Soils & Soil Physical PropertiesUnit 2.1 | Part 2 –15i. Structure
Organic matter acts like glue that helps hold soil aggregates together. These will hold
even upon wetting.
ii. Available Water Capacity (AWC)
Organic matter helps bind water to the soil, keeping it from being lost through
percolation and making it available to plants for uptake. This is especially important
in sandy soils.
iii. Cation Exchange Capacity (CEC) (see description of CEC under Texture)
Like clay particles, organic matter particles have a negative charge and thus attract,
hold, and release cations necessary for plant growth and reproduction
iv. Binding plant toxins
Organic molecules can bind up dome ions that are toxic to plants
c) Relationship to climate
i. The amount of organic matter soil can hold is really in equilibrium, like a bathtub that
is receiving water at the same rate that water is going down the drain—the water
molecules are moving in and out of the bathtub but the total volume in the tub
stays constant. Organic matter added beyond a soil’s equilibrium is “overflow,” and is
broken down to carbon dioxide and water.
ii. There is a maximum amount of organic matter that any particular soil can hold,
and that amount is inversely proportional to soil temperature and moisture: wetter
and colder soils can hold higher quantities of organic matter than warmer and drier
soils. For example, peat soils of northern Canada and Europe have very high organic
matter while soils of the Southwest U.S. tend to be very low in organic matter.
iii. Some researchers and growers are looking at the potential of organic and
conservation tillage practices to increase soil OM. Increasing the total C content of
the soil through high C inputs and minimal soil disturbance may increase the amount
of OM in equilibrium, sequestering the C and offsetting emissions of CO 2.
- Color
Soil color varies with parent material, how long the soil has been forming, and the
environment itself
a) Describing soil color
Soil color is described using Munsell Color Notation (show Munsell Soil Color Book). The
notation breaks soil color into hue (the particular color), value (how light or dark the
color is), and chroma (how washed out or intense it is). The Munsell Soil books have
color chips that allow scientists to precisely describe and compare soils.
b) Meaning of soil color
i. Drainage and wetness (also called “redoximorphic features”) (show samples)
Greenish, bluish, and gray colors in the soil indicate wetness. These colors may occur
as the dominant color (the matrix) or in patches (mottles). The colors are caused by
the reduction of iron by bacteria in anaerobic conditions, when the bacteria get the
electrons they need for energy from iron rather than from oxygen. These colors will
persist even if the area is drained.
Lecture 2: Soil Properties