30 
SIE J. B. LAWES AND PROFESSOR J. H. GILBERT ON THE 
shaken, in the case of the water, the malic acid, and the acetic acid extracts, a given 
quantity was drawn off at the expiration of one hour. The remainders of these, and 
the mixtures of the other acid solutions, were frequently shaken, finally left to settle, 
and after contact for between two and three days the extracts were decanted ofiP, and 
the soils drained on a vacuum filter. The several extracts being filtered, portions were 
at once evaporated to dryness on a steam bath, and the nitrogen iu them was deter¬ 
mined by the soda-lime method. The actual quantity of nitrogen involved in each 
determination was, however, so small, that recourse was afterwards had to Kjeldahl’s 
method; and comparative results led to the conclusion that those obtained by it were 
the more trustworthy. Accordingly, only the general indications obtained by the 
soda-lime method are here given. 
In the case of the water, and of the malic and acetic acid solutions, nitrogen was 
taken up after 1 hour’s contact with the raw clay subsoil; the most being taken up 
by the malic acid. In each case, after contact for between two and three days, the 
amounts of nitrogen in the extract were less than after only 1 hour’s contact. There 
had thus obviously been re-precipitation of nitrogenous matter at first taken up, and 
as the extracts showed scarcely any remaining acidity, the explanation seemed to be 
that the longer the contact, the more was the acid neutralised by the fixed bases of 
the subsoil. Of the five organic acid solutions left in contact between two and three 
days, the malic retained the most nitrogen ; next came the acetic and tartaric, then 
the citric, and lastly the oxalic. 
In a second series of experiments, besides the same five acids as before, formic acid 
was included. The acid solutions were however now twice as strong as those used 
in the first series. In the case of the malic acid, the periods of contact were 1 hour 
and 48 hours, and in that of each of the others 1 hour and 24 hours. In each case 
the acidity of the solution was much reduced by contact with the subsoil, and m each 
the more the longer the contact. Again the malic acid took up the largest amount 
of nitrogen ; and with the malic, and the formic acids, less was found in the extracts 
after the longer periods of contact. With the oxalic acid, however, in a striking degree, 
and less in that of the tartaric, the amount of nitrogen taken up was greater after 
24 hours’ than after 1 hour’s contact, probably owing to the precipitation of the lime 
in these cases. 
It was next decided, with the view of getting larger amounts of nitrogen taken up, 
to make 3 series of experiments as follows :— 
1. With double the quantity of subsoil to a given volume of the acid solution. 
2. With double the quantity of acid solution to a given quantity of the subsoil. 
3. To add a second quantity of the acid solution to the already once extracted 
subsoil. 
Further, it was decided to experiment with malic acid only ; and for comparison 
with the results on the subsoil, to make parallel experiments with the surface soil of 
the Medicago sativa plot. Lastly, duplicate determinations of the nitrogen in the 
