Soil Basics: Adhesion and Cohesion


[MUSIC] Now, what does water do in the soil? Water does all sorts of
wonderful things in the soil. Actually, it’s very important for plant
growth, it can regulate soil temperature. You have to ask yourself,
well how do nutrients get to plant roots? Well, they move through the water,
and finally, the last thing that water can be involved
with is in organic matter decay. If you don’t have water there, you don’t have decay of soil
organic matter, for example. So let’s talk about the water molecule,
the water molecule is a dipolar molecule, it consists
of two hydrogens and an oxygen. It’s dipolar because one side of
the molecule has a positive charge, and the other side of the molecule
has sort of a negative charge, hence, it is called a dipolar molecule. So water molecules actually
stick together, one molecule attaches to another molecule through
a process called hydrogen bonding. One thing I want to talk about right
now is this concept of adhesion and cohesion, what does it mean? So adhesion is the absorption of
the water molecule to the sides of a, could be a container, it could be
the absorption of water to soil, through a process called
called the matrix potential. Keep in mind that the water
molecule is a dipolar molecule, it’s got a partial positive side,
and a partial negative side. And also remember, keep in mind that
the charge on soils is negative. So what’s going to happen is that
the water molecule is going to, the positive charges on the water
molecule are going to absorb the negative charges on the soil, through a process
called the matrix potential. So what’s happening here is, I’ve taken
this screen, and I’ve dunked it in this pan of water, so water’s being held
in this screen through adhesion. And then,
if we tip the screen over onto its side, or up and down, what you’ll notice is that now there’s water dripping out of this
screen, well, why is that happening? It’s happening because of cohesion, the linking together of these water
molecules are forming a long chain. And these water molecules are pulling
other water molecules out of this screen through
a process called cohesion. [MUSIC]

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