64 BIOLOGY OF PLANTS. 



organs of absorption in a most vigorous state, but, at that pe- 

 riod, the pores are partly closed up, and the nourishment 

 must pass in at the roots ; and partly to the kind of nourish- 

 ment which the soil alone is capable of furnishing. Hence, 



(1) In the application of manures, we may derive, from 

 the above facts, the most important practical rules ; the kind 

 and quantity depending upon the time when the crop matures 

 its seeds. If the crop is winter rye, or any of the smaller 

 grains, which mature their seeds in July or August, green 

 manures should not be applied, because the process of fer- 

 mentation yields abundance of carbonic acid, which power- 

 fully stimulates and increases the stalks and leaves, but is in- 

 jurious to the formation of the grain. This process will be 

 most active when the kernel of early grains is maturing, and 

 the appropriate nutriment, which goes to the seed, will not 

 then be prepared in the soil ; hence there will be abundance 

 of straw, with but little grain. But if the crop ripens its seed 

 in September, like corn and most hoed crops, green manures 

 are far preferable, because the fermentation will be most ac- 

 tive, when the stalks and leaves require its influence, and 

 the nutriment, which is formed in the soil, by this process, 

 will be ready for the formation of the grain, by the time the 

 seed requires it. 



(2) The above facts explain the reason why crops exhaust 

 the soil more when permitted to mature their seeds, than when 

 cut green ; hence, crops cut for fodder, as grass, should not 

 be left to mature their seeds, in consequence of their exhaust- 

 ing effects upon the manures in the soil ; hence, too, the utility 

 of ploughing in green crops, because food is thus taken from 

 the atmosphere, and added to the soil, 



(3) Finally, from the same principle may be inferred the 

 utility of " soiling," that is, of keeping farm stock on green 

 crops, during the summer season. The green crops, deriving 

 their support mostly from the atmosphere, exhaust the soil but 

 little, while their conversion into manure in the stables, adds 



