Soil Moisture Or Soil Water

SOIL MOISTURE OR SOIL WATER: MEANING, SOURCES AND OCCURRENCE In a hypothetical medium-textured soil, moisture constitutes about 25% by volume; the value is generally lower and higher in coarse- and fine-textured soils respectively, all other factors remaining constant. Under natural field conditions, the soil is never devoid of moisture, which exists as films of water at the peripheries of the solid phase. Soil moisture is actually water in the soil; the only difference is that it is neither in its pure state nor in ponded condition. As such, the terms ‘moisture’ and ‘water’ are regularly used interchangeably with respect to the soil. Since water is a universal solvent, its dissolution of solutes in a mineralrich system like the soil is naturally inevitable. Therefore, the use of ‘soil water’ should be with caution, to denote that which ordinarily has some solutes (mainly minerals) dissolved in it, and which is not ponded in any occlusions in the soil. In other words, if not connoting the soil moisture properties (such as movement and energy status) and its availability or utilization by crop plants, the use of ‘moisture’ would be preferable. Soil moisture is that constituent of the soil that makes it characteristically wet. It represents the liquid phase component of the soil – as a three-phase (solid, liquid, and gas) system. Sources of moisture to the soil include precipitation (especially rainfall, snowfall, and throughfall), irrigation, and groundwater recharge. With rainfall, overland flow sometimes occurs, depending on some characteristics of the rainfall, as well as on the topographic, structural, and cover management features of the soil. This overland flow, termed runoff, runoff does not enter the soil from where it emanates. If, however, the rainwater is impounded in depressions or a reasonable proportion is trapped by some techniques geared primarily towards conserving soil water, the ‘harvested’ runoff automatically becomes part of the soil moisture, provided the soil physical conditions favour infiltration. Strictly speaking, water lost to runoff never constitutes in soil moisture, since soil moisture refers to the water already infiltrated into, and is part of, the soil. Runoff is, therefore, considered an important avenue of loss of rainwater from surface of the soil. On the other hand, deep percolation is another important avenue of unproductive loss of soil moisture, and is influenced mainly by soil texture and climate, being more pronounced in sandy soils and in the wetter regions. Based on the foregoing, it is more informative to talk of effective rather than total rainfall. Effective rainfall is simply total rainfall less runoff and deep percolation. Sunday E. Obalum Department of Soil Science University of Nigeria, Nsukka Nigeria