276 ° BULLETIN OF THE BUSSEY INSTITUTION. 
have been taken off a given plot of land, year after year, in crops that 
were either not manured at all, or were manured with saline mixtures 
that contained no compound of nitrogen. ‘The results of many experi- 
ments that have been made with gypsum and other sulphates, point in 
the same direction. It has been noticed again and again that the sul- 
phates of lime, of magnesia, of soda, and of potash, tend to increase 
the growth not only of leafy plants, btit of the leafy parts of plants, 
very much in the same way that the nitrogenous manures do, though 
less decidedly ; and analysis has shown that plants manured with gyp- 
sum, or with Epsom salt, are often richer in nitrogenous constituents 
than those grown upon adjacent land that had not been manured. But 
it appears from the experiments of Dehérain,* that the sulphates 
neither promote the formation of nitrates nor of ammonium salts in 
the soil. 
The old system of. leaving land fallow, and in general the planting 
of the so-called fallow-crops before grain, undoubtedly depend in good 
part upon changes in the condition of the soil-nitrogen which those 
practices promote. ‘The high estimation in which the better kinds of 
vegetable mould have always been held by practical men, as attested, 
for example, in a very forcible way‘by the stringent laws against the 
raking of leaves and the like from wood-land, that prevail in many 
parts of Europe, is another item of evidence not to be overlooked. 
All considerations relating to the form or manner in which the soil- 
nitrogen is commonly consumed by plants have purposely been made 
subordinate, in this paper, to the exhibition of the empirical fact that, 
under certain conditions, such nitrogen can be used by plants in some 
way. ‘There is no longer any doubt but that nitrogen may be taken 
up by plants in several, perhaps in many, different forms. Of late 
years, the idea continually gains ground, that the inert nitrogen of the 
soil is usually changed to the condition of a nitrate before it is taken 
up by plants; and there are a number of experiments, notably those 
of Boussingault, Cloez, and Bretschneider,f which prove that nitrates 
are easily formed through the oxidation of the soil-nitrogen, and that 
they are exceedingly important for the growth both of wild plants and 
* See his “ Cours de Chimie Agricole,” Paris, 1878, p. 427. 
t+ See Professor Johnson’s “‘ How Crops Feed,” New York, 1870, pp. 254 to- 
288. 
