Get Information on Soil-Plant-Atmosphere Continuum

The soil-plant-atmosphere continuum refers to the many interconnections between the physical and organic components of the landscape.

The atmosphere provides the major energy and hydrologic inputs and also creates the conditions (wind, water, vapour, and heat) for a large percentage of the output (evaporation) from a location. Plants respond to the conditions provided by the atmosphere, although the atmosphere too is influenced by albedo (the reflection of light energy from the Earth’s surface) and water vapour handling properties of plants.

Soils play a significant role in the hydrological cycle because of their water-holding capacities. Due to their requirement for water, various plants are both influenced by water availability and influence the amount of water in soils. Plants also play a role in soil formation.

The drainage basin has been the key unit of landscape that has provided opportunities for experimental observations of the relationships between soils, plants, and the atmosphere, and their influence on the movement of water (G.S. Brush 1982). The next two sections examine these relationships.

The Soil Factor

The basic point of reference to begin a discussion of the movement of water in the landscape is soil. Soil has varying capacity to absorb water. Infiltration of water depends upon several physical properties of soil.

Particle size is the most important. If the particles are sand sized, water generally infiltrates rapidly. Smaller particles such as silt and clay tend to pack together and have much reduced infiltration rates.

The existence of roots, soil fauna, and organic matter can moderate the infiltration rate considerably. Because of the relationship between volume and surface area, particle size strongly influences water-holding capacity.

Larger particles such as sand absorb water freely and then, because of lack of surface area, are unable to retain soil moisture. Clays generally have a higher water-holding capacity because of the large surface area of the clay particles.

But, if infliltration is poor, and spaces between clays particles too confined, then water-holding capacity is also reduced. Silts and loams (that is, mixtures of clay and sand-sized particles) have the largest amount of available water holding capacity reserve for plant growth.

Three different types of bonding between water and soil particles are important to plants. Gravity water is that which occurs in liquid state and fills the spaces as it permeates the soil.

This water is freely available to plants and moves through the soil under the influence of gravity. Capillary water clings to small particles and fills small spaces of the soil. It is also available to plants as it slowly moves into the water.

It is so tightly bound to soil particles by molecular forces that it is unavailable to plants which wilt without other water. This water does not move through the soil (Brady 1974).

Soil releases water to streams and groundwater when two conditions are met. The soil must have sufficient water, and the water must be under the influence of gravity. When precipitation occurs at a rate that is equal to or greater than the rate of soil infiltration, the excess water flows under the influence of gravity as runoff toward the stream.

If more water infiltrates the soils than can be held by it, the surplus flows as groundwater and reappears in springs and streams, or enters an underground Aquifer.