REVIEW AND CONCLUSION. 371 
twice the amount of magnesia, sulphuric acid, and phos- 
phoric acid, and the same quantity of lime. These facts’ 
show how 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 that 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 particular 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 chemical 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 atmosphere 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 
the air, (pp. 122-135). Again, the atmospheric nitrogen 
is assimilated by the soil in the shape of ammonia, ni- 
trates, and the amide-like matters of humus, (pp. 254-265). 
The 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- 
