|MadSci Network: Environment/Ecology|
This question turned out to be a little harder to answer than I thought, but I found a lot of information about soil and water at these net addresses:
the Florida Agricultural System
The United States geologic Survey"
Soil is little pieces of rock mixed with little pieces of dead plants and animals. The size of the pieces of rock makes a big difference in how much water can be held. When the pieces of rock are about 2mm down to about .05 mm then we call that sand, if the pieces are even smaller -- about .05 mm down to .002 mm then we call that silt, and if the pieces are even smaller than .002mm then we call that clay.
There are spaces between the little particles of rock. In sandy soil the spaces are fairly big, but in clay soil the spaces are very small, so sandy soil has more space for water than clay.
But if the spaces are big that means the water can run out pretty quickly. So, sandy soil can soak up a lot of water but it runs off quickly; clay soil can't hold much water within the soil, but since the spaces are small the water can't leak through very well. (That is why we put clay at the bottom of a little pond, to seal up the pond by preventing water from leaking through).
Most soils are made of a mixture of sand, silt, clay and dead plants and animals. When the soil is mixed, the sifferent kinds of particles stick together into little clumps called "peds". Cracks and little tunnels or channels form between the clumps. These little tunnels can hold a lot of water and are help to allow water to drain through the soil.
When the soil gets mashed down (like, in a path where a lot of people have walked, or in a construction site where heavy trucks have driven on it) then the little tunnels and spaces get squeezed together and the water can not soak through the soil. Usually that means that plants can't grow too well on that spot.
You can do some great experiments on soil and water. Try to get some big cans (like coffee cans)and get an adult to poke some holes in the bottom. Fill the cans with different kinds of soil. Then soak them with a lot of water so they hold as much water as possible. In a few minutes when they finish dripping you can weigh them. Then leave them in a dry spot for a few days and check the weight each day until they are completely dried out. How much water did they hold? How long did it takes for them to dry (some might take a lot longer than others!). When you put water back in, how much can they hold before they start dripping?
Thanks for the great question. I learned a lot by answering it!
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