Editing Jethro Tull (agriculturist) (section)

===Growing soil===
Tull considered soil to be the sole food of plants. "Too much nitre," Tull wrote,<ref>''Tull's Book on Husbandry'', p. 13.</ref> "corrodes a plant, too much water drowns it, too much air dries the roots of it, too much heat burns it; but too much earth a plant can never have, unless it be therein wholly buried: too much earth or too fine can never possibly be given to their roots, for they never receive so much of it as to surfeit the plant." Again, he declares elsewhere, "That which nourishes and augments a plant is the true food of it. Every plant is earth, and the growth and true increase of a plant is the addition of more earth." And in his chapter on the "Pasture of Plants", Tull told his readers with great gravity that "this pasturage is the inner or internal superficies ''[sic]'' of the earth; or, which is the same thing, it is the superficies of the pores, cavities, or interstices of the divided parts of the earth, which are of two sorts, natural and artificial. The mouths or lacteals of roots take their pabulum, being fine particles of earth, from the superficies of the pores or cavities, wherein their roots are included."<ref name="CWJ 1844">"Earth" in: ''The Farmer's Encyclopædia, and Dictionary of Rural Affairs'', by Cuthbert W. Johnson, 1844, pp. 428–429.</ref>

Tull wrote with enthusiasm and carried his admiration of the powers of the earth to support vegetation too far; he was deceived, in fact, by the effects of his finely pulverising system of tillage, and did not sufficiently attend to the fact that there are many other substances in the commonly cultivated soils of the farmer besides the earths, and that so far from their being always the chief constituents of the soil, they very often form the smallest portion of even a highly productive field.<ref name="CWJ 1844"/>

That the four earths of which all cultivated soils are composed are all the necessary food or constituents of vegetables has, long since Tull wrote, been decided by accurate investigations of chemists. Of these, lime, either as a carbonate or an acetate or a sulphate, is by far the most generally present in plants; indeed, in one form or another, it is rarely absent from them. The presence of silica (flint) is almost equally general. Magnesia is less usually present, or, at least, it exists in smaller proportions; and the same remark applies to alumina (clay).<ref name="CWJ 1844"/>