Nitrification in Acid Sorls. 197 
approaching clay and including a good many unworn flint stones—it differs 
in one essential respect from them, in that it contains but a small propor- 
tion of calcium carbonate, a substance which is usually present in the 
surface layer of the Rothamsted soil to the extent of from 2 to 5 per cent. 
It has been shown* that the calcium carbonate at Rothamsted only exists 
in the surface soil and is of artificial origin, being due to repeated “ chalking ” 
during the eighteenth century and earlier with chalk rock drawn from 
10 to 12 feet below the surface. As the land in question was in grass 
before the practice had become general, it cannot have been often subjected 
to the chalking process, at any rate the surface soil now contains little calcium 
carbonate, 0°5 per cent. at the most, and the lower depths show less than 
01 per cent. On Plot 11-1, which has received the largest amount of 
ammonium salts, the proportion is less, though the chalk has not been 
entirely removed, 0-16 per cent. being still present in the surface layer. 
But that the acidity of the grass soils is a consequence of the com- 
parative absence of calcium carbonate there can be little doubt, since the 
soil of the arable fields at Rothamsted, which contains from 2 to 5 per cent. 
of calcium carbonate, is still neutral, though the same amounts of ammonium 
salts have been applied over even longer periods. 
To ascertain if the nitrifying organisms were present, small quantities 
of the soil were introduced into a suitable sterile culture medium containing 
an ammonium salt and other inorganic nutrients, together with a sufficiency 
of calcium or magnesium carbonate. Flasks containing 100 e.c. of such a. 
solution were seeded with 0:2 to 0°5 gramme of soil, and were maintained 
at 30° C. with the usual precautions; after a month the liquids were tested 
for nitrates, nitrites, and ammonia. It is well known that nitrate or nitrite 
will be formed during such a period of incubation if any of the proper 
organisms have been introduced with the soil; as a rule, with normal soils. 
the whole of the ammonia will be oxidised. 
Samples were taken on six different occasions between October, 1904, and 
February, 1907, mostly from the surface soil only, but once down to 
135 cm.; they were drawn from the continuously unmanured plot, from two 
plots receiving 400 lb. per acre per annum of ammonium salts, from one 
which received 600 lb. of ammonium salts, and from one plot to which: 
sodium nitrate is applied in place of ammonium salts. Other samples were 
taken from the sections of these plots to which 2000 lb. per acre of quicklime 
had been applied in January, 1903. 
On summarising the results, it appears that the eal of the unmanured 
plot from a depth of 10 to 15 cm. set up nitrification in seven out of eight. 
* Hall and Miller, ‘ Roy. Soc. Proc.,’ B, 1905, vol. 77, p. 8. 
