

EFFECT OF PHYSICAL CONDITION. 93 



considering, are likely to come under our notice. I merely 

 enumerate four more or less obvious causes of such modification. 



First) Though the substance we add it may be potash or 

 soda, for example, or a substance containing nitrogen may be 

 present in the soil to which we add it, yet it may not be in such 

 a state of combination or of solubility as readily to minister to 

 the wants of the crop that is growing upon it. The application 

 we make, therefore, may produce a much more striking effect 

 than its quantity, compared with what is already present in the 

 surface soil, would warrant us to expect and this because of the 

 chemical or physical condition in which we apply it being such 

 as to make it more immediately available to the nourishment of 

 the plant. 



Second, The converse may also be true. A large quantity 

 of a substance laid on the land in one physical condition, may 

 produce a less sensible effect than a smaller quantity applied, or 

 already existing, in the land in another physical condition. 

 Such we know to be the case with bones, which, when reduced 

 by means of sulphuric acid, produce a more immediate and 

 much more striking effect, than when used in the state of ordi- 

 nary bone-dust. 



The state of chemical combination in which it exists also 

 modifies the special action of a substance. Such is the case with 

 quicklime, compared with common marl or chalk, and such also 

 is the case with the various states of combination in which mag- 

 nesia, potash, soda, ammonia, and the nitric, sulphuric, and 

 phosphoric acids, may be presented to the roots of plants through 

 the medium of the soil. 



It is obvious, therefore, that the physical condition, and the 

 state of chemical combination, in which we apply a given sub- 

 stance to our fields, will materially modify the effect it would 

 be expected to produce if we attended only to the relative 

 quantities in which it exists in the soil, and in the application 

 we make to it. 



Third, This question of the influence of chemical combination 

 is deserving of further and more profound investigation. We 

 have seen that substances must be soluble if they are directly 

 to promote vegetable growth. But will a nutritive substance, 



