Soils - Part 2: Physical Properties of Soil and Soil Water Glossary


Soil separates that are grouped into a unit. Aggregates influence bulk density, porosity and pore size.  Principal types of aggregates are platy, prismatic, columnar, blocky and granular.  An aggregate possesses solids and pore space.

bulk density

The dry weight of soil per unit volume of soil; most often expressed on a soil volume basis, rather than on a particle basis. Bulk density considers both the solids and the pore space; whereas, particle density considers only the mineral solids.


As a soil separate, clay refers to mineral soil particles which are less than 0.02 millimeters in diameter. As a soil textural class, clay refers to soil material that is 40 percent or more clay, less than 45 percent sand, and less than 40 percent silt.


Precipitation, temperature and other aspects of climate affect how specific soils are formed.

field capacity

Soil water content after the soil has been saturated and allowed to drain freely for 24 to 48 hours. Free drainage occurs because of the force of gravity pulling on the water. When water stops draining, we know that the remaining water is held in the soil with a force greater than that of gravity.


Water movement in the soil.  Pore space in soil is the conduit that allows water to infiltrate and percolate (downward movement of water through the soil).

mechanical analysis

The laboratory procedure used to identify soil separates.

organic matter

Material that contains carbon and is found in the soil. Most soil organic matter comes from previously living organisms.  Temperature and moisture are the two main factors affecting its development.

particle density

The weight of an individual soil particle per unit volume. Particle density is usually expressed in units of grams per cubic centimeter (g/cm2). Bulk density considers both the solids and the pore space; whereas, particle density considers only the mineral solids.


The downward movement of water through the soil, made possible by pore space in the soil.

pore space

Spaces in soil, between the mineral and organic matter, that are filled with water or air.


The volume of soil voids that can be filled by water and/or air; inversely related to bulk density.  Porosity is also known as "pore space."


Individual rock or mineral fragments in a soil that range from 0.05 to 2.0 millimeters in diameter. Most sand grains consist of quartz, but they can be of any mineral composition.  Sand is also the textural class name of any soil that contains 85 percent or more sand and no more than 10 percent clay.


A soil inorganic separate in the range of 2 to 50 micrometers (or 0.002 to 0.05 mm.). Silt is smaller than sand but larger than clay.


The top layer of the Earth’s surface, consisting of four major components: air, water, organic matter and mineral matter. There are three categories of soil particles--sand, silt and clay--which are called "soil separates."

soil separates

Categories of soil particles—sand, silt and clay—divided by particle size.  The proportion of different soil separates in a field defines its soil texture.

soil series

A unit of soil classification determined by studying horizon characteristics, such as: number of horizons, color, thickness, texture, erosion phase, slope, organic content and depth to hardpan. All soils given the same soil series name possess the same characteristics across the landscape.

soil structure

The arrangement of soil separates into units called soil aggregates.

soil texture

Determined by the proportion of different soil separates--sand, silt and loam--in a soil.

water-holding capacity

The ability of a soil to hold water; varies by soil texture. Medium textured soils (fine sandy loam, silt loam and silty clay loam) have the highest water-holding capacity.