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The Pamphlet Collection of Sir Robert Stout: Volume 73

Silica in Plants

Silica in Plants.

There is a great difference of opinion among agricultural experts as to the part that silica plays in plants. Silica itself—the oxide of an element called silicon—we have in abundance in the rocks and soils of the earth, where it occurs as quartz or rock-crystal. Sea-sand is the same thing (silica), coloured occasionally with oxide of iron, and mixed usually with a small proportion of fragments of shells. It occurs in abundance in all kinds of clay. The common rocks of our mountains—granite, mica-schists, basalts—all the different kinds of clay, and all the soils of our fields cos- page 47 tain irom 10 to 70 per cent, of silica. Indeed, it may be said that, of all the substances that enter into the composition of our rocks, clays, and soils, silica is the most abundant. There is no need, therefore, for applying silica directly to the soil.

But silica as it exists in sand and in the rocks is virtually insoluble in pure water. It may be, as some botanists suppose, that plants exude from their root-hairs some acid liquid which can extract and dissolve silica from rocky materials in the soil, and thus render it available for plant-food. However that may be, it is well enough known that caustic lime (slaked lime) or wood ashes and ammoniacal and, indeed, all alkaline manures have the property of rendering a portion of the silica in soil soluble. The same function is probably performed also by superphosphates and all other acid manures.

It is in the grasses (oats, wheat, barley, maize, rye, sugar-cane, &c., and the pasture grasses) that silica is found in largest quantities; and it is in the straw of these that it is most abundant. Its functions in the plant are not clearly understood, the general opinion being, however, that it gives stiffness and strength to the stems, enabling them to stand upright, carrying a great weight of grain-bearing head. Some experiments with maize and other kinds of corn crops have proved, however, that these can grow big and strong-stalked and mature, as well as carry a large head, in soil quite destitute of silica, the plants receiving none of that ingredient except from the particles of dust that are floating in the air in which they grow.

Other experiments tried on oats with a dressing of soluble silica have led to the conclusion that it benefits the plant by enabling it to take up and assimilate other elements of plant-food. Griffiths recommends an admixture of soluble silicates (silicate of potash or of soda) with other manures for grain crops and pasture.

The following table shows the quantity of silica in 100lb. of the ash of the common cultivation plants:—
Wheat (straw) 65
Wheat (grain only)
Oats (grain), Wynter Blyth 44
Barley (grain), Wynter Blyth 27
Potatoes (tubers), Griffiths 5
Potatoes (shaws), Griffiths
Turnips (roots), Griffiths 1
Turnips (shaws, leaves), Griffiths 2
Mangels (roots), Griffiths 9
Meadow-hay 38

The lecturer would not recommend the direct application of silica in any form as a manure, believing that, by the liberal use of lime and superphosphates (not both together, of course), enough silica will always be available for the requirements of the plants.