158 THE ART OF CULTURE. 



them is, that they do not extract the alkalies of the 

 soil, and only a very small quantity of phosphates. 



It is evident that two plants growing beside each 

 other will mutually injure one another, if they 

 withdraw the same food from the soil. Hence it 

 is not surprising that the Matricaria chamomilla, 

 and Spartium scoparium, impede the growth of 

 corn, when it is considered that both yield from 

 7 to 7*43 per cent, of ashes, which contain $ of 

 carbonate of potash. The darnel, and the Erigeron 

 acre, blossom and bear fruit at the same time as 

 the corn, so that when growing mingled with it, 

 they will partake of the component parts of the 

 soil, and in proportion to the vigour of their growth, 

 that of the corn must decrease ; for what one 

 receives, the others are deprived of. Plants will, 

 on the contrary, thrive beside each other, either 

 when the substances necessary for their growth 

 which they extract from the soil are of different 

 kinds, or when they themselves are not both 

 in the same stages of development at the same 

 time. 



On a soil, for example, which contains potash, 

 both wheat and tobacco may be reared in succes- 

 sion, because the latter plant does not require 

 phosphates, salts which are invariably present in 

 wheat, but requires only alkalies, and food con- 

 taining nitrogen. 



According to the analysis of Posselt and jReimann, 

 10,000 parts of the leaves of the tobacco-plant 



