=I. Physical Properties of a Soil.=--The physical properties of a soil are generally admitted to have a very important bearing on its fertility. This has been long practically recognised, and perhaps has in the past been unduly exalted in importance, at the expense of the no less important functions of the chemical.[33] The reason of this is doubtless to be ascribed to the fact that it is much easier to study the physical properties of a soil than it is to study the chemical; and that, while we are in possession of a very large amount of useful information with regard to the former, we are at present only on the threshold of our knowledge of the latter. _Variety of Soils._ It is a matter of common observation that soils differ widely in their mechanical nature. The early recognition of this fact is evidenced by the large number of technical terms which have been long in vogue among farmers descriptive of these differences. Thus soils are in the habit of being described as "heavy," "light," "stiff," "strong," "warm," "cold," "wet," "damp," "peaty," "clayey," "sandy," "loamy," &c., &c. _Absorptive Power for Water._ One of the most important of the physical properties of a soil is its power to absorb water. Water to the plant economy is just as important and necessary as it is to the animal economy. Consequently it is of primary importance to examine into the conditions which regulate the absorption of this important plant-food by the soil. By the absorptive power of a soil is meant its capacity for drinking in any water with which its particles may come in contact. This power depends, first, on the predominance of its proximate constituents--viz., _sand_, _clay_, _carbonate of lime_, and _humus_; and secondly on the fineness of the soil-particles. _Absorptive Power of Sand, Clay, Humus._ First, then, with regard to the absorptive power of sand, clay, and humus. Of these, sand possesses this power to the least extent, clay to a greater extent, while humus possesses it most of all.[34] The extent, therefore, of the absorptive power of a soil depends very much on the proportions in which it possesses these three ingredients. The more sandy a soil is, the less will its power be of absorbing water; and this, there is little doubt, is one of the reasons why a sandy soil is, as a rule, an unfertile soil. Of course there are other and even more important reasons; but that this absorptive power has an important bearing on the question is conclusively proved by the fact that sandy soils are more fertile in a climate where rain is frequent than in one where much dry weather prevails. The incapacity of a sandy soil to absorb a large quantity of moisture is not fraught with such evil effects to the crops in the former case, because it is counteracted by the climatic conditions, which obviate the necessity, in a soil, of possessing great absorptive powers. The converse, of course, we may mention in passing, holds good of clayey soils.