468 Abstract Report of Arjricultural Discussions. 
men who have tried it on a pretty extensive scale. AVe leam from 
farmers residing in the neighbourhood of Edinburgh that they can 
realise by the application of sewage from 2bl. to 40Z. per acre — the 
average perhaps is about 25/. per acre. But if we calculate the 
value of the dressings applied, as has been done by Dr. Hoffman 
and other chemists, and, for illustration' sake, by me to-day, we 
shall find that the calculated value of the fertilising constituents 
comes to something like 75/. or 80/., whilst the profit realised is 
only 25/. ; which shows plainly the exaggerated nature of these cal- 
culations. If we look rather to the produce than to the price set 
upon the constituents of sewage, it will be found that its fertilising 
value is on an average perhaps one halfpenny a ton. 
Moreover we learn from the practical experience of men who 
appl}' the sewage under the most favourable circumstances that the 
produce rises just in proportion to the quantity applied. To get a 
material advantage from the application of sewage it should go 
through the soil. Those soils will be most benefited by its use 
which act merely as the vehicles for holding the manure. We must 
never think of storing up the liquid manure in the soil. The soil 
does not hold such fertilising matters. 
A great deal has been said of the powers of soils to absorb manuring 
matters ; and it is true that all soils, not even the most sandy soils 
excepted, have the power of rendering insoluble to a great extent the 
soluble fertilising matters that we usually find in manures ; but they 
have not the power of rendering them completely insoluble, and from 
very dilute liquids they take away very little indeed. If time per- 
mitted, I could refer you to some experiments which I have made 
with a view of ascertaining whether soils have the power of retaining 
soluble matters to any extent ; but it may suffice to state in a general 
way that the weaker the solution the less is the soil capable of retain- 
ing the soluble matter. Thus, in operating with very dilute solution 
of ammonia, I find that hardly any ammonia is retained by the soil ; 
and again, that the proportion of phosphoric acid which is left in 
the liquid after passing through the soil is just as large as it was 
before it was applied. 
By filtering very dilute liquids, such as sewage, through soils 
which, like clay soils, contain potash, you may even take out the 
potash. This was the case with an experiment which I made 
on Mr. Mechi's soil. By filtering some of his tank liquid through 
his clay land I actually obtained more potash in the liquid that 
filtered through the soil than was contained in the tank liquid 
itself; thus showing plainly that the fertilising matters from 
very dilute liquids are not retained in the soil; and that we 
must not, therefore, calculate upon storing in the land during win- 
ter the fertilising matters of sewage. If we are to derive benefit 
from the practical application of the sewage of towns, we must apply 
it in large quantities, and get an immediate return in the course 
of four, five, or six weeks. Then we may give a new dose of 
manure with advantage, and so on. But with so dilute a liquid, 
which absolutely contains a considerable amount of fertilising mat- 
