It is the want of drainage, however, and of the free access of air, that most frequently renders subsoils for a time injurious to vegetation. Let the lands be well drained—let the subsoils be washed for a few years by the rain-water passing through them,—and there are few of those which are clayey in their nature that may not ultimately be brought to the surface, not only with safety, but with advantage to the soil. 4. _Ploughing._—Other benefits, again, attend upon the ordinary ploughings, hoeings, and workings of the land. Its parts are more minutely divided—the air gets access to every particle—it is rendered lighter, more open, more permeable to the roots. The vegetable matter it contains decomposes more rapidly by a constant turning of the soil, so that wherever the fibres of the roots penetrate, they find organic food provided for them, and an abundant supply of the oxygen of the atmosphere to aid in preparing it. The production of ammonia and of nitric acid also (see pages 33 to 36), and the absorption of one or both from the air, take place to a greater extent, the finer the soil is pulverised, and the more it has been exposed to the action of the atmosphere. The general advantage, indeed, to be derived from the constant working of the soil, may be inferred from the fact, that Tull reaped twelve successive crops of wheat from the same land by the repeated use of the plough and the horse-hoe. There are few soils so stubborn as not to shew themselves grateful in proportion to the amount of this kind of labour that may be bestowed upon them. 5. _Mixing._—It has been shewn (page 114), that the physical properties of the soil have an important influence upon its average fertility. The admixture of pure sand with clay soils produces an alteration which is often beneficial, and which is wholly physical. The sand merely opens the pores of the clay, and makes it more permeable to the air. The admixture of clay with sandy or peaty soils, however, produces both a physical and a chemical alteration. The clay not only consolidates and gives body to the sand or peat, but it also mixes with them certain earthy and saline substances useful or necessary to the plant, which neither the sand nor peat might originally contain in sufficient abundance. It thus alters its chemical constitution, and fits it for nourishing new races of plants. Such is the case also with admixtures of marl, of shell-sand, and of lime. They slightly consolidate the sands and open the clays, and thus improve the mechanical texture of both kinds of soil, but their main operation is chemical; and the almost universal benefit they produce depends upon the new chemical element they introduce into the constitution of the soil.