8 OF THE FOOD OF PLANTS. Parti. 



weeds rot and add to its fertility. In fuch poor loofe lands, thefe 

 rich particles fink lower than the roots of common grafs reach, and 

 are thereby heaped up till the land is plowed, and then they be- 

 come the food of wheat, &c. If tap-rooted gralles grow in it, 

 their roots enrich the ground greatly when they putrify there ; 

 befides that fuch plants drop many of their leaves, which likewife 

 rot, and by fliading the earth, greatly mellow it. 



Another oblervation which does not agree with the identity of 

 food we have hitherto fuppofed, is, that mallin corn, which is a 

 mixture of wheat and rye, lucceeds in fuch light foils, better than 

 wheat alone ; and that a mixture of oats and vetches thrives very 

 well, where oats alone would fcarcely grow. 



To this it maybe anfwered, that thofe mixtures of wheat and rye, 

 or oats and vetches, thrive better together, than wheat or oats would 

 do alone, becaufe the rye and vetches, being quick growers in fuch 

 ground, fheiter the wheat and oats from the fcorching fun and dry- 

 ing winds, till they have attained fulhcient ftrength to refill them. 



From what has been faid, M. Duhamel concludes ; 



-I . That many plants of different fpecies feed on nearly the fame 

 fubftance. 



2. That there is no plant which does not rob thofe that are within 

 its reach, of feme part of their food. 



^. That the foil which once is good for one kind of plant, will 

 always be able to fupply it with food, provided it be cultivated 

 properly. 



If thefe propofitions are true, it follows that there is no necefiity 

 to change the fpecies of plants from one year to another, on account 

 of the different nourilhment which the earth is fuppofed to yield 

 them. 



It cannot however be denied, but that, in following the common 

 principles of agriculture, there is an advantage in fowing different 

 plants fucceffively in the fame land. But this may be owing to 

 three caufes, very different from thole to which we have oppofed 

 feveral obje<5lions. 



The firft, is the quantity of food which is necelfary to fome plants. . 



The fecond, is the particolar conftitution of each plant ; fome be- 

 ing more tender than others. 



The third, the degree of tillage which each plant requires. 



All plants do not draw a like quant't/ of nourilTiment from the 



earth. A proof of this is, that there ?ro poor light grounds which 



2 produce 



