Soil Physical PropertiesUnit 2.1 | 17A new practice related to this is being employed to reduce greenhouse gas emissions.
Carbon sequestration is the practice of burying organic matter deep in the soil to
maximize the amount of organic carbon it contains. To the extent that the soil can hold
the organic carbon in equilibrium, it will reduce emissions of CO 2 from the soil.
- color
a) How it is measured
Munsell Color Notation (show color book)
b) Significance of/indicator of:
i. drainage and wetness (Redoximorphic features) (show samples)
Greenish, bluish, and gray colors in the soil indicate wetness. These colors may occur
as the dominant color (matrix) or in patches (mottles). The colors are caused by the
reduction of iron by bacteria in anaerobic conditions. (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, so they serve as indicators, not proof.
Bright colors (reds and yellows), on the other hand, indicate well-drained soils. you
don’t want to have free water (water in excess of the available water capacity) within
the rooting depth of your plants during the growing season. It is possible for a soil
with bright colors to still have wetness problems if the groundwater is moving fast
enough and if it has sufficient oxygen or if it is too cold for biological activity.
ii. Organic matter
dark colors in the soil usually indicate organic matter. They may also indicate wetness
(remember, wetter soils can accumulate more organic matter). Sometimes, the color
may be derived from the parent material. This is often the case in soils derived from
basic (dark-colored) igneous rock.
- Soil depth
It is important to know the depth of the soil. The depth determines how far the roots can
grow and how much water the soil can hold. depth is measured to the shallowest root-
limiting layer. Some things that may limit depth are:
a) Bedrock
b) densely compacted material (tillage pan)
Tillage pans are formed when farm implements repeatedly pass through the soil at the
same depth. This action causes soil particles to be pressed closer together, reducing
the amount of pore space and the size of the pores. Consequently, these pans have
permeability rates lower than that of the soil above and below it.
A soil may be plowed or ripped to tear up natural or tillage pans and to increase the
pore space in the soil. Also, deep-rooted cover crops might be used (see Unit 1.6,
Selecting and Using Cover Crops). In a gardening context, double digging might be used
(see Unit 1. 2, Garden and Field Tillage and Cultivation).
The benefits of using some kind of tillage to break up soil compaction don’t last forever.
And while it is more difficult to break up compaction in a finer textured soil, the benefits
will last longer than they will in a coarse textured soil. In a coarse textured soil, such as a
sandy loam, most of the pore space added by plowing or ripping will be lost by the end
of one cropping season.
c) Natural hardpans (soil cemented by iron, lime, gypsum, silica, etc.) (Show example)
d) Strongly contrasting textures (pot effect; usually found in floodplain soils)
e) Water tables
Students’ Lecture Outline