Soil Science Glossary

  • Bulk Density of Soil - (mass of soil)/(volume of soil) typically in g/cm3. The test is performed by hammering a hallow steel cylinder into the soil until its top is flush with the ground level. The soil is removed from the cylinder, and weighed. The equations is computed with the internal volume of the cylinder. The average density of soil particles is 2.7g/cm3.

  • Field Capacity - Is the water content of the soil after it has been saturated, and excess water has been drained away, and the rate of downward movement of the water has decreased substantially.

  • Gravitational Water - The water which runs out of the soil immediately after it has been saturated.

  • Gravimetric Water Content (GWC) - (moist weight - dry weight)/(dry weight)

  • Loam is a mixture of clay, sand and/or silt.

  • Particle Density - Density of a given mineral assuming no pore space.

  • Mineral/Substance Density (g/cm3) Density (lbs/ft3)
    Water 1.0 62.5
    Quartz 2.65 166
    Feldspars 2.5-2.7 156-169
    Micas 2.7-3.0 169-188
    Clay minerals 2.0-3.0 125-188

  • Plant Available Water - This is the amount of water in the soil which can be extracted by a plant. It is calculated as the Field Capacity minus the Wilting Point.

  • Porosity - BD%= (wet weight (g)-dry weight (g))/(soil volume(cm3)). Or, BD%= (1- Bulk Density/2.7)*100. 1 minus the bulk density divided by the average density of soil particles.

  • Potentiometer - See Tensiometer.

  • Resistance Blocks - Two electrodes are embedded into a block of gypsum, fiberglass or other material, and the resistance across the block is measured, as an indication of soil moisture. The medium acts as a buffer to salts and other chemicals. Unfortunately, the results of these devices are greatly affected by salinity and other chemicals.

  • Root Depth - Root depth is important in calculating how much water is available to the plant. The deeper the roots, the more available water.

  • Soil Particle Sizes
    Soil Particle Type UK US International System
    Stones/Gravel >2.0mm >2.0mm >2.0mm
    Coarse Sand 2.0 to 0.2mm 2.0 to 0.2mm 2.0 to 0.2mm
    Fine Sand 0.2 to .06mm 0.2 to .05mm 0.2 to .02mm
    Silt 60 to 2 um 50 to 2 um 20 to 2um
    Clay <2um <2um <2um

  • Tensiometer - Acts like an artificial root. It's a plastic tube with a vacuum gauge at one end and a porous clay cup at the other end, and water inside the tube. When inserted into the soil, water is pulled out through the porous clay until an equilibrium is reached, at which point the vacuum gauge can be read to indicate the moisture of the soil.

  • Tilth - How easy it is to till a soil.

  • Water Content - (moist weight - dry weight)/(dry weight)

  • Water Holding Capacity - The amount of water that soil can hold, similar to the Field Capacity.

  • Wilting Point - occurs with the wter content of soil is so low that plan toots can no longer adequate water to sustain life.

  • Volumetric Water Content (VWC) - VWC= (Wet Soil Volume - Dry Soil Volume)/Dry Soil Volume. Or, VWC= GWC*Soil Bulk Density/Water Density.