Soils are typically about 45% mineral, 5% organic matter, and 50% voids (or pores) of which half is occupied by water and half by air or gas. The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other.
Pore space allows for the infiltration and movement of air and water, both of which are critical for life in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.
Soil hydrology stands at the forefront of soil health due to its critical importance in regulating physical, chemical and biological processes in soils. Soil-water interactions are closely related to and create both positive and negative feedbacks with soil characteristics, landscape features and management practices that are closely tied to soil health.
Knowing the hydrological behavior of soils is essential for managing and protecting natural and agricultural ecosystems. Soil hydrological behavior determines crop responses to water and nutrients provided by irrigation and fertilization. Soil hydrology also controls deep percolation fluxes of water and nutrients, as well as water and nutrient runoff. Thus, it impacts the quality of soil, surface and groundwater resources.
Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed. Water is also key to the dissolution, precipitation and leaching of minerals from the soil profile. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.
Soils supply plants with nutrients, most of which are held in place by particles of clay and organic matter. Nutrients may be absorbed on clay mineral surfaces, bound within clay minerals (absorbed), or bound within organic compounds as part of the living organisms or dead soil organic matter. These bound nutrients interact with soil water to buffer the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.
Plant nutrient availability is affected by soil pH, which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acid) where weathering is more advanced.
Most plant nutrients, with the exception of nitrogen, originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute nitric acid and ammonia, but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria.
Major Soil Properties
Other Soil Properties
Soil Bulk Density
In general, the greater the soil density, the less water it will hold and the slower water will move through it. There will often be soil horizons that will be denser than others giving the soil different hydrological properties with depth. Occasionally, water will stop or slow down and rest on a dense, less permeable layer of soil. This phenomenon is called perched water. If two soil sensors 20 cm apart have very different soil moisture readings, chances are that one of the probes is residing in perched water.
There is also a relationship between soil bulk density and the complex dielectric permittivity. The bulk density is associated with the density of a soil ped or a soil core sample. The particle density is the density of an individual soil particle such as a grain of sand. The two densities should not be confused with one another. Because the dielectric permittivity of dry soil is a function of both the bulk and particle densities, the soil density often creates the need for soil-specific calibrations.
Shrink/swell clays belong to the soil taxonomic order vertisol and are composed of smectite clays. These clays have a large ion exchange capacity and will shrink and swell seasonally with water content. The seasonal expansion and contraction homogenizes the top soil and the subsoil. As the clay shrinks during a drying period, the soil will crack open and form large crevasses or fissures. If a fissure forms in the measurement volume of a soil sensor, the probe will signal average the fissure and potentially generate biased results. If the fissure fills with water, the soil moisture measurement will be high, and if the fissure is dry the soil moisture measurement will be lower than expected. If the sensor measurements are being affected by shrink/swell clays, it is recommend to relocate the probe to an adjacent location.
Rock and Pebbles
Usually it will be obvious if a rock is encountered during an installation of a soil sensor. Never use excessive force to insert the probe into the soil. Some soils will have a distribution of pebbles. If a pebble finds its way between the probe’s mental tines, it will create an area in the measurement volume that will not contain water. The probe’s moisture measurement will be artificially lowered. This is of particular concern with soil profile probes, because the presence of a rock will generally not be known since the a hole is dug or augered from the surface.
If the pebble is an anomaly, relocating the probe would provide more representative soil measurements. However, if it is revealed from the soil survey that there exists a random distribution of pebbles, a pebble between the tines may provide realistic measurements because of the way pebbles influence soil hydrology.
Organisms such as plants and burrowing animals can homogenize soil and dislodge soil probes. A tree root can grow between the tines affecting the measurements and in some cases, tree roots can bring a buried soil probe to the soil surface. Each of these instances will have an effect on the accuracy of the measurements.
The cable leading to the probe may also become a tasty treat for some animals. If communication between the data logger and the probe fails, check the cable for damage. Wherever possible, a metal or PVC conduit can help protect the cable.
A soil ped is a single unit of soil structure. Ped shapes include granular, platy, blocky and prismatic and ped sizes can range from 1mm granules to 10cm prisms. The preferential pathway water travels through soil is between the peds. This is evident by clay film coatings that develop around a ped. The clay film precursors become dissolved in the pore water; as the pore water subsides the clay film precursors fall out of solution and adhere to the surface of the peds creating the clay film. The clay film will often delay the infiltration of water into the ped. Thus as the wetting front moves down into the soil, the regions between the peds will be the preferential water pathway. As the wetting front moves through the soil column the soil moisture measurements may be temporarily biased by the peds. For example, if the soil probe’s measurement volume is residing entirely in a single ped, the probe would not detect the wetting front until the water infiltrates the ped. Likewise, if the sensing volume is residing between several peds, the soil moisture measurements will reflect the movement of water between the peds. During installation, if a horizon has thick clay films around the peds, you may want to use daily averages of soil moisture reading to accommodate soil moisture variations in the peds.