REVIEW AND CONCLUSION. 367 
be detected by the tests employed. This soil was mostly 
naked and destitute of vegetation, and its composition 
shows the absence of any crop-producing power. 
Relative Importance of the Ingredients of the Soil. 
—From the general point of view of vegetable nutrition, 
all those ingredients of the soil which act as food to the 
plant, are equally important as they are equally indispens- 
able. Absence of any one of the substances which water- 
culture demonstrates must be presented to the roots of a 
plant so that it shall grow, is fatal to the productiveness 
of a soil. 
Thus regarded, oxide of iron is as important as phos- 
phoric acid, and chlorine (for the crops which require it) 
is no less valuable than potash. Practically, however, 
the relative importance of the nutritive elements is meas- 
ured by their comparative abundance. Those which, like 
oxide of iron, are rarely deficient, are for that reason less 
prominent among the factors of a crop. If any single 
substance, be it phosphoric acid, or sulphuric acid, or pot- 
ash, or magnesia, is lacking in a given soil at a certain 
time, that substance is then and for that soil the most im- 
portant ingredient. From the point of view of natural 
abundance, we may safely state that, on the whole, availa- 
ble nitrogen and phosphoric acid are the most important 
ingredients of the soil, and potash, perhaps, takes the next 
rank. These are, most commonly, the substances whose 
absence or deficiency impairs fertility, and are those 
which, when added as fertilizers, produce the most frequent 
and remarkable increase of productiveness. In a multi- 
tude of special cases, however, sulphuric acid or lime, or 
magnesia, assumes the chief prominence, while in many in- 
stances it is scarcely possible to make out a greater crop- 
producing value for one of these substances over several 
others. Again, those ingredients of the soil which could 
be spared for all that they immediately contribute to the 
