SOIL FERTILITY The fertility of the soil is its power to support plant life, and the phrase has meaning because the general requirements of most cultivated plants in any particular region are very similar. A fertile soil has the following properties:—I. Sufficient depth to allow full root development. 2. Supplies sufficient moisture with unfailing regularity. 3. Supplies sufficient air for the roots. 4. Has suitable temperature. 5. Supplies adequate nutrients. 6. Has a suitable reaction. 7. Is free from any harmful factor. No soil is perfectly fertile, and the problem of the agriculturist is to discover the defects from studies of the soil and of the symptoms exhibited by the plant, and then to devise suitable remedies. Depth of soil is improved by deep cultivation, by removing a pan or layer of rock or by drainage to lower the water level. Water supply is improved: (I) by direct addition of water by irriga tion, (2) by reducing loss of water: this can be done (a) by maintaining a fine layer of soil on the surface to act as a mulch to reduce evaporation : (b) in tropical countries by growing leafy crops to shade the earth from the sun's rays: (c) by incorpo rating organic matter (e.g., farmyard manure) with the soil, thereby increasing the amount of colloid material and the power of absorbing water and preventing it from soaking through the soil. The folding of sheep on the land has the same effect and is one of the reasons why sheep are so valuable for the arable cultivation of light lands. Air supply is ensured by adequate cultivation, by drainage to remove excess of water and by the use of sufficient lime to keep the clay properly flocculated. Temperature is usually best improved by drainage : something can be done, however, by setting up the land in ridges and by dressings of soot. Nutrients are supplied in artificial manures, the use of which in proper amounts given at the most suitable time may greatly enhance yields. Many methods have been devised for analysing soil with a view of determining its probable fertiliser requirements: some are purely chemical and depend on the use of acid solvents, others, such as Neubauer's, use the seedling as the extracting agent : none is entirely successful though some in expert hands give useful information. The reaction is ascertained by determining the hydrogen ion concentration (pH value) of the soil, if this is neutral there is usually nothing to be done: if acid there are two methods of procedure: (I) the growth of crops (e.g., rye, oats, alsike clover) specially suited to or tolerant of acidity, (2) the addition of lime or limestone to make the soil neutral. From the pH value alone it is not possible to estimate how much lime is required : several chemical methods have, however, been devised for this purpose. When the soil is alkaline the most suitable remedies are to add calcium sulphate or free sulphur, then to wash out the soluble salt taking care that the drainage is adequate. Harmful factors include (I) marked excess or deficiency of clay : in neither case is the soil usually cultivable; (2) excess of soluble salts: this is remedied by flooding with water and providing adequate drainage—an example is the reclamation of Lake Aboukir near Alexandria (3) in soils actually or recently waterlogged, sulphides, ferrous compounds and other reduction products : the remedy here is cultivation and aeration; (4) disease organisms and pests.
See D. K. Glinka, Die Bodentypen; E. J. Russell, Soil Conditions and Plant Growth. (E. J. R.)
Soil and Disease.—The influence of different kinds of soil as a factor in the production of disease has long been debated in regard not only to the nature and number of the micro-organisms they contain, but also to the amount of moisture and air in them and their capacity for heat. The moisture in soil is derived from the rain and the ground-water. Above the level of the ground water the soil is kept moist by capillary attraction and by evapora tion of the water below, by rainfall, and by movements of the ground-water; on the other hand, the upper layers are constantly losing moisture by evaporation from the surface and through vege tation. When the ground-water rises it forces air out of the soil; when it falls again it leaves the soil moist and full of air. The nature of the soil largely influences the amount of moisture it takes up or retains. In regard to water, all soils have two actions —namely, permeability and absorbability. Permeability is prac tically identical with the speed at which percolation takes place; through clay it is slow, but increases in rapidity through marls, loams, limestones, chalks, coarse gravels and fine sands, reaching a maximum in soil saturated with moisture. The amount of mois ture retained depends mainly upon the absorbability of the soil, is greater for soils which consist of fine particles and increases with the amount of organic substances present. Above the level of the ground-water all soils contain air, varying in amount with the looseness of the soil. Some sands contain as much as 5o% of air of nearly the same composition as atmospheric air. The oxy gen, however, decreases with the depth, while the carbon dioxide increases.
Among the most noteworthy workers at the problems involved in the question of the influence of soil in the production of disease were von Foder, Pettenkofer, Levy, Fleck, von Naegeli, Schlee sing, Muntz and Warrington. The study of epidemic and endemic diseases generally brought to light facts which were held strongly to suggest that an intimate association exists between the soil and the appearance and propagation of certain diseases ; still the role played by the soil was not, and even yet, is not so well under stood as to make it possible to separate the factors and dogmatize oa their modes of action and possible effects. The general evidence indicated that the specific bacteria of cholera discharges, for example, are capable of a much longer existence in the superficial soil layers than was supposed ; consequently it is necessary to guard against pollution of the soil, and through it against the probable contamination of both water and air. But it was the dampness of the soil and its temperature that were incriminated. The incidence of diphtheria and of typhoid fever, too, were regarded as associated with dampness of soil from consideration of the behaviour of B. diphtheria and of B. typhosus under appro priate experimental conditions. At the present time, the incidence is ascribed to the relative presence of carriers (q.v.) of the re spective bacilli, and the influence of the soil becomes merged into the effect produced by soil and climatic factors on general health apart from the possibilities that they afford for leading to con tamination of water supplies. The level of the ground-water and the liability of disease-producing bacteria in the soil, subjects to which great attention was given at the beginning of the century, hardly enter into modern discussions on the behaviour of epidemics.