40 CONDITION OF THE SOIL SELECTED. 



preferred for comparative trials, which are intended to test the 

 special efficacy of different fertilising substances, as on such a 

 soil their precise effects are likely to be more clearly and easily 

 seen. 



4. But the condition of the soil selected is of much import- 

 ance. Its richness or poverty in organic matter ; its fulness 

 of manure, or the contrary ; the kind and quantity of manure 

 added in past years, and the crops grown upon it ; its clearness 

 or freedom from weeds ; its past history, in short, and its pre- 

 sent treatment, are considerations from which its condition may 

 be deduced, and which, when rightly weighed, will not only 

 explain an apparently anomalous result when obtained, but will 

 often demonstrate beforehand what the effect of this or that 

 substance is likely to be. 



It is not, as many have hitherto supposed, equally favourable 

 or equally indifferent to all the substances we may wish to 

 compare, whether we select this or that piece of land for our 

 experiments. If twenty men are to be employed to dig a piece 

 of land, the chance is the same to all, whatever be the nature of 

 the land j and the quantity dug by each in an equally efficient 

 manner, in a given time, is a fair measure of his comparative 

 strength, industry, and skill. But it is very different with sub- 

 stances which are to be mixed with the soil, for the purpose of 

 comparing their effects upon a growing plant, which requires a 

 certain portion of each of them to enable it to build up its own 

 stem and leaves. These substances contain, or are themselves 

 the very things which should or do exist in the soil itself, and it 

 is only in proportion as one or other of them is deficient in the 

 soil, in a state in which the plant can make use of it, that its 

 influence becomes perceptible. The exact or absolute influence 

 of a substance upon vegetation, or even its comparative influence, 

 can only be estimated by experiments upon a soil in which one 

 or all are equally deficient, or are deficient in known degrees, 

 so that the quantity of each can be proportioned to this known 

 deficiency. 



But a practical and economical result is not so difficult to be 

 arrived at as this precise theoretical one. If, when two sub- 

 stances are tried by the farmer upon his field under like circum- 



