1S46.J Rotation of Crops. 309 



of crop which would be obtained from each of the plants tried 

 under these two systems, but also the quantity of inorganic mat- 

 ters extracted in each case from the soil, and the chemical consti- 

 tution of the latter, which had furnished these ingredients. The 

 plants experimented upon were spurge, potatoes, barley, turnips, 

 hemp, flax, beans, tobacco, poppies, buckwheat, clover, oats, beet, 

 mint, endive, and parsley. 



From a chemical examination of the crops. Dr. Daubeny con- 

 cludes, first, that the falling off of a crop after repetition depends, 

 in some degree, on the less ready supply of certain of the inor- 

 ganic ingredients which it requires for its constitution; not but 

 that two crops equally well supplied by the soil with these ingre- 

 dients may take up different quantities of them, according as their 

 own development is more or less favored by the presence of or- 

 ganic matter in the soil in a state of decomposition. 



Secondly, that it is possible that a field may be unproductive, 

 although possessing abundance of all the ingredients required by 

 the crop, owing to their not being in a suflficiently soluble form, 

 and therefore not directly available for the purposes of vegetation; 

 so that in such a case the agriculturist has his choice of three 

 methods — the first, that of imparting to the soil, by the aid of a 

 manure, a sufficient quantity of these ingredients in a state to be 

 immediately taken up; the second, that of waiting until the ac- 

 tion of decomposing agents disengages a fresh portion of those 

 ingredients from the soil (as by letting the land remain fallow); 

 and the third, that of accelerating this decomposition by mecha- 

 nical and chemical means. 



Thirdly, that it is probable that in most districts a sufficient 

 supply of phosphoric acid and of alkali, for the purposes of agri- 

 culture, lies locked up within the bowels of the earth, which 

 might be set at liberty, and rendered available by the application 

 of the artificial means above alluded to. 



Fourthly, that the aim of Nature seems to be, to bring into this 

 soluble, and therefore available condition, these inorganic sub- 

 stances, by animal and vegetable decomposition, and, therefore, 

 that we are counteracting her beneficial efforts when we waste 

 the products of this decomposition by a want of due care in the 

 preservation of the various excrementitious matters at our dis- 

 posal. 



Fifthly, that although we cannot deny that plants possess the 

 power of substituting certain mineral ingredients for others, yet 

 that the limits of this faculty are still imperfectly known, and the 

 degree in which their healthy condition is affected by the change 

 is still a matter for further investigation. 



Lastly, that the composition of various plants, as given in this 

 paper, differs so widely from that reported by Sprengel and others, 



