SOIL EXHAUSTION AND ROTATION OF CROPS. 153 



An annual plant, again, one that is sown in the spring, or 

 in the fall, perhaps, and is harvested within 'a year, other 

 things being equal, will be different in its relation to the soil, 

 from a biennial plant, which lives two years, or a perennial 

 plant, which keeps along indefinitely. Now, our ordinary 

 grains are annuals, as we cultivate them; the clover plant is 

 a biennial more nearly than any thing else. When it grows 

 vigorously, it is usually spent in about the second year. "We 

 may not call it properly a biennial in a botanical sense, but 

 in an agricultural sense it is a two-year old plant. We can- 

 not depend^ ordinarily, upon having much clover from tlie sow- 

 ing of 1872, later than 1874, except as the result of self-seed- 

 ing. Our natural grasses are perennial ; they live, we do not 

 definitely know how long. Their mode of propagation, besides 

 from seed, is by root-suckers ; the old root dies, but in the 

 meantime it has propagated a numerous family, which succeeds 

 it. and the race is kept up without the trouble of sowing any 

 seed or giving any attention to the matter at all. Tliese dis- 

 tinctions make an obvious difference in the relation of the 

 three kinds of plants to the sul)ject of rotation of crops. 



We have thus considered the plant itself, its roots, foliage, 

 and manner of growth ; now let us look more closely at what 

 remains when the crop is removed. This matter came up in- 

 cidentally, and a little out of order, yesterday, as I referred 

 to the tables on the board. When I raise a crop and harvest 

 it, I leave, of course, the roots in the soil, I leave the stubble 

 on the surface. If each crop were taken out of the soil com- 

 pletely, root as well as branch, so that nothing of it were left 

 in the field, the effect of any crop upon the soil would be 

 measured simply by 'what we took away. But we leave a 

 great deal in the soil. Ever since farming has been practised, 

 the value of what is left on and in the soil has been, to some 

 extent, appreciated, but we have not known accurately the 

 quantities or the relative proportion of those substances. We 

 have known that clover leaves much more than wheat, but 

 the precise relation we have not understood as we understand 

 it now, and we do not understand it now as we ought to and 

 as we shall understand it after further investigation. I re- 



