|Basalt has very tiny crystals that when weathered produce clay soils. The crystals in basalt are so small they typically can not be seen.|
So how can you identify the soil parent material and what soil texture comes from it?
1. Find the most common type of rock at a particular location.
2. Looking closely at the rock, does it have larger or smaller crystals or particle sizes?
3. The larger the crystals/particles the more sandy the soil. The smaller the crystals the more clay the soil will have.
The above rules generally hold true near mountains or where bedrock is close to the surface. It does not hold true in a few cases.
1. If the predominate rock is limestone. Limestone doesn't weather into soil particles, it rather is dissolved by water and washed away. For this reason, limestone doesn't make soil texture like other rocks but certain types of soil will form from it.
2. If you are very close to a stream or river. If you find lots of roundish rocks in the area, they are alluvial, or water, deposited. In this case the soil will typically have a lot of clay.
|If you find a lot of rocks in an area that look roundish like this you can know the area has alluvial, or water deposited, soil.|
So using rocks can be very useful when trying to understand a habitat and determining how soil is influencing plants and animals. In doing this the best book I have found in identifying rocks and their textures is "Smithsonian Handbooks: Rocks and Minerals". This book has the clearest, most useful pictures of any rock and mineral book I have looked at. It also has extremely useful information organized in a very easy to use and understand way. One of my favorite features along with the great photos is the "grain size" information that goes along with each rock. Grain size is the rock texture that we discussed above and fine grain size will result in fine clay soils, medium grain size a silt soil (half way between sand and clay), and a course grain size a sandy soil.
While this post discussed how to identify soils by looking at rocks I will have another post on how to identify soils directly by touch and later show how these things relate to the plant and animal life in a habitat. Understanding and using rocks and soil textures are extremely useful when trying to find specific plants and even animals. I personally have used rocks to help locate plants as well as places to find birds and animals. But it works best for plants.