PHYSICAL PROPERTIES. The physical proper ties of soils which are of special importance are color, weight, fineness of division or texture, structure or arrangement of particles. adhesive ness, and relations to gases, heat, moisture, and dissolved solids. Variations in these properties determine to a large extent the productiveness of soils. Good tilth and texture with their ac companiments of good water conditions, aeration, and temperature are fully as essential to plant growth as an adequate supply of plant food. Physical properties of soils are, however, so largely dependent upon their natural character, and can lie modified to such a limited extent by man, that it is of the greatest importance in practice carefully to select soils with special reference to the suitability of their physical characteristics to the crop to be grown.
The physical properties of soils are determined to a large extent by the proportions they con Lain of stones, grave], sand, clay, lime, and or ganic matter. A soil containing much sand is dry, warm. and easy to work, but as a rule is naturally poor and has little absorptive power for water and fertilizing matter. A soil in which clay predominates is apt to be cold, wet, and difficult to till. but to have a high absorptive power not only for water, but for fertilizing matter as well. Clayey soils generally contain more plant food than sandy soils. II umus makes soils light in weight and dark in color and greatly increases their absorptive power. Lime not only has value as a plant food, but improves the structure of both clayey and sandy soils and corrects acidity. It also promotes the de composition of organic matter and aids nitrifi cation.
Mechanical analysis, which separates the par ticles of a soil into six or more grades of fine ness ranging from stones and gravel through sand and silt to clay, furnishes a valuable means of securing data for judging of the physical properties of soils. The productiveness of a soil depends very largely upon its texture and struc ture. i.e. the size of the particles and their ar rangement. These determine very largely the circulation of water and gases, the solution and retention of plant food, and the growth of plant roots. When the grains are single or separated
the soil is said to have a puddled structure. while a compounding of the soil grains gives a floccu lated structure. The latter is desirable in all good soils. as it increases the pore space and facilitates the circulation of air and water through the soil mass. Flocculation may at times be_enused by frost action, hut more fre quently is produced by the action of lime. Fer tilizers vary in their action on soils, some, like nitrate of soda, producing puddHng, while others produce flocculation. The finer the soil particles the greater the injurious effects of puddling, clay soils suffering from this cause more than sandy soils. Puddling increases the water-retaining power, and retards percolation. but may ac celerate capillary rise of water in the soil layers. Flocculation of the particles decreases the re tention of water, aids percolation, and may re tard evaporation. Water more easily from a coarse to a fine layer than from a fine to a coarse one, a fact taken advantage of by the farmer when he firms the soil by rolling and then loosens the surface by harrowing. which de stroys the capillary spaces and so checks the escape of water into the air. The water is thu-s held near the surface, where it is readily acces sible to the roots of plants. The action of the mulch (q.v.) depends upon this principle. In humid regions the clay particles of the soil are n.uall• washed down to a layer several inches below the surface, the surface layer being called the soil proper. and the lower one the subsoil. In arid regions this difference does not exist. hut the fine clay particles are evenly distributed throughout the soil layers.
Soils vary widely in their absorption power for water and for fertilizing matter, a property frequently due in clayey and humus soils to the presence of colloid substances. Of the three principal fertilizing constituents—nitrogen, phos phoric acid, and potash—soils apparently have the least retentive power for nitrogen (in the form of nitrate) and the greatest for phosphoric acid.