in all soils; it reaches 

 toxic concentrations in semiarid climates in 

 soils evolved from Cretaceous shales. Cer- 

 tain plants concentrate selenitmi; consump- 



Table 17. Relation of "Hardness" of Drinking Water to Soundness of Teeth in German 



Children (Hesse, 1924) 



"German degrees of hardness," in which 1 = 10 g CaO per m' of water. 



A few animals (the fire salamander of 

 Germany is one) avoid calcium soils. Con- 

 trariwise, many animals, especially many 

 snails and mammals, flourish best on soils 

 rich in calcium. Deer that grow new antlers 

 annually are favorably affected, and the 

 bones of other animals tend to be decidedly 

 heavier in limestone soils than in regions 

 based on igneous rock. Human teeth pro- 

 vide an almost diagrammatic demonstration, 

 as shown in Table 17, where the increasing 

 amount of calcium is indicated by the de- 

 gree of "hardness" of the water. 



Trace concentrations of chemicals may be 

 as important in soils as in aquatic habitats. 

 Cobalt, for example, is present in most soils 

 in concentrations up to some 10 or 15 ppm. 

 Plants do not seem to be affected by the 

 lack of cobalt, but some animals are. Sheep 



tion of these by cattle gives rise to "alkali 

 disease," a fairly common disturbance in 

 some of the drier sections of the western 

 United States (McMurtrey and Robinson, 

 1938). 



Other elements more or less important in 

 trace concentrations in soil include boron, 

 iodine, manganese, molybdenum, zinc, ar- 

 senic, barium, cadmium, chromium, Uthium, 

 rubidium, strontium, vanadium, and per- 

 haps lead (Brenchley, 1943). 



At the other extreme, salts may collect 

 in the soil, especially in the surface layer, 

 to such an extent that the soil becomes 

 saline. Roughly speaking, all saline soils are 

 referred to as alkali soils, regardless of their 

 pH. They occur in basin-like areas where 

 water accumulates in soil that is overlaid 

 by a drying atmosphere as in arid or sub- 



