474 



flourish, where another would fail. The same kinds of plants cannot 

 be cultivated in succession on the same soil for any length of time 

 without declining in productiveness. Some plants, as flax for example, 

 will not bear a repetition on the same soil oftener than once in five 

 years. It has been supposed that plants themselves assimilate, and 

 consume in their growth, certain ingredients in the soil necessary to 

 the perfection of the plant, which should not be repeated on the same 

 ground until this material is again supplied. But this is not all. No 

 artificial supply of any ascertainable ingredient can control this general 

 law of the necessity of a change in the rotation, growing out of other 

 circumstances. DecandoUe suggested, and may be said to have estab- 

 lished, another theory, namely, that plants excrete from the roots cer- 

 tain substances, which are innutritions or hurtful to the same kind of 

 plant in succession, but which may serve as the food of other plants. 

 But there are difliculties, in respect to this subject, upon which we 

 cannot dwell, which Liebig's theory solves with remarkable ability and 

 equal reasonableness. The exudations or excretions of plants may be 

 considered of two kinds. Plants, as we have before said, have no se- 

 lection in their food but take up with little discrimination what is acces- 

 sible to their organs of nutrition, and in a condition to be absorbed. 

 They consequently may take up many things, which they can assimi- 

 late but in part, or not at all. These are exuded, and may serve as the 

 food of other plants of a difl^erent character. But there is another class 

 of excretions, or properly speaking excrements, which are purely the 

 result of the vital action of the plants, and which, in the form of gum 

 or otherwise, after having served the purpose designed in the nutrition 

 of the plants, pass off" by the appropriate organs into the soil. These, 

 of course, cannot serve as the food of the same kind of plants, or of any 

 other in their present condition ; and these go to assist in forming the 

 humus of the soil. In their unchanged condition, these excrements are 

 pernicious to the kind of plants from which they were discharged, and, 

 it may be, to others ; and after becoming converted into humus, under 

 the operation of air and moisture, the eftects are the same as those of 

 humus. 



After all, where the crops are removed from the soil in the forms of 

 seeds, roots, and leaves, the soil is of course deprived of many of the 

 constituents requisite to a healthful and productive vegetation. The 

 substances removed are then to be supplied by manure. The seed of 

 the plant contains within itself the food, which it first requires in order 

 to the protrusion of its radicles. The humus in the soil will give out its 



