6 Uie History of a Field newly laid down to Permanent Grass. 
the staWe for some hours daily. Upon the whole, therefore 
there does not seem to be conclusive ground for supposing that 
town dung is very much richer than ordinary farmyard manure. 
So far as the nitrogen is concerned, however, the estimates 
given below are based on the same percentage as we assume for 
farmyard manin-e, namely 0'G4 ; and also on the alternative 
assumption of 0'80 per cent. In reference to this point it may 
be added that the average of determinations made at Rotham- 
sted, in eight samples of box-dung made at Woburn, was 0-736 
per cent. 
Table III. (opposite) gives approximate estimates of the 
amounts of nitrogen, phosphoric acid, and potash, in the manures 
applied, and in the crops removed, assuming average percentages 
of those constituents in the manures and crops as follows : — 
Percentage of nitrogen: in the dung 0'G4 or 0*80, in the guano 
12-0, in the nitrate of soda 15-5, and in the hay 1-5. Percentage 
of phosphoric acid : in the dung 0-23, in the superphosphate 
16-0, in the guano 12'0, and in the hay 0-4. Percentage of 
potash: in the dung 0'53,in the sulphate of potash 50'0, in the 
guano 2'5, and in the hay 1"6. 
First as to the nitrogen : — It is seen that, even adopting the 
lower percentage of nitrogen in the dung, the calculations show 
more nitrogen supplied in the manures than removed in crops 
over the first, the second, and the total periods, but rather less 
over the third period. The details show that the amount of 
nitrogen supplied in the dung was higher in each succeeding 
period ; but that supplied in artificial manures was only about 
half as much over the third as over either of the preceding 
periods ; whilst the amount removed in the crops gradually in- 
creased from 47 lbs. per acre per annum over the first period, 
to 57 lbs. over the second, and 72-4 lbs. over the third period. 
Nevertheless, as already said, there is, over the total period 
of 23 years, an excess of nitrogen supplied in manure, which, 
as the Table shows, amounted to an average of 7'9 lbs. per acre 
per annum. 
Adopting the higher percentage of nitrogen in the dung, 
however, there is an excess supplied in manure, compared with 
that taken oif in crops, over each of the three periods ; and the 
excess amounted to an average of about 20 lbs. (20-1), per acre 
per annum, over the 23 years. 
Next as to the plwsjplwric acid: — The estimates show an 
excess supplied in the manure over that taken off in crops, over 
each of the three periods, and an average excess of 14"9 lbs. 
per acre per annum over the 23 years. In fact, over each 
individual period, and the total period, the dung alone is esti- 
