16 The Ilistory of a Field newly laid down to Permanent OrasS, 
already said, no samples were taken at the commencement ; but 
having taken so many samples of our arable soils to the depth 
of 9 inches, and determined the nitrogen in them, we believe we 
may safely assume that, at the commencement, the arable surface- 
soil would, to that depth, contain about 0"14 per cent, of nitrogen. 
But the average weight of fine dry soil, to the depth of 9 
inches, would, when arable, weigh about 2,400,000 lbs. per acre, 
instead of about 2,000,000 lbs. as in the case of the land after it 
had been so many years under grass. We have, therefore, to 
estimate what would probably be the percentage of nitrogen in 
the upper 2,000,000 lbs. of the surface arable soil. That is to 
soy, we have, in the first place, to deduct 400,000 lbs., or one- 
sixth of the original weight, and to deduct the amount of nitrogen 
it would contain from the total, and calculate the percentage 
in the remaining upper 2,000,000 lbs. Taking the original 
2,400,000 lbs. at 0-14 per cent, of nitrogen, the total amount to 
the depth of 9 inches would be 3,360 lbs. of nitrogen per acre. 
Assuming the lower inch and a half, or one-sixth =400,000 lbs., 
which would be below the reach of the plough, to contain only 
0-08 per cent, of nitrogen, the total amount of nitrogen in it 
would be 320 lbs. per acre ; and deducting this from the total 
3,360 lbs., there remain 3,040 lbs. in the upper 2,000,000 lbs. of 
the fine dry soil of the arable land at the commencement, and 
this would correspond to a percentage of 0-152 in the 2,000,000 lbs. 
of surface-soil.' 
We have thus an estimate of the percentage, and of the total 
amount, of nitrogen in 2,000,000 lbs. of fine dry arable surface- 
soil in 1856, when the land was taken in hand ; we have actual 
determinations in the surface-soil, weighing rather over 2,000,000 
lbs. in 1879 ; and we have actual determinations in 1888, when, 
however, the fine diy soil to the depth of 9 inches amounted to 
little more than 1,900,000 lbs. Before, therefore, we can make 
any accurate comparison of the amounts of nitrogen at the differ- 
ent dates, we must correct the results of the actual determina- 
tions, so as to show the amount in 2,000,000 lbs. For example, 
we assume that the about one-twentieth, or nearly 100,000 lbs. 
deficiency of weight per acre in 1 888, being subsoil, but influenced 
more or less by the perennial vegetation, would contain about 0'09 
per cent, of nitrogen, and the calculation is as follows: — 91,000 
lbs. of subsoil, at 0-09 per cent. nitrogen=82 lbs. of nitrogen per 
acre, which added to 4,604 lbs., the amount by actual determi- 
nation in 1,908,978 lbs., gives a total of 4,686 lbs. nitrogen in 
' It is perhaps more probable that these estimates of the percentage, and 
of the actual amount, of nitrogen in the surface soil in 1856, are too high than 
too low. 
