269 
PLANT DISTRIBUTION AND BASIC RATIOS. 
W. H. PEARSALL, D.SC., F.L.S. 
Numerous enquiries as to the nature of ‘ sour ’ soils have 
led to the enunciation of various theories of soil acidity. 
Most of these theories explain in an attractive manner some 
single aspect of this complex subject, and most of them 
equally fail to give any clue as to how soil sourness affects 
plant life and the distribution of vegetation. The most 
reliable index of soil sourness still appears to be the character 
of the vegetation the soil bears, a vegetation in which ling, 
bilberry, cottongrass and the matgrass, Nardus, are usually 
predominant. 
Using modern colorimetric methods (see Fisher, Journ., 
Agric. Sci. XI., 1921) of estimating soil acidity in terms of 
hydrogen ion concentration, the writer has been unable to 
find any very marked correlation between the acidity of a soil 
and the vegetation it bore. As a general rule, the heathy 
plants mentioned above are usually found on the more acid 
soils, while others (e.g., Mercurialis , Br achy podium) are 
common on the less acid soils. Widely exceptional cases, 
however, upset any exact generalisation. Moreover, when 
plants are grown in water cultures, no observable effects 
seem to be produced by changes of acidity of the order of those 
found in natural soils. It thus appears unprofitable to suppose 
that acidity alone is producing the observed changes in 
vegetation found on sour soils in nature. 
Another idea, which has recently been restated by Clements, 
is that deficiency of oxygen is the characteristic feature of 
sour soils. To this one may object that waterlogged soils 
which are deficient in oxygen frequently exist, and yet do 
not bear heathy vegetation. Conversely, some examples at 
least exist of soils bearing this type of vegetation, and yet 
being well aerated. As an example of a not infrequent case, 
one often finds in the Lake District Calluna and Nardus 
growing abundantly on stream gravels, bathed either by 
stream or lake waters which are well aerated and nearly 
neutral in reaction. The peculiar feature about these waters 
is that they contain a high ratio of potassium and sodium to 
calcium and magnesium salts — and that in similar places 
near other lakes, where this ratio falls below 1*5, other plants 
colonise the stream gravels, these being plants usually found 
on normal fertile soils. 
It seems that in this case, absence of calcium is the char- 
acteristic feature of the habitat and not acidity or oxygen 
deficiency. The application of this idea to all cases of soil 
sourness as known in the north of England seems quite justi- 
1922 Aug.-Sept. 
