Soil
There are a couple of other factors of soil in addition to what was mentioned in section
8-2 that affect runoff. They are porosity and permeability. Porosity refers to the amount of
space between the sediments. The greater the porosity, the more water the soil can hold. If
you think about a pile of fairly large rocks, you could see spaces between the rocks. Ok, so you
may be thinking, what does a pile of rocks have to do with soil. Well, soil is like the pile of
rocks, there are spaces between the sediments of the soil. They are just a lot smaller. Some
soils are made from larger sediments like sand, so sandy soils will soak up water faster and
allow less to runoff. Other soils like clay, which are really small sediments, will produce greater
runoff.
Somewhat related to porosity is permeability. Permeability refers to how fast water
drains through the soil. When soil particles have a lot of space between them, water will flow
easily through it and drain quickly. Water moves quickly thorough sand. When it rains on
sandy soil, it will dry quickly. This can be bad if you are trying to grow plants on sandy soil
because it doesn’t hold water well and becomes dry. Clay soil has poor permeability and
because if that it holds water. Once water gets into it, it dries very slowly. Besides the type of
soil affecting permeability, soil compaction does too. Soil compaction refers to how much the
soil is packed together. Farmers are concerned with soil compaction. When water stands on
fields and the water does not drain, farmers cannot work in the fields. Also if the water stands
to long, it can harm the crops. Part of the reason farm vehicles have such large tires is to
reduce soil compaction.
Define porosity
What affects soil porosity?
Define permeability
How does compaction affect soils?