ECOLOGICAL RELATIONS OF SOIL 



223 



1936). From previous comments regarding 

 some of the effects of calcium on soil struc- 

 ture (p. 221), it is not surprising to find 

 that in acid soil, with reduced calcium con- 

 tent, the flocculation of clay may be de- 

 stroyed with a consequent increase in con- 

 tained water and a decrease in aeration. 

 Such soils tend to be in poor physical con- 

 dition and are heavy and relatively cold. At 

 least a part of the relation of plants and 

 animals to acid soil is not to the H ion con- 



available pH range, but those with cal- 

 careous shells are hmited to the more alka 

 line soils where calcium is more abundant; 

 available granite and quartzite regions 

 (acid soils) have few species, basaltic soils 

 (intermediate in pH) have a richer fauna, 

 and limestone areas (alkaline soils with lo- 

 cal acid situations) have most species and 

 individuals. Even for these snails, the cor- 

 relation between soil pH and distribution 

 is imperfect, since within an area of 2 



Fig. 56. Numoer of species of snails in Ireland in relation to soil pH. (Redrawn from Atkins 



and Labour.) 



centration as such, but to accompanying 

 calcium deficiency and altered physical 

 properties. 



Recorded pH values for soils lie between 

 2.2 and 9.6, inclusive (Russell, 1937), 

 values below 4.5 and above 8.5 are unusual. 

 Volcanic ash is practically neutral. In the 

 rainy tropics the soil tends to be acid; dry 

 areas are frequently alkaline. The suggested 

 generalization is unsafe, even when purely 

 local variations are disregarded, because of 

 differences in the original soil from one 

 place to another. The soil profile shows dif- 

 ferences in reaction that are at least loosely 

 correlated with humidity. In a humid cli- 

 mate the upper layers of the soil tend to be 

 more acid than those below; in arid climates 

 the reverse tends to be true (Arrhenius, 

 1922). Soil reaction is often a limiting fac- 

 tor in the distribution of land snails. In 

 Ireland, snails were found to be more 

 numerous at pH 7 to 8 than at other H ion 

 concentrations, with the number of species 

 greatest at 7.0 (Fig. 56). Irish land snails 

 with hyaline shells occur throughout the 



square miles certain species may be absent 

 from one locality, though abundant in 

 others with a similar pH, chemical content, 

 and aspect, but differing in exposure to 

 wind (Atkins and Lebour, 1923). 



The relation between earthworms and 

 soil reaction is intimate and may be 

 summed up briefly. In Ohio, earthworms 

 live in soils with a pH range of 4.5 to 8.4, 

 inclusive; around Chicago the range is from 

 5.6 to 8.3. The reaction of most soils is 

 from 4.5 to 8.5; hence the H ion evidence 

 indicates that earthworms live in soil— not a 

 startling conclusion. Again, there are some 

 species differences in pH toleration, and 

 earthworms usually are most numerous in 

 somewhat alkaline soils; the mode in Ohio 

 is about pH 8; near Chicago it is somewhat 

 less, and apparently in England the opti- 

 mum lies about pH 7.2 (Salisbury, 1923). 



PRINCIPLE OF PARTIAL EQUIVALENCE 



Consideration of the role of soil in the 

 life of plants and animals brings to light an 

 ecological principle of some importance that 



