The colloidal character of the soil is associated mainly with the finest particles ; the clay fraction therefore is of special inter est. If a mechanical analysis of a heavy soil in good tilth were made without any preliminary treatment of the soil to break up the natural aggregates of particles, the clay fraction would be much less in amount than the true value, since each undisinte grated aggregate would behave as a single large particle. The examination of the best means of dispersing these aggregates into their constituent particles, which is essential for purposes of me chanical analysis, has thrown much light on the mechanism of compound particle formation. Calcium carbonate—a normal con stituent of most soils—and organic matter both act as cementing agents and are removed by treatment with dilute acid and hydro gen peroxide respectively. Even when this is done the full per centage of the clay fraction is not obtained in the mechanical analysis unless a deflocculating agent such as ammonia is added to the water, when the finest clay particles remain in suspension indefinitely. The compound particle therefore may be regarded as a loose porous aggregate of single soil particles, held together partly by the mechanical cementing action of organic matter and calcium carbonate and partly by physicochemical forces, analo gous to those producing the phenomena of flocculation in many other colloidal materials. Part of the beneficial effect on soil fer tility due to additions of chalk, or lime, and organic manures, is due to the improved tilth that results.
Soil Moisture.—The water requirements of vegetation are very considerable: for each unit of dry weight produced the plant transpires several hundred units of water, that must be taken up by the roots from the moisture in the soil. The moisture content of the soil represents the balance between supplies and withdrawals. Supplies are represented by rainfall (and irrigation where this is practiced) and by water raised by capillary action from the lower depths of the soil; the losses are due to evapora tion at the soil surface, transpiration by vegetation, and down ward percolation under the action of gravity. It is evident that the losses from causes other than transpiration must be reduced as much as possible if an adequate reserve of soil moisture is to be available for the plant. A soil in good tilth has the maximum water holding capacity; the compound particles act as sponges and provide a kind of water reservoir, while the comparatively large interstices between the adjacent aggregates permit of rapid drainage of excess water after periods of heavy rain. It was formerly thought that this percolation water could be brought back to the top soil by capillary action from a considerable depth, thus constituting a large reservoir of available moisture.
Careful experiments show that this is not so. For distances greater than about 3 feet the return of water by capillary action is so slow as to be negligible. Hence the only moisture supply available for plants is that in the soil layers permeated by roots and a layer about three feet thick below this. A long series of records from the percolation gauges at Rothamsted shows that on the average about 5o% of the total rainfall passes below the 5 foot depth, and this result emphasizes the necessity, even in a country of adequate and well distributed rainfall, for conserving by appro priate cultivations the soil moisture against direct loss by evaporation.
The soil material even in the continuous or rock form is a poor conductor, and in the discrete particle form its conductivity is smaller still, since, apart from radiation and convection, heat can only pass from particle to particle through the points of contact. The presence of water as an annulus around these points of contact, improves the conductivity, although water itself has a low conductivity and high specific heat; the thermal contact is improved by the presence of the water film and the higher heat conductivity of the soil material is brought into play to an extent more than sufficient to compensate for the low conductivity of the water. Hence a soil that is in the optimum condition of moisture content is also in the best condition for heat conduction. Again, the existence on the surface of a dry mulch of soil, which has a very low conductivity, protects the soil below from extremes of heat and cold and preserves it at a steadier temperature, to the advantage of the vegetation. In general, the average soil temperature in the upper layers of soil in temperate regions varies from 20° C in the summer to 5° C in the winter.