Week 3
Chapter 4
ESPM 120
Response to Minute Papers
After going through the minute papers, I picked out several of the main topics that I
wanted to go into more detail for you. Since it is impossible to cover all of the details of
the chapters in class, my intent is that this response should help clarify some of your
questions.
In general, flocculation is good for a soil because it is the process that promotes
aggregation of soil particles into peds (also called aggregates). This helps create
macropores which allow more water and air movement through a soil – benefiting plants
and organisms living there. As described in class, ions with greater charge (such as Ca2+,
Mg2+, and Al3+) do a better job flocculating soil particles than ions with a single charge
(e.g. Na1+). Single charged ions are not strong enough to hold clay particles together or
form a “bridge” between them. Also Na1+ ions have a larger radius so the clay particles
aren’t able to get very close, which also leads to dispersion (breaking up of soil
aggregates).
Macropores are the larger soil pores (>0.08mm) and micropores are the smaller pores
(<0.08mm) in a soil. Macropores can be formed 1) between sand particles, 2) between
aggregates (interped pores), and 3) by organisms (biopores). Micropores are found
within aggregates. Micropores can be too small for root penetration or for organisms to
live within, and are usually filled with water to the exclusion of air. Macropores are
important for ready movement of air and drainage of water. Both soil structure (shape:
spheroidal, plate-like, block-like, prism-like) and soil texture influence the balance
between macropores and micropores. Also, the size, shape, and interconnection of soil
pores, rather than their combined volume, are of greatest importance in determining soil
drainage, aeration, and other such processes. See Table 4.5 for more details.
A soil textural class is a grouping of soil textural units based on the relative proportions
of the various soil separates (i.e. the percent sand, silt, clay). There are three broad
textural classes: sand, clay, and loam. Clays contain >40% clay, <45 % sand, and <40%
silt. Loams have a moderate amount of sand, silt, and clay; containing 7-27% clay, 2850% silt, and 23-52% sand. The definition of loam is a mixture of sand, silt, and clay
particles that exhibit the properties of those separates in about equal proportions (i.e.
because clay has such a large influence on the properties of a soil, a loam does not have
equal amounts of the three separates – see percentages above).