Soluble vs. Insoluble Cane Manures. 339 
likely to be present, free alumina, it is especially liable 
to form insoluble compounds, which as manurial materi- 
als are considered to be practically valueless. Two 
reasons that should tend to limit the use of superphos- 
phate in British Guiana, are, firstly, that being a power- 
fully acid substance in itself, it increases the already 
excessive acidity of the soil ; and secondly, it is probably 
productive of great waste of phosphorus by forming the 
above mentioned inert compounds with the iron (and 
perhaps alumina) existing so abundantly in the clay soil 
of the colony. The validity of the first reason is self- 
evident, in support of the second, the following experi- 
ments recently carried out at the Government Labora- 
tory may be submitted. 
A sample of superphosphate was prepared by acting 
upon tri-calcium phosphate with half its weight of sul- 
phuric acid. This was dissolved in water, and the solu- 
tion diluted until iooo parts contained i part of phos- 
phoric acid in the form of superphosphate. There were 
also added to the solution, 10 parts per iooo of crystal- 
lized sodium acetate to "destroy" any free sulphuric 
acid that might be present, and 20 parts per iooo of 
glacial acetic acid to prevent any action of lime on the 
phosphate. 
Experiment I. — 500 parts of air-dried new soil from 
Pin. Lusignan were mixed with iooo parts of the super- 
phosphate solution. The mixture was made in a corked 
flask, and shaken at intervals for five days. The solu- 
tion was then filtered off from the soil, and the amount 
of phosphoric remaining dissolved in it, determined. This 
was found to be only "09 part per iooo, which, allowing 
for the water present in the soil (81 per cent.), showed 
UU 
