126 Monerj-value of Niglit-Soil and other Manures. 
With this estimate of 45 lbs. per head yearly Mr. Lawes 
agrees ; and Professor Way gave substantially the same answer 
when in round numbers he stated to the Committee on Sewaare 
of Towns the amount at 1000 grains per head per day, equal 
to 1 lb. per week. Liebig's estimate for the phosphates, of about 
4J lbs. per head, valued at 3f/. per lb., would only give a money- 
value of 14fZ. for that important item. He does not mention 
nitrogen in this passage ; neither can I tell the comparative value 
' as between this substance and phosphate which would meet 
with his sanction and approval ; but he can hardly make a 
greater, if so great, a distinction between the two as our English 
chemists. 
The proportion existing between the phosphates and ammonia 
in excreta, as found in sewage, is, I believe, considered pretty 
constant ; so that if the amount of nitrogen is determined, that of 
the other element may be safely estimated, the ammonia being 
double the phosphates, according to Dr. Hofman's calculations.* 
So, then, as to bulk and relative proportions the authorities of 
different schools appear to agree, and the question of price or 
valuation only remains. 
On this point, speaking as a practical farmer, I am disposed to 
complain that all our chemists put on the constituents of manure 
a value in some cases too high, in others almost imaginary. In 
truth, the question of value does not belong to science but to 
commerce ; and all the advice which a scientific man, as such, 
can offer will be on the comparative value of different sources 
of the same fertilizer. 
To the eye of Science all elements of plant-food have an 
equal importance — are equally essential ; their value only varies 
according to the difficulty experienced in securing a supply, and 
of this difficulty Commerce is the sole arbiter, the purchaser 
limiting the course of trade by refraining from an unremunerative 
purchase. Yet from (questions of value the philosopher cannot 
stand quite aloof; and therefore from time to time, startled, it 
may be, by local abuses, he sounds an alarm, in which his want 
of familiarity with the marts is as striking as his scientific 
knowledge is admitted. Rut the practical man, not much 
moved by general accusations of wasting plant-food or the mineral 
constituents of plants, Avill review his requirements in detail, 
when the list will appear to be neither very long nor very 
formidable. 
On the nitrogen he will have kept a pretty sharp look-out, as 
an element of which the waste is considerable and the difficulty 
of supply great. In regard to phosphate, he will probably have 
been for some years a greater buyer tlian seller, and he will thank 
* See ' First Keport of the Select Committee on Sewage,' p. 26. 
