Why Dry Peat Moss Repels Water ..An easily understood explanation.
Every gardener has had to deal with the frustration of trying to wet dry peat moss. The stuff tenaciously repels water. Peat-based potting mixes can contain dry voids even after hours of soaking, Dry peat and peat-based mixes would rather float on top of water then soak it up. What is it about peat moss that enables it to hold up to 15 times its weight in water when damp, yet stubbornly refuse to drink up a single drop when dry? To understand peat moss's dual personality we need to understand a little chemistry and physics.
Chemically, water is what is called a polar molecule. If you could see one it would look like a "V" with an oxygen atom at the point of the V and a hydrogen atom at the end of each arm. Because oxygen and hydrogen atoms have different electrical natures the molecule as a whole isn't electrically symmetric: the oxygen end has a slightly negative charge and the hydrogen end a slightly positive charge.
This makes it behave like a magnet: attracting other polar molecules with opposite charges attracting each other. This is what creates the surface tension force that pulls water droplets into spheres. The organic molecules that make up peat moss are symmetric with respect to charge so they don't have any charged "ends" to attract the water. In effect they repel water molecules, though in fact it's more accurate to say that they simply don't attract them.
Structurally, peat moss is similar to a sponge with millions of tiny spaces that can hold emormous quantities of water. The problem is that its chemical nature repels water more than its sponge-like nature attracts it.
When it's dry, the chemestry of peat moss makes the tips of its fibers repel the water so that it can't reach into the spaces between them. But, if you get it just a little damp (either by mechanically forcing a little water into it by mixing or by placing it in an area of high humidity where individual water molecules as vapor can infiltrate their way into it) then there are some water molecules embedded in the fibers of the peat moss and their polar nature attracts other water molecules, overpowering the peat's chemical force of repulsion (or more correctly: disinterest.) It's sort of like priming a pump: adding a little water enables it to pump a lot a water.
One way potting soil companies overcome the problem of dry peat moss's repulsion of water is by adding a wetting agent to their mixes. Wetting agents, or surfactants, are molecules with one end that is attracted to water and the other that's attracted to nonpolar molecules like those in peat moss. They act like glue that holds two surfaces together that would otherwise repel each other. The wetting agent only needs to attract and hold a little water in the peat moss. After that, the process of hydrating the peat carries on by itself as explained in the previous paragraph.
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