Insoluble silicates 83.90 81.34 Soluble silica 0.08 0.02 Peroxide of iron 4.45 6.68 Alumina 2.40 3.00 Lime 1.23 1.33 Magnesia 0.45 0.25 Potash 0.20 0.22 Soda 0.07 0.09 Sulphuric acid 0.05 0.08 Phosphoric acid 0.38 0.07 Carbonic acid 0.09 0.34 Chlorine trace trace Humic acid 0.42 0.43 Humine ... 0.10 Insoluble organic matters 3.70 3.61 Water 2.54 2.52 ---- ---- 99.96 100.08 Nitrogen 0.15 1.15 In this instance such difference as exists is rather in favour of the soil on which clover fails, but it is exceedingly trifling; and it is necessary to seek an explanation in the special properties of its mechanical constituents. These properties are partly mechanical and partly chemical, and in both ways exercise an important influence on the fertility of the soil. Sand and clay, the most important of the mechanical constituents, confer on the soil diametrically opposite properties; the former, when present in large quantity, producing what are designated as light, the latter stiff or heavy soils. The hard indestructible siliceous grains, of which sand is composed, form a soil of an open texture, through which water readily permeates; while clay, from its fine state of division, and peculiar adhesiveness or plasticity, gives it a close-textured and retentive character, and their proper intermixture produces a light fertile loam, each tempering the peculiar properties of the other. Indeed, their mixture is manifestly essential, for sand alone contains little or none of the essential ingredients of plants; and if present in large quantity, the openness of the soil is excessive, water flows through it with rapidity, manures are rapidly wasted, and on the accession of drought, the plants growing upon it soon languish and die. Clay, on the other hand, is by itself equally objectionable; the closeness of its texture prevents the spreading of the roots of plants, and the access of carbonic acid, which, as we have already seen, is so important an agent in the changes occurring in the soil. In fact, a pure clay, that is to say, a clay unmixed with sand, even though it may contain all the essential constituents of the plant, is for this reason unfertile. Practically, of course, these extreme cases rarely occur; the heaviest clay soils being mixtures of true clay with sand, and the most sandy containing their proportion of clay; but frequently the