For promoting the Vegetation of many plants, clay is a necessary ingredient in the soil, with the exception of those* species called ar-gilla aerata, or lac lunce, and ar-gilla apyra, or porcelain, and other white, fermenting clays, for which no use has hitherto been discovered in agriculture. By its cohesion, clay retains humidity, on which, perhaps, its fertilizing property chiefly depends.
In its pure state, clay is unfit for the purposes of vegetation, on account of the great adhesion of argillaceous particles, which cannot be penetrated by the tender fibres of roots; but, when mixed with calcareous earth, and siliceous sand, or marl, it is much improved, and of great use in tillage.
It is commonly believed, that lumps of clay, in a moist state, may be rendered more friable, by exposing them to frost; which, by expanding the water they contain, and converting it into ice, is sup-posed to cause a farther separation of the clayey particles. This notion, however, appears to be erroneous ; for, unless the frost be very sudden, it will probably be attended with a contrary effect. Mr. KirwIn observes, that clay, in its dry state, absorbs more than twice its weight of water, before it parts with that fluid, and retains it, in the open air, more tenaciously than other earths; but, in a freezing cold, clay contracts more than other soils, and, as it were, squeeze* out its water in a greater than usual . proportion.
As clay, by the great cohesion of its particles, is not well adapted to the growth of roots, Dr. Darwin remarks, that it may, in some degree, be corrected, by frequently exposing the air confined in its interstices ; for instance, by turning it over with the plough, or spade. Another method is, by planting, in a clayey soil, first, those vegetables which are known to thrive in it, such as beans; and if their roots be afterwards left to putrify in the clay, they render the mass less cohesive, and enrich, rather than impoverish, the land. When clay abounds with vitriolic acid, so as to be convertible into alum, it becomes very unfavourable to vegetation, and checks the growth of trees, as well as of herbaceous plants, by corroding the fine extremities of their roots. This injurious quality may be most ef-fectually remedied, in gardens, by wood-ashes, or soap-suds; and, in fields, by mixing with such clay, lime, powdered chalk, or the sweepings of roads consisting of lime-stone.
All clay soils, as they retain too much water, are apt to chill the plants in moist seasons; on the contrary, in dry weather, they become hard, and obstruct vegetation. They naturally produce weeds, goose-grass, thistles, poppies, etc.; but some will yield clover and ryegrass ; and, if well manured, bear the best grain. Such soils are more advantageously manured than any other lands; the most proper that can be selected for this purpose, is horse or pigeons' dung, malt-dust, chalk, etc.
Clay-ground is naturally steril, because it adheres together in in masses. This defect may, however, be remedied, by mixing with it burnt clay; which tends to correct the cold nature of the soil, and will, by proper tillage, yield most excellent crops.
A remarkable instance of rural industry, in rendering a wet clayey soil uncommonly productive, occurs in the 28th volume of the Annals of Agriculture. The land was two perches in width, and gently arched up, so that the crown of the ridge was about 2, or 1\ feet higher than the bottom of the furrow. These ridges were gently rounded off, so as to describe the form of a segment of a very large circle, then disposed into double beds, and well manured. The fertility of the soil was farther promoted, by adapting the course of crops to its nature; namely, by sowing, 1st, beans; 2d, wheat; and, 3d, clover. In this succession, the beans were set upon a clover-lay, which saved much time, in preparing the land after the common way; and being sown just before, or immediately after, Christmas, they were ready to be hoed in the dry weather, usually occurring towards the end of February, or the beginning of March: by this management, they were brought so forward, that they could be cut in July or August. It is an error in agriculture, that beans cannot be left too long on the ground. They should be harvested while most of the pods are quite green; by which means a fine sample is secured, and the straw rendered Imcomparably better. After the beans, wheat was sown; and over that, in the month of March, or April, from 15 to 20lbs. of clover-seed per acre, which, in the following year, was mown twice for hay. These crops are particularly valuable on strong soils, where oats and barley never thrive well; and even if a large crop of either should be raised, it would be of a very inferior quality. Hence we recommend a similar course to be pursued, as the labour and expence necessarily incurred, will be amply compensated by perseverance and industry.