BULLETIN OF THE BUSSEY INSTITUTION. 21 
difficultly soluble.. In this vicinity we are not justified in allowing 
more than the price of an equivalent weight of spent bone-black for 
any insoluble or difficultly soluble phosphoric acid with which a super- 
phosphate may be contaminated. 
_ In case the proportion of soluble phosphoric acid in a superphos- 
phate, as indicated by analysis, is very small, the intelligent farmer 
will reject that material altogether, since it does not contain the chem- 
ical agent he seeks. 4 
On the other hand, when the proportion of insoluble phosphoric 
acid in a superphosphate is small, it seems but justice to allow the 
manufacturer something for the benefit land may receive from the 
addition to it of a “durable” fertilizer, such as insoluble phosphate 
of lime (from whatever source) is known to be. 
It will probably be found in most cases in which impure superphos- 
phates are employed as manure, that the practical benefit derivable 
from the mixture of reduced and undecomposed phosphates, which 
contains the “insoluble phosphoric acid ” of the fertilizer in question, 
may be very fairly represented by an amount of spent bone-black con- 
taining that much phosphoric acid. 
It might seem at first sight that the experience of farmers as to the 
value of most American superphosphates must be similar to that of 
the chemists, and equally conclusive to the minds of the farmers ; but 
for obvious reasons this is not the case. Few, if any, superphosphates 
are wholly worthless ; most of them do produce a certain beneficial 
effect when applied to moderately good land ; thns the purchaser of a 
sample said to be similar to No. X. of the foregoing list obtained re- 
sults from using it in his garden which seemed to him highly favor- 
able ; the only trouble is that the benefit obtained is incommensurate 
with the money paid out. The farmer, in order to come to a definite 
decision upon a point like this, would have to make careful compara- 
tive experiments such as he seldom has time to attend to; and the 
common result seems to be that by repeated trials of various fertilizers, 
each as worthless as the others, he practically becomes habituated to 
the use of materials which he has no good reason to esteem. 
At all events, the continued sale, year after year, of enormous quan- 
tities of very poor materials shows conclusively that there must be 
hosts of farmers who are still unconscious that their money could be 
spent to better purpose. It is by analysis alone that the disreputable 
