=115. Mechanical analysis.= From the above it is evident that, with the same rainfall, coarse soils will be relatively dry, and fine soils correspondingly moist. However, this difference in holard is somewhat counterbalanced by the fact that the chresard is much greater in the former than in the latter. The basis of these relations can be obtained only from a study of the texture of the soil. The usual method of doing this is by mechanical analysis. This is far from satisfactory, since the use of the sieves often brings about the disaggregation of groups of particles which act as units in the soil. Furthermore, the analysis affords no exact evidence of the compactness of the soil in nature, and tests of capillarity and porosity made with soil samples out of position are open to serious error. Nevertheless, mechanical analyses furnish results of some value by making it possible to compare soils upon the basis of texture. For ecological purposes, minute analyses are undesirable; their value in any work is doubtful. A separation of soil into gravel, sand, and silt-clay is sufficient, since the relative proportion of these will explain the holard and chresard of the soil concerned. The latter are also affected in rich soils, especially of forests, by the organic matter present. If this is in a finely divided condition, the amount is determined by calcining. When a definite layer of leafmold is present, as in forests and thickets, its water-value is found separately, since its power of retaining water is altogether out of proportion to its weight.