Liquid Manure. 9 



The power of soils to absorb potash presents us with a striking 

 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 phosphoric 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. 



