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Soils 102

By Edited Jan 5, 2016 0 0

What is soil made of?

Soil is made up of three components

  1. Mineral
  2. Organic
  3. Pore(empty) Space

These three in various combinations give us our different types of soils that range from a sandy beaches to the clayey and mucky soils at the bottom of river or lake.  I will start off with going into more detail about the mineral components of soil which are divided into three parts according to size



Clay the smallest of the soil particles will be less than .0002 mm in size which is to small to be seen under a standard microscope.  To put this another way the width of a human hair is .1 mm  

The clay’s in a soil helps control some of the most important parts of how a soil functions, including its water holding capacity and the exchange of ion between soil particles and soil that is in solution(a.k.a dissovled in water).  The exchange of ions in a soil is crucial to how fertile the soil is and the clay content of the soil regulations how well ions are exchanged



The middle bother so to speak is between .002mm - .5 millimeters in size and can be seen with the naked eye. It will feel like flour or powdered sugar to touch not smooth like clay or

Loess Hills, Iowa Photo from wikipedia commons taken by Bill Whittker
gritty like sand. Silt is commonly the soil particle that is easily carried by the wind and as such makes up quite a bit of windblown particles we find in the air. This tendency for silt to blow can lead to it easily eroding away and travelling to a new area. The Loess Hills in Iowa are a large mound of deposited silt that has been blow off of the Great Plains over thousands of years.



Perhaps the part of soil that most people are familiar with due to its many useful and sometimes irritating characteristics.  Sand will range from 0.5mm-2.0 mm in size and is easily visible to the naked eye.  Due to its large particle size and pore space sand will let water infiltrate through it much easier than clays or silts.  The amount of sand in a soil can be crucial in determining what types of plants can be grown.  In addition sand and its larger cousin gravel are integral in road building, house construction making glass and many other applications.      


Organic Matter

Without organic matter many of the functions of soil that we take for granted would simply not be possible.  The amount of organic matter in a soil affects how well it holds water, recycle nutrients through the soil, recovers from disturbance and it provides habitat for many important soil dwelling organisms.  Organic matter as the name suggest is the living or dead matter that is left behind by plants, animals, bacteria and fungi.  Different types of

Prairie Grasses-Photo from Wikipedia Commons and was taken by Robert Lawton
organic matter help build different types of soils.  If you look that the prairie soil where the principal vegetation is grass with is deep, dense, fibrous roots.  This root structure lead to the development of a thick black topsoil or O horizon in the prairie ecosystem.  While in contrast to this a forest soil will be thinner due to the majority of the organic matter begin deposited on the surface and not in the soil.


Pore Space

Last but not least with we have the poor space, probably the most important nothing you will ever encounter.  The amount of pore space in a soil determines how well a soil drains and holds water.  The bigger the pore the better the drainage and vice versa.  This can affect many things about how we go about utilizing the soil, it determines where we build to what we plant and where we may be able to find a aquifer. 

One little know fact about pore space is that soils high in clay actually have more pore space than soils high in sand. 


I know that’s what I thought too the first time my soil professor said that.  Here is how he explained it; Clay has a higher amount of pore space because there are more particles for there to be space between.  Since there can be more clay particles in the same amount of space as sand there are more clay edges.  The more edges you have the more room there is between many little edges than a couple big edges.  If you add up the space between the clay you will find there is more there what is between the sand.  The diagram below illustrates my point.

Clay and Sand Pore Space

 For me this explanation just brought up the question of:

If there is more space between the clay than the sand then how come water pools on clays but goes through sand?

That has to do with the size of the pores.  There is more pore space in the clay but pores are bigger in the sand.  The water essential gets stuck on the space between the clay and only slowly squeezes through it while on the sand it can slide right through without a problem. 

With those three building blocks you can build any soil in the world but thankfully Mother Nature has done that for us.  I hope I have made learning about soil and soil science a bit more enjoyable for you and I look forward to continuing my articles on soil and soil sciences




Smith , Thomas M., and Robert Leo Smith . Elements of Ecology. 6. San Francisco: Pearson Education Inc., 2006. Print.





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