Soil
water is of great importance because of the many biological activities and
chemical reactions that require water, and also because of the erosion of soil
by water. For example, water is something that is very abundant in Minnesota,
but the amount that exists in soils during the growing season will have a
tremendous impact on crop yields.
A water molecule is formed when two atoms of hydrogen bond covalently with one atom of oxygen. In a covalent bond electrons are shared between atoms. In water the sharing is not equal. The oxygen atom attracts the electrons more strongly than the hydrogen. This gives water an asymmetrical distribution of charge. Molecules that have ends with partial negative and positive charges are known as polar molecules. It is this polar property that allows water to separate polar solute molecules and explains why water can dissolve so many substances. A water molecule is therefore not a neutral molecule.
Note that the positive charges from the two hydrogens tend to be near one
side of the molecule, and the negative charges from oxygen are nearer the other
side. The positive side of one water molecule is attracted to the negative side
of the next water molecule.
The attraction for one water molecule to another is called cohesion
Adhesion is when water molecules are attracted to other types of molecules, such as glass, soil, metal, or plant leaves since the other surfaces also have charges. Adhesion forces are stronger than cohesion forces. The water beads on the leaves because the leaves have a waxy coating, which weakens the adhesive forces of the water molecules toward the leaf surface. The droplets hang together due to cohesive forces of the water molecules for each other, rather than spreading out into a thin film of water.
If we place water in a pan and then put a capillary
tube in the water, water will rise inside the tube. The smaller the tube, the
more adhesive forces are in play and the higher the water rises. In big tubes,
the cohesive forces pull the water down into the tube and keep it from rising.
See Capillary Tube
When water is added to the soil, because of the attractive force of the soil
particles for water, water spreads out in a thin film around the soil
particles.
Capillary water in soils refers to the water in small pores that is connected to a free water surface, or water in a dish , a lake or the water table. The smaller the pores, the higher the water will rise above the water table. The higher the rise, the tighter the water will be held to soil particles to overcome the force of gravity.
For an interesting look at water go to this PDF file from the Minnesota
Volunteer Magazine. The Wonder of Water by
Mary Hoff 
Soil Water Chapter 2
Soil Water Chapters
© Terence H. Cooper & Regents of the University of Minnesota, 2006The University of Minnesota is an equal opportunity educator and employer.
