why do some soils have high porosity but low permeability
Earth is called the 'blue planet' because of the large amount of water on its surface. Most of that is salt water, and the rest is freshwater. We usually think of freshwater being in lakes and rivers, but you might be surprised to learn that about 98. 5% of all the fresh water on Earth is actually found underground! That's right, all the lakes, streams, rivers, and other freshwater bodies on the surface only make up 1. 5% of Earth's total fresh water. Collectively we refer to all that underground water as
groundwater. Much like surface water, groundwater doesn't stay in just one place. It moves through deep channels underground as well as through the soil itself. How does it do this? To understand, it might help to think about soil or sediments as a jar of mixed nuts. When you look at the jar you see all different kinds of nuts mixed together but also air spaces between the nuts. These spaces in between the nuts are called pores, and we find the same thing in soil and sediments. Porosity is the volume of those pores relative to the total space, and it's a good measure of how much water the ground can hold. It also tells us a lot about the type of soil or sediment we're looking at. Let's go back to our jar of nuts. If you have a jar full of large, uniform Brazil nuts, then you are going to have a lot of space in between and therefore a high porosity.
Therefore, if you have a soil or sediment that is pretty uniform in terms of the particle size and shape you can expect it to have a high porosity as well. But if you have many different kinds of nuts in the jar, and some smaller nuts can fit in between the larger ones you will have a lower porosity because there will be less open space. The same is true again for soil or sediment with multiple types of particles of different size and shape. Porosity is great for telling us how much water the soil or sediment can hold, but it's also important to know how that water can move through those spaces underground. For this we use a term called permeability, which describes how easily water flows. Let's think about this in terms of drinking through a straw. If you have a large open straw you can drink your water much more easily than if you have a small thin straw like a coffee stirrer. If the pores in the soil are large and well connected, water runs through them with great ease like it does with the large straw. But if the pores are small and not well connected then water has a more difficult time flowing, much like with the thin coffee stirrer. But just because a soil has a high porosity doesn't necessarily mean it has a high permeability as well.
For example, if the pores are not well connected then it will still be difficult for the water to flow no matter the size of the pores. Surface water movement causes rocks and landscapes to erode because as the water moves it picks up particles and sediments and transports them. The same thing happens underground as groundwater moves through the soil and rocks there. Additionally, rainwater, which is a major supplier of groundwater, is naturally slightly acidic because it reacts with carbon dioxide in the air and soil. This produces carbonic acid, which can dissolve certain rocks like limestone underground, carving out a unique underground landscape. The answer from Dr. Baboo is exceptional. P I denote from your question the interest in good or bad for soils. P The combination of soils with water and air, provide for land based life and habitat. P Although as humans, we may favor some soil types and characteristics for our own benefits and desires, the diversity of soils provides a wide range in habitat and conditions that contributes to diversity ofPflora and fauna. PPBad soils then would be unproductive, barren and unable to support native life forms. P Bad things for soils would be factors that alter soil function and productivity, or its ability to recover from activities or events in a reasonable period.
P Soil condition and function has some natural range of variability as it responds to various circumstances of wet, dry, wildfire, severe storms, vegetation, etc. P When activities or actions take a soil out of its range for normal function and resilience, it presents a loss in natural function that result inPchanges of various types in habitat, productivity, and uses. P These changesPcan be more commonly bad but sometimes good, depending on who might be rating the changes. P A farmer may rate change different from wildlife biologist, hydrologist, forester, landscape architect, environmentalist, or urban developer. P Some change in soils in land management is avoidable,Ptemporary, can be mitigated, while other changes constitute alterations that are difficult to reverse and may persist for extended periods. P It may be most important to understand when changes are made that cannot be easily or naturally reversed. P Soil scientists experienced in land managementPare perhaps the most prepared to suggest what to expect from changes to soils, and ways to adjust activities and actions to limit change, offer suggestions for recognizing soil capability, mitigation or improvement to soil conditions that can help landowners and managersPmake more informed decisions.
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