126 



AND ON DIFFERENT CROPS. 



mixed with the soil, it be strewed in spring over the young green 

 leaves. Third, A greater advantage if, at the same time, the 

 soil is deficient in lime or in sulphuric acid ; and., fourth. The 

 greatest of all if the soil is deficient in both. 



3. Why it does not act equally on all crops. I explain this, 

 and, indeed, its general effects, by the conjecture for I do not 

 wish to state it more strongly at present that the compound 

 which we call gypsum contains its constituents in a state in 

 which they are specially adapted to enter into and aid in the 

 growth of particular plants. It is not enough, I believe, to 

 place within the reach of our crops the various elementary 

 bodies or binary compounds which we find in plants. These 

 are, we now know, very nearly the same in all. It is chiefly 

 in proportion that the mineral constituents of plants differ. We 

 must discover and apply these substances also in the state of 

 combination in which they are severally best suited to enter 

 into to find admission perhaps I ought to say into the roots of 

 each particular genus or species of plant, and to be physiologi- 

 cally, chemically, and structurally useful to it after they obtain 

 admission into its sap. 



We appear to have ascertained by trial that, in a great 

 variety of circumstances, gypsum has this special adaptation to 

 certain plants. If this be so, it is an ultimate fact of much 

 interest, which explains many anomalies, and may point the 

 way to other interesting discoveries of a similar kind. I have 

 already briefly alluded to this topic,* and shall have occasion, in 

 a succeeding chapter, to return to it, to consider what evidence 

 we possess in favour of the opinion I have thrown out, and 

 what experiments may be suggested with the view of inves- 

 tigating and shedding further light upon it. 



3. Suggestions for experiments with gypsum, applied alone. 



The field of experiment with this substance is very wide. I 

 shall advert to a few of the points it may be most interesting to 

 investigate. 



1. To the practical farmer it may be recommended for trial 

 in small experiments on his different soils, and especially on 



* See Chapters IV. and V. 



