SOIL EXHAUSTION AND ROTATION OF CROPS. 203 



than any thing else. When it growte 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 cannot depend, ordinarily, upon having 

 much clover from the sowing of 1812, later than 1874, except as 

 the result of self-seeding. 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 trouble of sow- 

 ing any seed or giving any attention to the matter at all. These 

 distinctions make an obvious difference in the relation of the three 

 kinds of plants to the subject 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 incidentally, 

 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 completely, 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 practiced, the value of what is left on and in the soil has 

 been, to some extent, appeciated, but we have not known accur- 

 ately 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 uuderstand it now as we ought to and as we 

 shall understand it after further investigation. I referred yester- 

 day to the table of Dr. Weiske, of Proskau, which gives the 

 ingredients of the stubble and roots of various crops remaining on 

 and in an acre of land after harvest. (See page 194.) This is 

 the first, or nearly the first, exact experiment of the kind that has 

 ever been made, and these observations must be repeated here 

 and there, on different soils, before we can get entirely trust- 

 worthy data, to enable us to make a satisfactory calculation. 

 Still, these first results will serve a very good purpose. 



In the case of rye, for instance, you have 3400 lbs. of dry vege- 

 table matter remaining in the soil to the acre. Ordinary rye straw 

 contains some fourteen per cent, of moisture. The vegetable 



