76 REPORTS ON THE STATE OF SCIENCE.—1918. 
Ramann adopts this view and gives up the expression ‘‘ acid 
soils,”’ using instead “‘ absorptiv ungesattigte Boden.’’ Kappen 
confirms the observations without entirely accepting the 
explanation. 
The physical explanation of acidity has not passed unchal- 
lenged. Rindall of Helsingfors, Sven Odén of Upsala, Tacke 
and Ehrenberg have each argued in favour of definite humic 
acids in peat. Truog finds, in the case of mineral soils, that 
equivalent amounts of different bases are required to neutralise 
the acid properties of the soil—which, if generally true, would 
be easier to explain by assuming an acid than an adsorption. 
It is always possible that both factors are at work. Evidence 
has been adduced by Christensen and by Schollenberger to show 
that acidity and base-absorbing power are not quite the same 
thing. Still more significant, measurements of the hydrogen ion 
concentration of soil extracts have been made, and show definite 
acidity. 
Pan formation. 
A pan is a layer of hard impermeable rock that gradually 
forms below the surface of the soil under certain conditions. Its 
effect is to cut off the soil above from the material below, and 
therefore to modify profoundly the movements of water and air, 
leading often to swampy conditions. The effect on vegetation 
becomes so marked that in agricultural practice the pan has to 
be removed, often at considerable trouble and expense. 
The conditions determining the formation of pan seem to be a 
supply of organic matter, permeability of soil, low content of 
soluble mineral matter, and absence of calcium carbonate. These 
conditions occur most frequently on light sandy soils where, for 
some reason, the water is held sufficiently near the surface. 
The older chemists explained the phenomena on purely 
chemical lines; it is unnecessary to go into the details of the 
various hypotheses put forward: in the main they involved 
alternate reductions and oxidations, or else solution in carbonic 
acid, and subsequent deposition. These hypotheses broke down 
on further examination, some necessary links failing to be 
realised when the experiment was made under natural conditions, 
Recent workers, therefore (Munst, Ramann, Morison, Sothers 
and Stremme) regard the whole process as a formation first of 
a “sol”? and then of a ‘‘ gel,” and Morison and Sothers suggest 
the following as the most probable course of events. 
It is well known that ‘‘ sols’? change to ‘‘ gels’’ in presence of 
small quantities of electrolytes, and conversely ‘‘ gels’? often 
change to ‘‘sols’’ when electrolytes are removed. In normal 
soils the conditions are favourable to gel formation, but when in 
these particular soils the upper layer of sand becomes denuded of 
its soluble material by the persistent washing of rain water, the 
conditions become favourable for the formation of sols of ferric 
hydroxide and of humus—or ferric humate, if one likes to put it 
in that way. Morison and Sothers actually obtained such 
sols* by persistent washing of ferric-humus gels. 
* As might be expected these did not give the ordinary iron reactions. 
