REVIEW AND CONCLUSION. 371 



twice the amount of magnesia, sulphuric acid, and phos- 

 phoric acid, and the same quantity of lime. These facts 

 show liow very far chemical analysis in its present state 

 is from being able to say definitely what any given soil 

 can supply to crops, although we owe nearly all our pre- 

 cise knowledge of vegetable nutrition directly or indi- 

 rectly to this art. 



The solvent effect of water on the soil, and the direct 

 action of roots, have been already discussed (pp. 309 to 

 328). It is unquestionably the fact tliat acids, like pure 

 water in Ulbricht's experiments (p. 324), dissolve the 

 more the longer they are in contact with a soil, and it is 

 evident that the question : How much a pai'ticular soil is 

 able to give to crops ? is one for which we not only have 

 no chemical answer at the present, but one that for many 

 years, and, perhaps, always can be answered only by the 

 method of experience — by appealing to the crop and not 

 to the soil. Chemical analysis is competent to inform us very 

 accurately as to the ultimate composition of the soil, but as 

 regards its proximate composition or its cliemical consti- 

 tution, there remains a vast and difficult Unknown, which 

 will yield only to very long and laborious investigation. 



Maintenance of a Supply of Plant-food. — By the recip- 

 rocal action of the atmosphei-e and the soil, the latter 

 keeps up its store of available nutritive matters. The 

 difficultly soluble silicates slowly yield alkalies, lime, and 

 magnesia, in soluble forms ; the sulphides are converted 

 into sulphates, and, generally, the minerals of the soil are 

 disintegrated and fluxed under the influence of the oxy- 

 gen, the water, the carbonic acid, and the nitric acid of 

 tlie air, (pp. 122-135). Again, the atmospheric nitrogen 

 is assimilated by the soil in tlie sliape of ammonia, ni- 

 trates, and the amide-like matters of humus, (pp. 254-265). 



Tlie rate of disintegration as well as that of nitrifica- 

 tion depends in part upon the chemical and physical char- 

 acters of the soil, and partly upon temperature and mete* 



