Visualizing Soil Properties: Water Infiltration
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Visualizing Soil Properties: Water Infiltration

(soft music) -[Robert Flynn] I’d like to show you some demonstrations
about soil, but using a sponge. A sponge is very similar to this soil, in the fact that it has pore spaces. These large holes in the sponge can represent larger
pores in our soil. It, also, has very small holes or small open spaces within the soil or within the sponge. So, very similar. And it, also, has all the solid material as what we would see in a soil: sands, silts, and clay’s. Very good way to demonstrate some soil property terminology that we oftentimes talk
about to our farmers and to our homeowners about how to manage soils
with water. So, I’m going to start adding water to this sponge. Soil, as it were. And as I add it, that water is going to start to go into the soil. Now, notice that as
it goes in, it”s not just a straight line down. It actually moves across. So, there’s a force of attraction here with all the solid particles in the soil. So, that force of attraction is oftentimes called matric potential, but it is a force of attraction to help move that water down and through that soil. So, as we add more
water, this is called infiltration. As the water infiltrates into the soil, it’s going
to infiltrate down through the soil profile. And now, as we keep adding that
water, another term to introduce is called percolation. So, our water is percolating through our soil. It’s filling up the large pores that are in
the soil. And then, as those pores drain, air fills up those pores. So, that’s going to be that percentage of the soil volume that is our air spaces. And then, there’s
some smaller pores. And then, they can drain over time. They take a little longer for them to release that water, that water to move through. And then, it fills with air, as well. And often, we don’t put enough water on. Some of our folks will irrigate their, soil but they don’t put enough water on and they leave
a dry zone down here. And if our plants are larger, they’re not going to have
access to the full volume of soil water. It’s actually limited by how much water
was put on. So, we want to be able to add enough water to fill up that profile, Where we expect our plants to get water from. As it comes along, with more water
being added, it’s going to start to reach the bottom of this top part of our soil. This water now can be referred to as leachate or drainage water, and it’s
going down into that second zone of soil. So, this sponge is going to represent my
subsoil, and this is where we–a lot of times when our irrigation water needs to
be managed, we want to be able to move that water down into this zone of the
soil which is the subsoil or the second foot of soil. Many roots will grow down
to this depth later in the season. And if we ever are short of water in the
topsoil, we want to certainly have enough water in that second foot in order for
the roots to take that up. So, as this percolates through, we’re now getting that water to drain down into the second foot, and this zone will start to fill up
with more water. I want to switch to just looking at our surface soil, and that top surface now is pretty much drained. And this is as much water as this soil can
hold. And we call that field capacity. That’s the amount of water that the soil
will hold after all these pores have opened up and drained that water down to the second foot. So, at this point, it’s field capacity, it’s the most water that a soil can hold. Clays hold the most total amount of water and sands hold the least. Then plants have then the ability to start extracting that water. And as it extracts that water, it doesn’t take much effort for that plant to remove the water. There’s many plants that are capable of
taking water out of the soil as it gets drier and drier. So, at some point though, some plants just cease to actually be able to take anymore water out. And this point at which plants cease to take water out of the soil is known as the
permanent wilting point. Now that number, or that amount of water that’s left in the soil, is different for different plants because there are other plants
that can exert a little bit more force to actually get more water out of the
soil. These are a little bit more of your drought tolerant type plants. There are other plants that can actually exert more force, in order, to get as much water
as possible out of there. But, even they will reach that permanent wilting point. Is the sponge totally dry? Is our soil totally dry? No. It’s still moist. You can feel that moisture on there. But, that water that’s left in the soil now is so tightly held to the solids within the soil that the plants cannot take it out
and use it. This then becomes permanent wilting point. Permanent meaning that, a
failure to replenish that moisture and that plant will certainly wilt and most
likely die. They’ve used up what’s known as the plant available water. This plant
available water varies by soil texture. And the way we calculate the amount of
plant available water is to look at how much water we held at field capacity, and we subtract out the water that’s held at permanent wilting point. Clays are, actually, much more similar to sands in the amount of plant available water. And oftentimes, we actually manage our water on our sands the same way as we water our clay soils. Because, there’s still a lot of water in a clay soil, but the amount of plant available water is so
tightly–it’s just not there. That plant-available water is tied up in all
that solid that we see in the clay soils. The loams and the silt loams have the
most amount of plant available water, that difference between field capacity
and permanent wilting point, and they tend to last longer between irrigations. A lot of soils are on slopes and without cover. A lot of our souls will dry out a
lot. And when you first add water, it may just run right off. See, just runs right off until that very dry soil starts to be wedded up, overcome that hydrophobicness. These soils can get so dry sometimes, that they’ll actually become
hydrophobic and need time in order to start to be able to absorb water. Now some of our soils, we could, actually also, have a cover on it, and our organic
matter that we put on the soils can, also, get quite dry. And they can, also, run off until they get moistened up. So, we have to be wary of runoff potential for these soils and take steps of soil conservation, in order, to slow this water
down so that it can start to go into the soil and not run off and bring soil
particles with it. So, these are the key properties of soil texture as it relates to water movement and pore spaces and how they have affect the water holding
capacities of soils. (light music)


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