276 ON THE CULTIVATION 



1. The necessity of an alteration of crops is presumed to be owing 

 to a peculiar excretion or discharge from the roots of plants, render- 

 ing the soil unfit, for a time, for the healthy growth of the same kind of 

 plant. 



2. After repeated cultivation of the same variety of plants, the soil 

 becomes exhausted of the ingredients which constitute the specific nou- 

 rishment of such plants. 



3. The failure (of the red clover in particular) has been attributed " to 

 a want of a certain degree of cohesiveness in the particles of the soil." 

 Hence the soil's power of retaining heat is diminished ; and all plants, 

 particularly clover, which are impatient of sudden changes of temperature, 

 are thus easily destroyed by the frost. 



Of tliese opinions, Mr. McTurk adopts that which bears the 

 stamp cf this inductive age, viz., that plants degenerate in soils which 

 do not supply them with a sufficiency of the elements of which they are 

 composed — of what, in animals, would be called their food. He proceeds 

 to consider this, as well as other circumstances, important to the successful 

 cultivation of the red clover.] 



It now remains for us to show that there are various causes 

 which have all more or less influence in occasioning- the failure 

 of the clover plant, and to endeavour to point out the means 

 by which the evil may be remedied. 



When we ascertain the composition of any plant, we will 

 find that the proportions of its org-anic and inorg-anic con- 

 stituents are, in man}' resjjects, ditierent from every other 

 plant, however nearly allied it may be to them all, and that 

 even the difterent parts of the same plant contain those 

 same constituents in different proportions. In so far as the 

 inorganic constituents are concerned, there is no source from 

 which they can be derived in sufficient abundance but the 

 soil ; and as these substances are found to exist in the soil in 

 very unequal cptantities, there is no doubt but that, while 

 one kind of plant is withdrawing- one substance in greater 

 amount, another is appropriating- another substance in a larg-er 

 degree ; and so it is with every crop. It is therefore evident 

 that, unless these substances are restored in quantities equal 

 to what have been abstracted, the soil must, in course of 

 time, be exhausted. It is also evident that, while some of 

 these substances readily accumulate in sufficient abundance 

 from the decomposition of matter which is continually taking- 

 place, others are very slowly restored, and some scarcely at 

 all, unless when contained in the manure which is from time 

 to time applied. It may be observed that, in the course of 

 the ordinary operations of husbandry, some of the more 

 soluble of these substances may be carried down by the rain 

 into the soil, while in a pulverized state, in g-reater abun- 

 dance than they are produced by natural causes, to a depth 



