140 
Liquid Manure. 
The power of soils to absorb potash presents us with a strikinji 
contrast to the apparent indifference of soils to absorb soda from 
its soluble combinations. 
8. It is likewise satisfactory to have in this experiment a direct 
proof of the power of the soil to take up phosplioric acid from 
soluble combinations with which it is brought in contact. 
Nearly the whole of the phosphoric acid originally contained in 
the manure was taken up by the soil. A much larger quantity 
of phosphoric acid] would have been- taken up by this soil, if 
a liquid richer in phosphoric acid had been passed through 
it. Experiments with soluble phosphates have shown that this 
is the case. My reason for alluding to this matter here is to 
guard the reader against the supposition that the quantity of 
phosphoric acid absorbed by the soil expresses its maximum 
absorptive power for phosphoric acid. The fact that some 
phosphoric acid was left in the liquid after 24 hours' contact 
with soil might readily give rise to such an opinion. 
Direct experiments have shown to me that the power of this 
soil to absorb phosphoric acid is very much greater than appears 
in this experiment. 
Although it is quite true that nearly the whole of the phosphoric 
acid contained in a liquid is retained by a moderate quantity of 
soil, it cannot be inferred from this that plants take up phos- 
phoric acid in the shape of an insoluble compound ; for a soil 
which is capable of absorbing a large quantity of phosphoric 
acid, when brought in contact with a liquid containing but a 
small proportion of the amount of phosphoric acid which the soil 
is capable of retaining, never completely removes the phosphoric 
acid. A certain quantity remains in solution — a quantity which 
I believe is sufficient to account for all the phosphoric acid which 
is found in the ashes of plants. 
In all probability it is the function of the soil to transform 
readily soluble compounds of phosphoric acid into combinations 
which are so little soluble in water that in common life they pass 
for insoluble, but which are still sufficiently soluble to supply the 
growing plant with the necessary amount of this kind of mineral 
food. This beautiful power of soils effectually prevents the waste 
which heavy rains infallibly would occasion, and compounds 
more conducive to the health of plants are formed ; for I believe 
we have sufficient evidence to show that all very soluble saline 
matters, however useful or necessary they may be for every kind 
of agricultural produce, impede the rapid growth of plants when 
they are presented too abundantly to the roots of plants. 
The next experiment was made with a soil from a permanent 
pasture. 
