This is a nice summary of the soil science subject. It is partly based upon the book "essential soil science; a clear and concise introduction to soil science" by M.R. Ashman and G. Puri and also of the lectures and powerpoints given at Avans hogeschool.
Soil structure
Lesson 2
Soil structure
Soil structure is the way in which sand, silt and clay particles bond together in larger units, called
aggregates.
The soil texture is determined by the shape and size of the minerals. We use the texture triangle
to determine the texture. The texture has a big influence on pore sizes and the capacity to hold
water.
Formation of aggregates:
Aggregate formation then aggregate stabilization. Stable aggregates resist breakdown
when subjected to pressure or wetting.
Micro-aggregates (<250microm) are mainly clay minerals, both deflocculated (individually
flowing around) and flocculates (clay minerals stacked together to form domains). The
flocculated state is the first step in forming aggregates. The most important thing for
formation of domains is the presence of cations with a charge of at least 2+. If there is a lot
of 1+ ions they will not be able to bind the clay minerals because they will surround the
clay mineral and then repel other clay minerals, but 2+ ions bind to two clay minerals and
binds them together.
The stabilizing agents are divided in two groups: Temporary (microbial and plant products,
like polysaccharide gum) and transient (plant roots and fungal hyphae) stabilizing agents.
These aggregates and stabilizing agents are dynamic and are slowly disintegrated, but as
old aggregates are resolved new ones form and are stabilized, this is a continuous process
Aggregate stability is important:
o Because if aggregates break down: no more pores in the soil, less gas exchange and
less water infiltration
o If water does not infiltrate it will form puddles on top of the soil and then evaporate,
leaving very dry soil: erosion
Water in soil
If all pores are filled with water we say the soil is saturated. Then we have the field capacity
which is referring to when the biggest pores are emptied, the water that is lost during this period
is the gravitational water. Unsaturated soil is when the soil has some empty pores. Eventually if
the soil drains even more, the water is held so tightly that it is not available for plants etc. this
water is referred to as residual water.
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