Absorb and retain Am/monia. 243 



There is, however, still another question which must be 

 satisfactorily explained, before we can have any very accu- 

 rate idea of the precise mode in which these changes are 

 effected ; and that is, the subsequent process whereby the 

 ammonia which the soil has absorbed is given off again, and 

 taken up by plants. All that is now shown is, that the 

 soil is able to fix a considerable quantity of ammonia, which 

 is retained by it with a good deal of force, so that mere sub- 

 sequent washing will not remove it. By whatever means 

 it is that the soil is able to do this, it is tolerably certain that 

 there exists some means whereby plants are able to avail 

 themselves of the ammonia thus arrested, and stored up by 

 the soil. Whether these means are simply chemical ; whether 

 they depend in any way on the influence of light, or whether 

 they are to be traced to the vital powers of the plant and the 

 peculiar structure of the roots, is not yet ascertained ; but 

 two things are plain ; firsjly, that the soil takes up from rain- 

 water all the ammonia which the latter contains ; and, sec- 

 ondly, that the soil again parts with some of this ammonia to 

 plants growing in it. 



The power of any soil thus to absorb ammonia is un- 

 questionably limited ; and if we continue to filter a weak 

 solution of ammonia through a portion of any soil, though 

 the water which comes through at first will not contain any 

 ammonia, yet, after a short time, and when the soil has 

 taken up as much as it is able to absorb, then the water 

 which passes through will be found to contain as much am- 

 monia as it did before it was thus filtered. The quantity 

 which the soil is able to retain is not great, but it is abun- 

 dantly sufficient for the wants of plants. Until we know the 

 mode in which clay subsequently parts with the ammonia 

 which it has absorbed, we shall not be able to devise the best 

 means of preventing the loss of that substance, or of making 

 the most of the power in question. There is, however, every 

 probability that by continued exposure to the air alone, much 

 of the ammonia which has been absorbed during a summer 

 shower, for example, is again slowly given off; an effect 

 which the mere gradual evaporation of the surface water 



