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Our Soil-Moisture Reservoir

Cecil Wadleigh, October 1, 1907 – February 18, 1997. Dr. Wadleigh was a very respected scientist who made numerous breakthroughs in the study of salinity and soil-moisture. At the time he wrote this article he was the Director of the Soil and Water Conservation Research Division, Agricultural Research Service, Beltsville, Maryland.

Here’s another trip into our past to learn from those whose shoulders we stand on.* We talk a lot about soil moisture and the importance of managing pastures so that they can absorb and hold precipitation whenever it comes. With this article from the October 1963 issue of “Soil Conservation,” Cecil Wadleigh, takes us deeper, covering how different soils make our job easier and harder.

Although most of the Nation’s growing concern over its water resources is centered on lakes and reservoirs and underground aquifers, the production of our basic needs, such as food and fiber and wood products, actually derives from the soil’s capacity to supply water to meet evaporative demands on vegetative lands.

On an average day, water in the root zone amounts to about 650 million acre-feet. Thus, water held in the soil-moisture reservoirs at a given time is nearly equal to one-half the total annual flow of the Nation’s streams – about 1,370 million acre-feet, or about 29 percent of average annual precipitation. Because about 3,380 million acre-feet of water – 71 percent of our water budget – is used annually in evapotranspiration from fields, forest, and rangeland, the soil-moisture reservoirs must be recharged the equivalent of five or more times during the average year.

Management of soil-moisture reservoirs not only is a key determinant in the production of man’s basic needs, but also is a significant determinant as to beneficial use of the major portion of the Nation’s water budget. The soil acts as a sponge and retains water against the downward pull of gravity. This retained moisture enables plants to use water and grow between rains or irrigations, and to survive during protracted droughts.

However, there is an important difference between a surface reservoir and the soil. Water in a conventional reservoir is free to run to the pump inlet and moves there more rapidly than the pump can extract it. Water in the soil is not completely free to move to the absorbing roots and it may move much more slowly than the plant roots can remove it. Thus, use of the water in the soil-moisture reservoir is dependent on growth and proliferation, multiplication of the roots – the “pump” – to reach moisture films absorbed on the surface of soil particles.

The availability of the soil moisture is limited by the depth of the rooting zone and the supply of available water per foot of depth. A sandy soil may hold less than 1 inch of available water per foot of depth; whereas a clay loam may hold 2 1/2 inches. Thus, a crop with roots penetrating but 1 foot in a sandy soil would have a moisture reservoir of only 1 inch of water; whereas a crop with roots penetrating 6 feet in a clay loam would have a reservoir of 15 inches.

On bright, summer days, solar energy will effect evopotranspiration of 0.2 – 0.35 inch of water per day. Hence, the capacity of the soil-moisture reservoir is a key factor in drought hazard and irrigation management.

The inherent nature of the crop plant has much to do with the characteristics of the root system. For example, alfalfa roots are capable of penetrating much deeper than those of Ladino clover. Roots of bermudagrass proliferate far more extensively than those of potatoes. Thus, the nature of the rooting behavior of crops may delimit the capacity and effectiveness of a soil-moisture reservoir.

There is little that can be done to alter the inherent water-holding capacity of a unit mass of a given soil. Puddling may decrease the proportion of larger pore spaces holding water at the lower tensions and thereby decrease the amount of available water. Significant increases in organic matter may increase water-holding capacity.There are numerous characteristics of soils which either inhibit or prohibit potnetial root development and thereby limit the moisture reservoir:

Shallow soils overlying bedrock or indurated layers have a correspondingly shallow soil-moisture reservoir. Similarly, when roots are stopped at plow depth by the plowpan, “traffic pan,” or claypan, the potential moisture reservoir in the subsoil is unavailable.

On many soils of the Atlantic Coastal Plain, roots of most crops will not penetrate the subsoil, because its acidity may record a pH of 3.8-4.2. Associated with soil acidity, there may occur toxic levels of aluminum and manganese in the subsoil that prevent root growth. Evidence also indicates that roots cannot penetrate a medium that is completely deficient in boron or calcium. Soluble salts in the soil mass frequently limit penetration and prliferation of rotts in the irrigated valleys of arid regions. In some arid areas, excess salts may have been leached away in times past, leaving high levels of exchangeable sodium. A sodic soil attracts calcium avidly and may actually remove it from growing root tips.

Since roots are living entities, their degree of growth is dependent upon the prevailing temperature of the soil. The prevalence of permafrost in some Alaskan valleys provide as an extreme example of conditions which adversely affect root penetration.

Roots of most land plants must have ample oxygen to grow, and even to absorb water; so good soil aeration is important in making the soil-moisture reservoir effective. Poor drainage curbs the development of root systems through lack of aeration, and in arid regions fosters the accumulation of salines in the surface soil.

All conservation farming practices which help the deep penetration and spreading of the roots of crop, range, and forest plants contribute to improved management and beneficial use of that enormous portion of the Nation’s water budget that is used in evapotranspiration. It also is evident that practices which affect the important infiltration of water into our soils, influence the rate of recharge of our soil-moisture reservoirs.

Illustration from the Library of Congress, public domain.

* From Wikipedia:

Standing on the shoulders of giants is a metaphor which means “Using the understanding gained by major thinkers who have gone before in order to make intellectual progress”.[1]

It is a metaphor of dwarfs standing on the shoulders of giants (Latin: nanos gigantum humeris insidentes) and expresses the meaning of “discovering truth by building on previous discoveries”.[2] This concept has been traced to the 12th century, attributed to Bernard of Chartres. Its most familiar expression in English is by Isaac Newton in 1675: “If I have seen further it is by standing on the shoulders of Giants.”

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