180 DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 



nerally impressed in favour of a theory which the celebrated De Candolle had so 

 ingeniously and eloquently maintained ; namely, that a soil became unfitted for sup- 

 porting a second crop of any given plant, in consequence of the deleterious influence 

 exerted upon it by juices excreted from the former one. 



My original object therefore was, — first, to detect, if possible, the chemical nature 

 of these supposed excretions ; and secondly, to demonstrate their poisonous influence, 

 by taking account of the expected diminution in the amount of the crop exposed to 

 them beyond that of another in which all the circumstances were the same, except- 

 ing the presence of the excretions in question. 



To accomplish these two objects, it seemed sufficient to set apart a number of plots 

 of ground uniform as to the quality and richness of its soil, planting one-half of the 

 number year after year with the same species of crop until the land no longer pro- 

 duced it, and the other moiety with crops of the same description, succeeding one 

 another in such a manner, that no one plot should receive the same twice during 

 the period of the continuance of these experiments, or at least within a short interval 

 of one another. 



By weighing the produce of each plot, reduced to the same uniform condition of 

 dryness, when it had arrived at maturity, I hoped to obtain data for computing, how 

 much of the expected diminution might be referred to the exhaustion of the ground, 

 and how much to the effect of excretions which the preceding crop had given out. 



The influence of seasons indeed is in all these cases one of the most important 

 elements in the calculation, yet by taking the average of a number of years, it was 

 hoped that this source of error might be eliminated, and that whilst the mean of the 

 crop obtained during the latter half of the period, as compared with that of the former 

 half, might suggest the rate of exhaustion brought about by the annual demand made 

 upon the resources of the soil, the diflference between the permanent and the shifting 

 crop in each instance might tend to show, in what degree the excretory function of 

 each plant contributed to the result. 



Assuming, therefore, on the faith of the then existing authorities, that soil would 

 soon become deficient in the food which was required for the plants grown in it, 

 and moreover that, even if not exhausted, it would become unsuitable to their 

 growth, by being contatninated with the excretions from preceding crops, I conceived 

 it unnecessary either to undertake an analysis of the soil itself at the commencement 

 of my labours, or to inquire into the chemical constitution of the crops which I had 

 obtained in the course of them. 



Supposing, as was then too hastily assumed, that the composition of each vegetable 

 was uniform, and had been already determined with sufficient precision, it should 

 follow, that the amount of produce ought in itself to be an index of the quantity of 

 inorganic matter abstracted from the soil, and that the number of crops obtained 

 before the soil became eff^ete would indicate the relative richness of the latter in 

 those ingredients which were essential to the growth of the plant in question. 



