ROTATION OF CROPS. 



23 



cumstances, two successive years with the 

 same species of crop. 



Pulverising and trenching may be re- 

 garded as a species of rotation, because 

 by them the surface of the soil may be 

 transposed, or the whole mass thoroughly 

 incorporated together ; thus presenting 

 to a great extent new food to plants, both 

 for facilitating the necessary chemical 

 changes, and the admission of rain and 

 air — the latter so important to the roots 

 of plants. Deeply pulverised soils are 

 increased in temperature, as well as ren- 

 dered more uniform in that respect. The 

 air admitted carries with it, during sum- 

 mer, heat from the sun, which is daily 

 accumulating, and retained for a length 

 of time, the earth being a bad conductor 

 of caloric. "The free admission of 

 atmospheric air to soil is also necessary 

 for the decomposition of humus, or orga- 

 nic matter, by which carbonic acid is 

 formed; and atmospheric air is also a 

 great source of nitrogen, which has been 

 lately found in all plants, and more espe- 

 cially in the spongioles of the roots. 

 — Suburban Hort. p. 35. "And hence," 

 says Liebeg, p. 1 90, " the great value of 

 animal manures to plants, all of which 

 contain nitrogen, but especially those of 

 carnivorous animals." 



From the comparative uncertainty, as 

 regards a correct analysis of plants, in 

 which chemists have left us, we cannot 

 see, not knowing the exact amount of the 

 ingredients of the soil which plants are 

 supposed to carry off with them (both 

 in their growing state, and more especially 

 when they have arrived at their fullest 

 maturity), what amount of those ingre- 

 dients should be added to the soil, to 

 make up for its supposed loss, or even 

 what these are. Professor Johnston, in 

 "Lectures on Agricultural Chemistry," 

 &c, p. 528, admits "that we scarcely 

 know as yet what any one entire plant, 

 when fully ripe, carries off from the soil;" 

 and in another part of these " Lectures," 

 he says " that our knowledge of the in- 

 organic constituents of plants is yet in 

 its infancy, and that our present opinion 

 upon the subject ought, therefore, to be 

 permitted to hang very loosely about 

 us." 



It appears, in connection with the rota- 

 tion of crops, as well as with the applica- 

 tion of manures, that too much stress has 



been laid upon vegetable chemistry, and 

 by far too little regard paid to vegetable 

 physiology ; the connection between both 

 is so intimate and important as to render 

 them, in the pursuit of true conclusions, 

 inseparable. " Intimately connected with 

 vegetable chemistry," says Mr Edward 

 Solly (in Rural Chemistry, p. 121), "is the 

 study of vegetable physiology : a know- 

 ledge of the one is essential to the perfect 

 comprehension of the other, for it is im- 

 possible well to understand the chemical 

 changes going on in the organs of plants, 

 if we are wholly ignorant of the forms and 

 structures of these organs ; and, on the 

 other hand, the most complete knowledge 

 of the anatomy of vegetables could never 

 lead any one to sound and correct con- 

 clusions respecting the nutrition of plants. 

 It is rather to be regretted that both 

 chemists and physiologists have appeared 

 to avoid availing themselves of the advan- 

 tages which both might have derived, by 

 studying the results which the others 

 had obtained. It is only by comparing 

 together the observations of both that 

 correct conclusions can be formed." The 

 researches of Grew, Malpighi, and Du- 

 hamel, did much in elucidating the struc- 

 ture of plants, and the modes by which 

 they derived sustenance ; indeed, they 

 may be said to have laid the foundation 

 of vegetable physiology. In more recent 

 times, Decaisne, De Candolle, Mirbel, 

 Dutrochet, and Brongniart, in France ; 

 Meyen, Mohl, Link, and Schleiden, in 

 Germany ; Amici in Italy ; and Knight, 

 Hooker, Henslow, Brown, Griffiths, and 

 Lindley, in England, by careful observa- 

 tions, and the advantages of improved 

 instruments, have reduced vegetable phy- 

 siology from a series of vague and uncertain 

 dogmas to a comparatively perfect system. 

 Little had been done since the days of 

 Priestley and Ingenhousz in the improve- 

 ment of agricultural chemistry, save the 

 labours of Sir Humphry Davy : these,how- 

 ever, an early death prevented him from 

 prosecuting, for we dare scarcely say 

 completing. It remained, therefore, for 

 Liebig to do almost single-handed for 

 vegetable chemistry what those we have 

 named above, with many others, had been 

 zealously engaged in doing for vegetable 

 physiology. In his "Organic Chemistry," 

 in tracing the sources by and from which 

 plants derive their food, he has strongly 



