34 
SIR J. B. LAWES AND PROFESSOR J. H. GILBERT ON THE 
action would not effect more resolution on the surfaces actually attacked ? Indeed, 
this must necessarily be the case if such an action is really quantitatively an important 
source of the nitrogen taken up by deep and strong rooting plants, with strongly acid 
sap. In illustration of this necessity it may be stated that, even if as much as 
20 parts of nitrogen were taken up per million of soil, as was the case in the last- 
mentioned experiments in the first and second extractions taken together, this would 
only represent 600 lbs. of nitrogen per acre to the depth examined, namely 
108 inches. 
Upon the whole, then, the experiments on the action of weak organic acid solutions 
on raw clay subsoil, or even on a poor surface soil, have not given results from which 
any very definite conclusions can be drawn, as to the probability that the action of 
roots on the soil, by virtue of their acid sap, is quantitatively an important source of 
the nitrogen of plants having an extended development of roots, of which the sap is 
strongly acid. 
That roots do attack certain mineral substances by virtue of their acid sap, was 
established by Sachs. He sowed seeds in a layer of sand on polished marble, 
dolomite, and osteolite, and he found that the polished surfaces were, so to speak, 
corroded, where in contact with the roots. In regard to these results, Sachs says : 
(‘Text-Book of Botany,’ 2nd English edition, p. 702) “every root has dissolved at 
the points of contact a small portion of the mineral by means of the acid water 
which permeates its outer cell walls,” It was to carbonic acid that Sachs attributed 
the action in these cases • but there seems no reason to suppose that other acids in 
the root-sap may not exert a similar action. The results which have hitherto been 
published have however reference only to the taking up of mineral substances from 
the soil by virtue of such an action; and so far as we are aware the possibility or 
probability that the nitrogen of the soil or subsoil is so taken up has not been 
considered. 
Provided it were clearly established that the organic nitrogen of the soil, and 
especially of the subsoil, was rendered soluble by the action of the acid sap of the 
root, the question would still remain, whether the nitrogenous body is merely 
dissolved, and taken up by the plant as such, as the evidence at command seems 
to show is probable in the case of the fungi, or whether the nitrogenous body, after 
being attacked by the acid, is subjected to further change before entering the plant ? 
To this point we shall recur presently. 
Since the experiments at Bothamsted, above referred to, on the character and the 
action of the root-sap were undertaken, a preliminary notice of experiments on the 
nitrogenous organic compounds of the soil has been published by Dr. G. Loges 
(‘Versuchs-Stationen,’ vol. 32, p. 201). He found that the hydrochloric acid extracts 
of soils rich in humus left on evaporation a residue containing a large proportion of 
the nitrogen of the original soil. In his notice he does not state the strength of the 
acid used ; but from the results it is to be concluded that it was somewdiat con- 
