200 Rrjjort of Experiments on tlie Groii tli of Red Cioi^er. 
well as the evidcnco of direct experiment, must lead to tlie con- 
clusion, that the view — that the organic compounds of the soil 
are only valuable to plants as a source of carbonic acid — requires 
modification. It is, indeed, probable, that some plants derive a 
considerable amount of their substance from carbon compounds 
other than carbonic acid, and that others depend for their carbon 
mainly, if not exclusively, upon carbonic acid. 
Those of our crops which, in the course of cultivation, are 
subjected to pretty natural conditions of growth, <md which 
accumulate the greater portion of their substance during the 
period at which the sun's rays are known to be most power- 
ful in influencing the decomposition of carbonic acid by plants, 
appear to depend chiefly on that source for their carbon. Those, 
on the other hand, Avhich are grown under somewhat abnormal 
conditions, and wliich store up a large amount of succulent pro- 
ducts of a comparatively low degree of elaboration, are probably 
partly dependent on other carbon compounds, yielded by the 
soil. The Leguminous crops, again, though generally coming 
more within the former than the latter category, still seem to be 
dependent, for luxuriant growth, more or less upon a supply, 
within the soil, of complex organic compounds. 
But whatever may be the precise result to which investigation 
may lead, in regard to the questions here involved, it may, at 
any rate, be pretty safely affirmed, that we shall not arrive at 
the true explanation of the phenomena upon which depend some 
of the most striking advantages of a rotation of crops, until we 
are better able than at present, to define the relations of the 
different crops to the different sources of carbon, and of nitrogen. 
The practical conclusions from the inquiry may he very briefly 
stated :- — 
When land is not what is called " clover-sick," the crop of 
Clover may frequently be increased by top-dressings of manure 
containing potash, and superphosphate of lime ; but the high 
price of salts of potash, and the uncertainty of the action of 
manures upon the crop, render the application of artificial 
manures for Clover a practice of doubtful economy. 
When land is what is called " clover-sick," none of the ordinary- 
manures, whether " artificial," or natural, can be relied upon to 
secure a crop. 
So far as our present knowledge goes, the only means of in- 
suring a good crop of Red Clover is to allow some years to elapse 
l)efore repeating the crop upon the same land. 
