Glossary - Agricultural Geophysics - page 391

Soil pH A measure of acidity or alkalinity of a soil. The soil pH is measured on a scale of 0 to 14,
with 0 being the extreme for acidity and 14 being the extreme for alkalinity. A soil pH of 7
reflects neutral conditions in which the soil is neither acidic nor alkaline. Plants and micro-
organisms do not tolerate extremes in soil pH.
Soil Productivity Zones Areas within a farm field having similar crop production potential.
Soil Proile Sequence of soil horizons (layers) from the surface down to the parent material. The
soil profile is derived from the parent material via weathering processes.
Soil Solution The aqueous phase of a soil containing dissolved ions, other solutes, and suspended
colloids.
Soil Volumetric Water Content Volume amount of water per unit volume of soil.
Specific Surface Total soil particle surface area per unit dry weight of soil.
Subsurface Drainage As applied to agriculture, refers to the use of jointed or perforated buried
pipes to remove excess water from the soil for the purpose of enhancing crop growth and
improving field trafficability.
S -Wave (Also called a secondary wave , shear wave , or traverse wave. ) A type of seismic body wave
having an elastic particle motion that is perpendicular to the direction of wave propagation.
There are two kinds of S-waves: the SV-wave and the SH-wave. The particle motion for an
SV-wave has a vertical component. SH-waves, on the other hand, have a particle motion that
is completely horizontal. S-waves are only capable of traveling through solid material, and
not liquids or gases. S-waves are slower than P-waves and have a velocity, V S , given by:
V S = µ
ρ
where μ is the rigidity modulus and ρ is density. The S-wave velocity, as indicated by the
equation above, is governed strictly by shear stress elastic behavior and density of the soil or
rock through which the S-wave travels.
Thematic Map A map that provides information on a single topic or theme. Examples include
maps of crop yield and soil volumetric water content.
Time Domain Reflectometry (TDR) A technique used primarily for in situ measurement of volu-
metric water content for a small volume of soil. The technique is also employed to measure
soil electrical conductivity. The probe used with the technique usually has two metal rod
waveguides that are inserted into the soil. The probe generates a high-frequency radar pulse
that travels along the waveguides, reflects at the ends of the waveguides, and then returns
along the original path. The travel time of the radar pulse is used to calculate the radar veloc-
ity in the soil. The radar velocity determines the soil dielectric constant, which, in turn, is
strongly correlated with the volumetric water content present in the soil.
Tomography As applied to geophysics, refers to techniques used to determine the spatial distribu-
tion of a soil or rock material physical property within a defined boundary. These techniques
are based on measurements of some form of energy transmitted through the area or volume
enclosed within the boundary. Furthermore, measurements are obtained for multiple energy
source and sensor positionings along the periphery of the area or volume being investigated.
Measurements are then input into image reconstruction computer software employing inver-
sion techniques to determine the physical property spatial distribution within the bounded
soil or rock material.
Tortuosity The ratio of the average path length a water or solute molecule travels in a porous
media from one point to another compared to the straight-line path length between the two
points. Tortuosity is an indicator of the amount of connectivity between pores within a soil
or rock.