ON SOILS. 31 



grow for many years successively in the same 

 spot ; but a careful attention even to this crop 

 will convince ev.en the most skeptical of the 

 universality of this law. If, for instance, a 

 field of Lucerne is planted, and no manure of 

 any kind is applied, it will be found after a few 

 years to have lost much of its fertility, and that 

 the produce is lessened considerably in amount; 

 but that, after being in this state for some time, 

 perhaps a year or two, the plant seems to re- 

 cover itself, and even an increased vegetation 

 takes place. 



This is to be accounted for thus — the plant 

 during the first year of its growth throws off a 

 quantity of excrement, which increasing year 

 by year in the soil faster than it is decomposed 

 by the air, at last is so prevalent, that the plant 

 can obtain scarcely any nutriment from the 

 soil, and consequently is enfeebled in its growth, 

 and throws off comparatively no excrementi- 

 tious matter to the soil. When this state of 

 affairs has arrived, the action of the air on this 

 matter gains on it, and in the course of one or 

 two seasons the whole of it is so reduced as to 

 act as a stimulus to the plant itself, and give 

 it apparently a new life, by supplying it with/ 

 a quantity of carbon for its assimilation. 



This decomposition may however be other- 

 wise affected by constantly turning the land, 

 and by adding small proportions of quick lime, 

 which would hasten the process of decay, and 

 thus prevent the barrenness of the plant from 

 taking place. 



Thus the rotation of the crops is necessary 



