18 The History of a Field newly laid down to Permanent Grass. 
rather more over the next thirteen, than over the total period 
of twenty-three years, from 1856 to 1879. The estimate for 
the conduding period from 1879 to 1888 (reckoned at ten 
years from the removal of the ci'op of 1878) is, however, founded 
on direct experimental data, and is probably very near the truth. 
The average annual gain over the 33 years, 1856 to 1888, is 
seen to be 50-0 lbs. per acre ; and the average over the 23 
years from 1866 to 1888, to which our records of manure and 
produce relate, is 51 '9 lbs. 
The gain so indicated is obviously independent of the visible 
and separated underground vegetable matter, roots, &c. The 
following Table (VII.) shows the amounts of roots per acre in the 
surface-soil, the percentage of nitrogen in them, and the amounts 
of nitrogen per acre, according to the results on each sample in 
1879 and in 1888 respectively: — 
Table VII. — Separated Roots per Acre, and Nitrogen in 
Them, per Cent, and per Acre. 
Sample 
Roots per acre 
Nitrogen in roots (air-dried) 
Per cent. 
Per acre 
1879 
1888 
1879 
1888 
1879 
1888 
lbs. 
m. 
per cent. 
per cent. 
Ibp. 
lbs. 
1 
10,400 
10,346 
0740 
0-771 
77-0 
79-8 
2 
12,741 
11,707 
0-696 
0-800 
88-7 
93-6 
3 
8,875 
7,623 
0 804 
0-805 
71-4 
61-4 
4 
11,810 
9,801 
0-795 
0-644 
93-9 
63-1 
5 
9,529 
12,523 
0-802 
0-731 
76-4 
91-6 
6 
16,008 
0-766 
122-6 
Mean 
11,561 
10,400 
0-767 
0-750 
88-3 
77-9 
Attention has already been called to the fact that, although 
the soil had undoubtedly increased in fertility during the last 
10 years, as proved both by the increased amounts of crop, and 
by the increased amount of nitrogen in the surface-soil, yet there 
was even a somewhat less amount of roots separated from the 
samples of soil in 1888 than in 1879. It was supposed pro- 
bable that part of the result might be due to the roots being 
more exhausted in September 1888, after the removal of two 
large crops, than in January 1879, when no hay-crop had been 
removed since the preceding summer. It may be, however, 
that the separation of fine vegetable matter was more complete 
in the case of the 1879 than in that of the 1888 samples; and, 
so far as this was so, a small portion of the increased amount of 
nitrogen found in the fine dry soil in 1888 would be due to 
