308 Extracts from the Journals. [April, 



different crops mentioned; but manures may be prepared accord- 

 ing to the invention for other plants than those mentioned; and, 

 if desired, manures may be made with greater exactness for those 

 plants which have been mentioned for particular cases, if the 

 matters of which the plants are composed and the quantities are 

 first ascertained, by burning the plants and analyzing the ashes, 

 and then combining the manure according to the analysis. The 

 manure so made is to be applied to the land in quantities as great 

 or greater than the quantities of the elements which have been 

 removed by the previous crop. It should be stated that, where 

 the straw of wheat and other similar plants, which require much 

 silicate of potash, is returned to the land as manure, that is con- 

 sidered to be the best means of restoring the requisite silicate of 

 potash to the land; in which case, in preparing the manures above 

 mentioned, the silicate of potash would be omitted. 



ROTATION OF CROPS. 



A paper has been read to the Royal Society, by Prof Daubeny, 

 " On the rotation of crops, and on the quantity of inorganic mat- 

 ters abstracted from the soil, by various plants under different 

 circumstances." 



The author was first led to undertake the researches, of which 

 a detailed account is given in this paper, by the expectation of 

 verifying the theory of De Candolle, in which the deterioration 

 experienced by most crops on their repetition was attributed to 

 the deleterious influence of their root excretions. For this pur- 

 pose, he set apart, ten years ago, a number of plots of ground in 

 the Botanic Garden at Oxford, uniform as to quality and richness, 

 one half of which was planted each year, up to the present time, 

 with the same species of crop, and the other half with the same 

 kinds succeeding each other in such a manner that no one plot 

 should receive the same crop twice during the time of the con- 

 tinuance of the experiments, or at least not within a short period 

 of one another. The difference in the produce obtained in the 

 two crops under these circumstances would, the author conceives, 

 represent the degree of influence ascribable to the root excretions. 



The results obtained during the first few years from these ex- 

 periments, as well as from the researches which had in the mean- 

 time been communicated to the world, by M. Braconnot and 

 others, on the same subject, led him in a measure to abandon this 

 theory, and to seek for some other mode of explaining the falling 

 off of crops on repetition. In order to clear up the matter, he 

 determined to ascertain, for a series of years, not only the amount 



