16 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



ail ocre : in a clay soil io aliout foui' at-roy. A soil absorbs water and 

 retains it against drainago in virliie of llio wet skin held on the surface 

 by the attraction of each particle. Not only does the soil thus refuse 

 to part \Nith water to the drains through tliis surface pull, but it also 

 liolds water against the roots of the plant, so that sometimes the plant 

 may be running sliort of water in a clay soil which still appears to be 

 comparativel\' moist. Again, this attraction of the surface of all solid 

 materials for water enables soils to suck up water from the subsoil 

 during a period of drought in quantities dependent u])on the grade of 

 the soil. Thus, the mechanical analysis determines one of the greatest 

 factors in the nutrition of the plant — namely, the supply of water. 



At the same time the temperatirre is also conditioned by the water 

 supply, because the great agent in maintaining soils at a low tempera- 

 ture is the amount of heat that is always being drawn away by the 

 evaporation of water from the surface of the soil. Hence, a soil which 

 holds a great deal of water close to the surface is always a cold soil, 

 and artihcial drainage is quite capable of making a difference of more 

 than a degree in the average temperature of such a soil. Again, the 

 moisture evaporating fi'om tlie soil determines to a considerable extent 

 the humidity of the air immediately above it — that is, of the air which 

 surrounds an ordinary plant; and, as every gardener accustomed to 

 the management of greenhouses is w^ell aware, the humidity of the air 

 is a potent factor in the well-being of a plant. 



The structure of the soil as revealed by mechanical analysis forms, 

 then, the chief of what we might call the causes determining the asso- 

 ciation of given plants with a given soil. We say the chief " because 

 on it hangs w^ater supply, temperature, and humidity. Of course, the 

 kind of climate set up by the structure of the soil may be greatly 

 modified, or even overridden, by those" other factors which determine 

 local climate — such as elevation, aspect, and slope. We know, for 

 example, that fields just at the bottom of a valley are subject to the 

 most violent alternations of temperature. On a sunny day the tem- 

 peratures will rise to a much higher degree at the bottom of the valley, 

 simply because the moist air there collected acts like the glass of the 

 greenhouse^ — lets the sun's rays through, but arrests the dark heat 

 rays reflected from the ground, and so causes the heat to accumulate. 

 On a still night, however, the valley temperatures are at the lowest, 

 because the chill air from all the neighbouring slopes becomes denser 

 and rolls down into the valley. These violent alternations of tempera- 

 ture may render the valley unsuited to the growth of certain plants, 

 independently of the occurrence in such places of early autumn and 

 late spring frosts, which in themselves render such a situation unsuit- 

 able for the commercial growth of fruit. 



Eeturning now to the practical problem, as it presents itself to the 

 gardener or fruit-grower, of whether a particular piece of land is suit- 

 able to his purpose, I think ] may fairly claim that the mechanical 

 analysis of the soil does give us information winch can be trusted if 

 it is used intelligently with dde consideration of the other local climatic 



