37 



evaporation, which takes place to a very great extent, 

 when too great an abundance of silica is present, or when 

 more than a due degree of porosity exists in its mecha- 

 nical texture ; and it should be sufficiently permeable, to 

 prevent any thing like excess of moisture, by stagnation. 

 Soils composed either principally, or almost entirely of 

 heath soil, or of vegetable mould, although very highly 

 recommended for the growth of the Cucumber in winter, 

 are nevertheless objectionable when applied alone, as 

 will be evident if the foregoing principles are taken into 

 view: it cannot however be assumed that the plants will 

 not grow in these soils, for they grow vigorously for a 

 period ; neither can it be asserted that such soils do not 

 contain the qualities which are necessary to administer 

 to the nutrition of plants, for it is scarcely possible 

 to conceive any substances which are more nutritious, or 

 whose application in this respect is more effectual ; but they 

 are objectionable, in consequence of their becoming soon 

 expended, and failing to maintain for any length of time, 

 an equable degree of moisture. The cause which tends 

 to produce this effect, is the porosity, or the want of 

 mechanical combination in the texture of the soil ; which 

 being highly favourable to evaporation, is liable to render 

 it speedily, and very materially dry, when exposed to the 

 influence of powerful solar heat : the frequent application 

 of water, does not entirely obviate the objection, for even 

 when so applied, it soon becomes again evaporated, and 

 thus tends to deteriorate the soil, and decrease its fer- 

 tility ; this it does, by taking up much of the soluble 

 matter contained in it, and conveying it by evaporation 

 into the atmosphere, instead of its being taken up by the 

 roots whilst in a fluid state, and applied to the plant as 

 a means of nutrition : when it is thus conveyed to the 

 atmosphere, the leaves though they are enabled to take 

 up a portion of their food from thence, are still incapaci- 

 tated to do so fully, and hence, much of the fertilizing 

 D 5 



