The soil moisture content of soil is the quantity of water it contains. Water content is used in a wide range of scientific and technical areas and is expressed as a ratio, which can range from 0 (completely dry) to the value of the materials’ porosity at saturation.
Moisture may be present as adsorbed moisture at internal surfaces and as capillary condensed water in small pores. At low relative humidity’s, moisture consists mainly of adsorbed water. At higher relative humidity’s, liquid water becomes more and more important, depending on the pore size.
In soil science, hydrology and agricultural sciences, water content has an important role for groundwater recharge, agriculture and soil chemistry. If the moisture content of a soil is optimum for plant growth, plants can readily absorb soil water. Not all the water, held in soil, is available to plants. Much of water remains in the soil as a thin film. Soil water dissolves salts and makes up the soil solution, which is important as medium for supply of nutrients to growing plants.
The amount of moisture or soil water is important to know because:
• Soil water serves as a solvent and carrier of food nutrients for plant growth
• The yield of a crop is more often determined by the amount of water available rather than the deficiency of other food nutrients
• Soil water acts as a nutrient itself
• Soil water regulates soil temperature
• Soil forming processes and weathering depend on water
• Microorganisms require water for their metabolic activities
• Soil water helps in chemical and biological activities of soil
• It is a principal constituent of the growing plant
• Water is essential for photosynthesis
The fact that soils hold water (moisture) is due to their colloidal properties and aggregation qualities. The water is held on the surface of the colloids and other particles and in the pores. The forces responsible for retention of water in the soil after the drainage has stopped are due to surface tension and surface attraction and are called surface moisture tension. This refers to the energy concept in moisture retention relationships. The force with which water is held is also termed as suction.
At Van Walt we recommend the easy and safe method of measuring soil moisture using TDR technology. Our award winning TRIME TDR devices generate a high-frequency-pulse (up to 1GHz) which propagates along the wave guides generating an electromagnetic field around the TRIME-probe. At the end of the wave guides, the pulse is reflected back to its source. The resulting transit time and dielectric constant are dependent on the moisture content of the material.
This highly complex but extremely easy to use technology should not be confused with capacitance or FDR techniques which, although acceptable for general relative measurements cannot be used for serious research. In particular accuracies can be seriously compromised in clay rich or very organic soils.
For more information on soil moisture technology please see below;