INTERCHANGE OF CROPS. 159 



contain 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 

 weights of wheat and tobacco must be as 97*7 : 16. 

 This difference is very considerable. The roots of 

 tobacco, as well as those of wheat, extract the phos- 

 phates contained in the soil, but they restore them 

 again, because they are not essentially necessary to 

 the development of the plant. 



OF THE INTERCHANGE OF CROPS, AND OF 

 MANURE. 



It has long since been found by experience, that 

 the growth of annual plants is rendered imperfect, 

 and their crops of fruit or herbs less abundant, 

 by cultivating them in successive years on the same 

 soil, and that, in spite of the loss of time, a greater 

 quantity of grain is obtained, when afield is allowed 

 to be uncultivated for a year. During this interval 

 of rest, the soil, in a great measure, regains its 

 original fertility. 



It has been further observed, that certain plants, 

 such as peas, clover, and flax, thrive on the same 

 soil only after a lapse of years ; whilst others, such 

 as hemp, tobacco, helianthus tuberosus, rye, and 



