Observations on Liebig^s " Organic Chemistry^* 127 



soils, if thin and bare, and will also on sandstone ; while oak 

 will not thrive so well in bare thin granite soils as when they are 

 deeper. Clovers are found to exhaust the soil in this country 

 more than wheat ; at least they cannot be renewed so often on 

 the same ground ; and no addition of lime, gypsum, or manure, 

 is found to remove the difficulty altogether. Yet the editor of 

 the Qiiarterly Journal of Agricxilture remarks that, if the clover 

 in the rotation is grown for four in place of two years, the dif- 

 ficulty vanishes ; what can be the reason of this ? If we say it 

 is in affording longer time for the first deposit of excrement to 

 be decomposed, what becomes of the last deposit, and how 

 would it not decompose more readily when the clovers were 

 removed ? There are many things in practice that baffle theory ; 

 this can only arise from errors in theory, if practice is correctly 

 stated. All are interested in setting things right, that we may 

 have right principles to guide us. I have seen much in what 

 the professor says, to prove the benefit of alkalies, more than 

 could be condensed in an essay ; and all are concerned in 

 proving such theories for themselves, before adopting them as 

 principles. If the inferences drawn are correct to their utmost 

 extent, we ought to alter our system much to advantage; if they 

 are to be modified, it can only be done by the aid of practical 

 men in digestinQ; them. 



On the next and last division, the Interchange of Crops and 

 Manure^ he commences by stating the benefits formerly de- 

 rived from fallows, the land by rest regaining its original fer- 

 tility ; some crops thriving only after a succession of years, as peas, 

 clover, and flax; others, as hemp, tobacco, rye, and oats, being 

 capable of following one another. Wheat, hops, turnips, hemp, 

 poppies, &c., he instances as crops which exhaust the ground. 

 The excrements of man and animals have been employed for 

 increasing the fertility of the soil : they restore certain constituents 

 removed by the former crops. It has been observed, however, 

 that even manure will not always restore fertility, and that 

 alternation of crops is necessary ; a change producing fertility 

 as well as fallow. It is evident, therefore, he remarks, that all 

 plants must give back to the soil something that is capable of 

 being used as food by the succeeding generation. Agriculture, 

 he says, has neglected to seek the aid of chemistry, in obtaining a 

 chemical knowledge of the food of plants and of manure ; it even 

 recoils with distrust and aversion, he complains, from the means 

 of assistance afforded by chemistry ; and some future generation 

 must reap the advantages which the present denies to itself. 

 Practical men should endeavour to do away with the necessity 

 for these complaints; they may be difficult to convince, but, if 

 once convinced, will look to their own interest. He next 

 notices DeCandolle's theory, that plants give out excrements, 



