PEOCEEDINGS OF THE CHEMICAL DEPARTMENT. 207 



sandy soil, or even Mr Harper's light gravelly soil, the average 

 produce in each case being — 



Tons. Cwts. 



Saltcoats, . . . 4 13 



Snawdon, . . . 5 17 



Dirleton, ... 8 6 



Now the differences in the chemical composition of the soils will 

 not explain this ; for there can be no doubt that the chemist 

 drawing his conclusions from analysis, and the farmer founding 

 on his practical knowledge of the qualities of soils, would concur 

 in the opinion that Mr Shirriff 's soil is decidedly superior to the 

 other two. It can scarcely be doubted that it is difference in 

 texture, coupled with the nature of the season, which caused the 

 inferior to surpass the better soils. The wet season was, in fact, 

 favourable to the lighter and more porous soils, through which 

 the water can percolate rapidly, and admit the supplies of air 

 which are necessary to the successful growth of all plants, and 

 unfavourable to the stiffer soils, whose pores remain closed up by 

 the superfluous moisture. The mechanical analyses confirm this 

 view ; for the percentage of clay is smallest in Mr Hope's soil, 

 and largest in Mr Shirriff's, while Mr Harper's is intermediate 

 between them. Of course, all this depends on season, for in a 

 dry year it is clear that the clay, owing to its power of retaining 

 moisture, would give the Saltcoats soil a decided advantage over 

 the other two. 



It must not be supposed, however, that the productiveness of 

 soil is always, or even often, independent of its chemical compo- 

 sition, for the very reverse of this is the fact ; but in the 

 particular case under consideration, all three soils being capable 

 of yielding a sufficient supply of food to the crop, their mechani- 

 cal state has affected the conditions under which these sub- 

 stances found access to the roots. And it is manifest that this 

 must in all cases exercise a very important influence, for the 

 great mass of the soil is only the vehicle through means of 

 which the actual food is presented to the roots. 



The problem which offers the greatest difficulty is, to determine 

 what part of the components of a soil do actually constitute the 

 food of the plants growing upon it. It seems unquestionable 

 that the substances insoluble in acids do not contribute to its 

 immediate fertility ; though it is highly probable that the gradual 

 changes which most minerals undergo in the course of time must 

 cause some part of them to pass from a passive to an active state. 

 Even of the portion soluble in acids but a small part is probably 

 accessible to the plant, but the determination of this point is 

 attended with great practical difficulties. The most obvious 

 inference to be drawn from known facts no doubt is, that those 





