THE CHEMICAL PRINCIPLES OF THE ROTATION OF CROPS. 
PRONOUNCED BEFORE THE AMERICAN AGRICULTURAL ASSOCIATION, MARCH 4th, 1846, BY 
D. P. GARDNER, M. D., 
HONORARY CONSULTING CHEMIST OF THE ASSOCIATION, MEMBER OF THE LYCEUM OF NATURAL HISTORY, ETC. 
FORMERLY PROF. OF CHEMISTRY AND NATURAL PHILOSOPHY IN HAMPDEN SIDNEY COLLEGE, VA. 
Mr. President and Gentlemen : 
It is necessary to premise this memoir by explaining that the Executive Committee had 
expected a communication from another gentleman and did not until a late hour throw the burden 
upon me, hut my desire to gratify them has induced me to hazard the criticisms of the Association— 
tempered, as I know they will he—by the circumstances of the case. I have selected the subject of 
the rotation of crops partly because opportunities have fallen in my way to witness some facts which 
are commonly overlooked by writers on this topic, and because I regard it as a question of pure 
chemistry. I propose to search after general principles only, for if these can be determined, particular 
cases or the rotation suited to any district of country will be determined by a little consideration. 
This is moreover the only way whereby the subject can he discussed so as to he of utility to the 
whole country, the agriculture of which it is the object of your association to advance. A local 
rotation is hampered with considerations of expediency, with the price of land and of labor, the 
merchantable crops, the profit or loss of grazing, which offer obstructions to reaching any generaliza¬ 
tion ; hut whereas every crop and agricultural process is profitable in some part of our widely 
extended country, it is proper that such considerations should be dismissed, and introduced only in 
reaching particular cases. I know that in this day practical disquisitions are considered superior to 
all others, hut if we make no effort to group facts scattered abundantly around us, the art can never 
advance. Your Association has the noble object in view of reaching principles in agriculture, and 
therefore I have no hesitation in presenting a theoretical memoir, the design of which is to attempt 
the deduction of the principles of rotation. 
4 
