84 HOW CROPS FEED. 
which would be taken from the field twice or thrice the 
above amount of nitrogen, although the period of growth 
of the two crops is about the same. Ulbricht found in 
his investigation of the clover plant (Vs. S¢., IV., p. 27) 
that the soil appears to have but little influence on the 
content of nitrogen of clover, or of its individual organs. 
These facts admit of another expression, viz.: Clover, 
though containing two or three times more nitrogen, and 
requiring correspondingly larger supplies of nitrates and 
ammonia than wheat, 7s able to supply itself much more 
easily than the latter crop. In parts of the Genesee wheat 
region, it is the custom to alternate clover with wheat, be- 
cause the decay of the clover stubble and roots admirably 
prepares the ground for the last-named crop. The same 
preparation might be had by the more expensive process 
of dressing with a highly nitrogenous manure, and it is 
scarcely to be doubted that it is the mitrogen gathered by 
the clover which insures the wheat crop that follows. It 
thus appears that the plant itself causes the formation in 
its neighborhood of assimilable compounds of nitrogen, 
and that some plants excel others in their power of accom- 
plishing this important result. 
On the supposition that ozone is emitted by plants, it is 
plain that those crops which produce the largest mass of 
foliage develop it most abundantly. By the action of 
this ozone, the nitrogen that bathes the leaves is convert- 
ed into nitric acid, which, in its turn, is absorbed by the 
plant. The foliage of clover, cut green, and of root crops, 
maintains its activity until the time the crop is gathered ; 
the supply of nitrates thus keeps pace with the wants of 
the plant. In case of grain crops, the functions of the fo- 
liage decline as the seed begins to develop, and the plant’s 
means of providing itself with assimilable nitrogen fail, 
although the need for it still exists. Furthermore, the 
clover cut for hay, leaves behind much more roots and 
stubble per acre than grain crops, and the cloyer stubble 
