134 



Power of Soils to absorb 3Ianure. 



for the retention of ammonia in soils. It matters not whether 

 one or more of these compounds is present in a soil, so that any- 

 one of them is there ; the ammonia added in manure, or derived 

 from the atmosphere, will equally be retained. Indeed, this list 

 of the order of these decompositions strikes me as of singular 

 interest, in indicating the care and solicitude of nature in the 

 preservation of the different substances which are essential to 

 the growth of plants, in direct relation, as far as we have been 

 able to learn, to their relative importance. Ammonia, for 

 instance, may well be considered as of the very first consequence 

 to vegetation, and, for its retention, four other alkaline bases are 

 made responsible. Next comes potash, which is also of very 

 , great agricultural importance ; whilst, at the other end of the list, 

 we have soda, for the retention of which no provision is made, 

 and which is liable to be displaced by salts of all the other bases. 

 Now, it so happens that almost all those chemists who have been 

 much engaged in the examination of the ashes of plants have come 

 to the conclusion that soda is not necessary to vegetation — that is 

 to say, not as a constituent of plants. In the seeds, which are the 

 only perfected parts of plants, soda rarely exists^ except in insig- 

 nificant quantity, and then only as common salt, present, probably, 

 from want of perfect maturity of the seed. In the succulent parts 

 of plants, it is true, soda is found in quantity, but in all proba- 

 bility it only there exists as a part of the unelaborated juice of 

 the plant, and in virtue of the great quantity of water contained 

 in it. It is most interesting, therefore, that not only is there 

 an absence of retentive power for this alkali, but it is also 

 made subservient to the preservation of all those that are of 

 importance. 



Lime, which stands next on the list, is again less cared for than 

 potash or ammonia, probably because its abundance in nature is 

 generally a sufficient security. 



It is necessary to state, in reference to these decompositions, 

 that the rule laid down only applies to the action of the salts of 

 different bases on the silicates. Thus, as has been said, sulphate 

 of lime cannot cause the displacement of ammonia from its 

 silicate ; but, on the other hand, the action of the caustic alkali- 

 lime itself would be very different, for not only would silicate of 

 ammonia be decomposed by lime, but the potash of silicate of 

 potash, and alumina, would also be displaced by it. 



This point is of great importance, because it is quite possible 

 that the evils of overliming of land may be due to the driving off 

 the ammonia of the soil, which constitutes, so to speak, its capital ; 

 and what may be only a useful application of lime to one soil may 

 be destructive to another ; because, from the smaller proportion of 

 other silicates for the lime to act upon and become absorbed in, 



