Reaction and Stic cession in Relation to the Plant Covering . 405 
samples where both the surface and subsurface agree in having the same 
carbonate content or none at all. Thus samples D 17 and D 18 from the 
Laboratory ridge both showed 0-05 per cent, carbonates, but whilst the upper 
four inches, with an average organic content of 0-53 per cent., had a p. H. of 
6.9, the lower level (4-8 in.), with an organic content of 0.35 per cent., had 
a p. H. of 7*i. The same feature is brought out by the samples B 6 and B 7, 
and A 1 and A 2. 
In view of such results where the humus has the same source of origin, 
the suggestion seems warranted that such discrepancies as occur between 
the organic content and the p. H. value, where carbonates are absent, may 
be, in part at least, attributable to differences of origin of the organic 
material ; in other words, may be influenced by the nature of the plant 
covering. Also the rate and state of decay are important factors. 
Determinations of humus obtained from different species show con- 
siderable differences as indicated below : 
Psamma ‘ leaves 
V 33 
33 35 
Senecio jacobaea 
Silene maritima 
Carex arenaria 
Cladonia 
very slightly decayed 
more decayed 
almost completely disintegrated 
very slightly decayed 
advanced decay 
partly decayed 
33 3 ) 
p. H. 5-4 -6-o 
p. H. 5 * 7 ~ 6*4 
p.H. 6*2-6. 8 
p. H. 5*6 
p. H. 7.3 
p.H. 6*7 
p.H. 5*4 
These estimations were based on too few examples to permit us to lay 
much stress on the actual values obtained, but the Cladonia ‘ humus ’ is clearly 
very acid, whilst the ‘ humus ’ formed by the pioneer species is clearly Jess 
so. Within certain limits we should expect the humus derived from a given 
species to have a definite reaction dependent on the degree of ionization of 
the substances capable of yielding hydrogen ions. Therefore on first con- 
sideration it might seem unlikely that the amount of organic matter present 
would influence the reaction. But the above data for Psamma show that 
the reaction varies with the stage of decay, and will therefore be influenced 
by the rapidity with which this proceeds. The amount of humus present is 
roughly a measure of the rate of decay, and hence a large organic content 
being associated with slow decomposition will involve a larger proportion of 
material in the earlier and more acid stage of decay. 
Another factor that must be considered is the presence of rabbits, 
which are constantly feeding on such plants as Psamma arenaria and 
Carex arenaria , and whose faeces consist very largely of partially digested 
fragments of their leaves. The faeces are deposited on the dune surface of 
all phases, but especially the younger. Here they become buried by 
subsequent accretion, whilst in the older phases they are swept by the wind 
into the rabbit holes. 
