160 THE ART OF CULTURE. 



those which are termed fallow-crops, and the cause 

 wherefore they do not exercise any injurious influ- 

 ence on corn which is cultivated immediately after 

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

 soil, and only a very small quantity of phosphates. 



It is evident that tw^o plants growing beside each 

 other will mutually injure one another, if they with- 

 draw the same food from the soil. Hence it is not 

 surprising that the wild chamomile (^Matricaria 

 Chamomilla) and Scotch broom (^Spartium Scopa- 

 riuTTi) impede the growth of corn, when it is con- 

 sidered that both yield from 7 to 7-43 per cent, of 

 ashes, which contain {^ of carbonate of potash. The 

 darnel and the fleabane (^Erigeron acre) blossom and 

 bear fruit at the same time as corn, so that when 

 growing mingled with it, they will partake of the 

 component parts of the soil, and in proportion to 

 the vigor of their growth, that of the corn must 

 decrease ; for what one receives, the others are 

 deprived of. Plants wdll, on the contrary, thrive 

 beside each other, either when the substances neces- 

 sary 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 succession, 

 because the latter plant does not require phosphates, 

 salts which are invariably present in wheat, but re- 

 quires only alkalies, and food containing nitrogen. 



According to the analysis of Posselt and Reimann, 

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

 tain 16 parts of phosphate of lime, 8*8 parts of 

 silica, and no magnesia ; whilst an equal quantity 

 of wheat straw contains 47*3 parts, and the same 

 quantity of the grain of wheat 99-45 parts of phos- 

 phates. (De Saussure.) 



Now, if we suppose that the grain of wheat is 

 equal to half the weight of its straw, then the quan- 

 tity of phosphates extracted from a soil by the same 



