Soil Components

The average healthy soil is composed of the following in their respective percentages:

• Liquid (soil water) 25%

• Air (mixture of gases) 25%

• Mineral matter 45%

• Organic matter 5 %

Soil compaction and other agricultural activities can change the composition of soil.

Soil Water

Water is a liquid (solvent) containing the nutrients needed by plants. It is in all chemical reactions that take place in the plant.

There are three major types of soil water :

Organic Matter

This comes about as a result of the decomposition of plant and animal waste. When it is fully decomposed it is called humus. Organic matter has four essential functions:

• supplies nutrients (N, P and K);

• improves soil structure - binding sandy soil and loosening clayey soils;

• encourages micro-organisms population to increase;

• increases water holding capacity in sandy soils.

Soil Profile

Soil Texture

This is the physical property/characteristics determined by the size and amount of particles and organic matter in the soil. It is the measure of coarseness and fineness of soil particles. The texture of soil has a major influence on its properties. Soil is said to have a coarse texture when sand is predominant. Clay soil is said to have a fine texture because the particle are fine. Particles are classified acoording to their sizes:

Gravel or Stone: particles larger than 2 mm.

Coarse Sand: particles gritty when rubbed between fingers;

not sticky when wet

Silt: smooth and powdery

Clay: smooth, sticky and plastic when moist.

N.B.: The texture of soil can be changed in special circumstances only, e.g. when a gardener adds sand to soil in a flower pot or seedbed. Nothing can be done to change the texture of soil under field conditions.

Soil texture can be determined qualitatively or quantitiatively using simple tests.

Soil Texture Video

Video Transcript

Hi. It’s Mr. Andersen and this is environmental science video 6. It is on soils. Soils are

incredibly important. It is where we grow our food. But they take along time to form.

We start with regular rock and then over time what we have is weathering. We have physical weathering where we break the rock down into smaller particles. We have chemical reactions or chemical weathering. We have biological weathering as well. And so it takes a long time for us to go from rock to soil, soil that we can grow our crops in. And the sad thing is that it can all be lost over night. And so rocks remember are made of minerals which are recycled on our planet using the rock cycle. They can undergo weathering both physical and chemical. And that produces the particles in the soil. It makes up about half of the soil. We also have the biosphere contributing life. We have the atmosphere and the hydrosphere.

And also a lot of time. And so all of these contribute to soil. We are going to have lots of different types of soil on our planet. One way to look at classified soil is to look at the different horizons or the layers in the soil. We could also look at the particle size in the soil. Going from large to small it goes from sand to silt to clay. Now that contributes to the porosity of the soil. How easy is it for the water to get down and bring water and nutrients to the roots of the plant? And then we also have the chemistry of the soil. Where did the parent rock come from? And one of the big things that is important in the chemistry is the CEC or the cation exchange capacity. How easily does that soil deliver important ions to the roots themselves. Conservation is incredibly important with soils. We have soil erosion where we are physically removing the soil. And then we also have salinization or salting of the soil which is contributing to soil loss. So on our planet remember we have a rock cycle where we can move from igneous rock, which are crystallized magma to sedimentary rock. So we have this weathering, erosion moves it and then we have this compaction that forms this sedimentary rocks like sandstone. And then we can have metamorphism where we are actually putting heat and pressure on that rock to convert it into a metamorphic rock. But it is weathering that contributes to our soils. First type of weathering is going to be physical weathering. This is an example of a rock that has been weathered physically. You can see it is just broken apart. So that is water. It could be ice wedging.

It could be the roots of a plant. But anything that increases the surface area of the rock, in other words anything that physically breaks it down into smaller bits, that is going to be physical weathering. And that is only half of it. We also have chemical weathering. So on this rock here you can see there is rust or oxidation going on on the outside of that rock. And so that chemical reaction breaks down a rock into particles of the soil as well. So if we look at this granite, the felspar here can react with naturally forming acids and form something called clay. And we will talk about the importance of clay in a little bit. Now we can talk about soil being three phases coming together. So we have solid, liquid and gas. If we look at the solid phase we are going to have the minerals of the soil itself. So that was the particles that came from the parent rock. We also have the organic materials. So the living the material. So roots would be an example or dead material.

We also have the hydrosphere so the water coming together. And then we are going to have the air. And so this is the breakdown of how much contributes to the soil. It is going to differ on what soil we have. But if you thing about it what is soil? It is the coming together of the lithosphere or the earth on our planet. It is also the air or the atmosphere and the hydrosphere. And then it is where we are headed next in this course. It is the biosphere, the living material. And so that is why soil is important. It is at this interface between all these different spheres on our planet. And if we look at how it is formed, bedrock is broken down, weathered over time, and we eventually get what are called the horizons of the soil. We could classify some of the major soil horizons.

At the top we are going to have what is called the O horizon. That is going to be the organic horizon. That is going to be where we have a mix of a lot of dead or dying material.

Below that we are going to have the A horizon. That is going to be our topsoil. That is going to be a nice mix of minerals and also all the organics from the horizon above. As we go below that we are going to have the B horizon which is the subsoil. Not a lot of organics found in here. We are still going to have minerals and nutrients that are pushed down from the soil layers above. And then finally we get down to the C horizon. And that is going to be where we have parent rock. Now in certain soil horizons we will also have an E horizon. And so that if eluviation taking place. In other words we have the movement of water down. It is pulling those minerals out. And we are just left with sand and silt, kind of this dry layer. Particle size contributes to what type of a soil we are talking about.

So this is a loam right here on the right side. So if we look at that soil, it is going to have varying sizes. So we could go from very big, like boulder to gravel, but eventually when we get to the level of the soil we have sand. Sand is going to be relatively large in the soil. We then have silt. And then finally we have clay. Clay is going to be particles that are smaller than 2000th of a millimeter. So really, really fine particles. Now what do those particles contribute to? It is the type of the soil and the porosity of that soil. So let’s say we take those three particles and fill up a container with sand, silt and clay. And then we fill it up with water? Well you can imagine what is going to happen. In the container that has sand it is going to drain out. Or we are going to have high porosity.

And that is going to take hours. In the silt it is going to take days. And in the clay it is going to take years. And so having a lot of clay can really stifle the movement of water into the pores where those roots need it. Now we can classify soil based on which of these particles we have. And so this chart takes a second to get used to. So this would be the clay on the left side, from 0 percent to 100 percent. And this would be the silt on the right side and then the sand down below. Remember is particle size sand is biggest. Then silt and then clay. And so you can see on this chart that anything that has 50 percent or higher clay, we just call that soil clay. And we are not going to have good drainage. And this is not going to be a great place to grow crops. What is the perfect soil? Well if we have about 20% clay and we have about 40% of sand and silt we have what is called a loam. And that is going to be a nice balance of all of those particle sizes.

Because clay is important. If we look at the cation exchange capacity, what is that? That is the ability of a soil to deliver important ions, important nutrients to the root itself. And the more clay we have, so this is going to be at the microscopic level, and the more organics we have, they are going to be attracting those cations and they are going to deliver it to the root itself. Another important property of soil that goes along with the CEC is going to be the base saturation. So how can these minerals buffer the acidity of the soil as it comes in as well? Because that acidity can damage the plants. Soil conservation you can see is incredibly important. It takes hundreds of years to form these soils. We can turn that around over night. So this is going to be soil erosion, where we are rinsing that top soil off. And so as a farmer you would want to mediate that. We can also have soil compaction. That is when if the soil is wet and we are driving on it with heavy machinery what we can is we can compact those pores. And so that is going to destroy that soil as well. And then we can have salinization, of increase is salt. So if we have plants growing on the soil then they are going to draw the water out. It is going to leave these natural salts behind. Normally not a problem because we are going to have rain water. Rain water is fresh water. And as we have that rain water it pushes those salts out. And so it is not going to damage the crops. Now what is the problem? If we start to irrigate.

So now we are going to use irrigation. And we are going to spray water on the crops. Where is that water coming from? It is not coming from the sky. It is coming from the soil itself. We are probably pumping it up. And so that means that those little droplets in the irrigation are going to have salts inside it. And so as that lands on the field we are going to increase the salt levels to the point where we cannot grow crops there anymore. And so how can we solve this problem? We could flush it out with freshwater. We could change the type of crops we have. Maybe get more salt tolerant crops. Or we could use crops that have bigger roots so that they can push that salt farther down. But you can see what I am getting at. It is this idea that soil is a non-renewable resource. It takes a long time to form and it is really hard to balance the proper chemistry and particle size in the soil. And this map shows us on areas on our planet where soil loss is vulnerable.

And so it is important that we conserve our soil. So did you learn all of this about soils? Can you pause the video here and fill in the empty boxes? Let me show you what goes there. So the rocks and minerals are weathered, both physically and chemically to produce soil. Along with the bio, atmosphere and hydrosphere, this takes a lot of time. Types of soils could be characterized by the horizons or the levels. So we have at the top O, A, sometimes E and then B and C. We have particle size, sand, silt and clay is going to be the smallest.

And that leads to the porosity of the soil. And then the chemistry is important. CEC or the cation exchange capacity is incredibly important. And I hope that was helpful.

Soil Structure

This refers to the arrangement of soil particles to form units or aggregates. These units have different sizes and shapes - platy, blocky, crumb, prismatic, columnar and granular. Soil structure improves when the land is ploughed.

Soil Structure Video

Video Transcript

All right! So this presentation is going to be on soil structure and sizing up soils. Okay. So I know a couple weeks ago we already went really hard into this conversation but what is soil all about?

We have soil that has different textures and each type of soil you touch is gonna feel different because they have different textures and you guys already know this so it's gonna be a review but the soil textures is the percentage of what we have. Sand, silt and clay particles and that will give you what's low texture that you have.

Soils have many different types of structure. Peds are the clumps of soil that form building blocks of the structure. In between peds there are pores which are the spaces between the peds. Pores are very important for moving water through the soil and having space for roots to grow through the soil.

So now, we're going to look at the different kinds of soil structure that you guys need to be able to identify.

So the first soil structure on the list is granular soil structure. It almost looks like Parmesan cheese. It has those different soil aggregates that are very crummy and cheese-like. Granular soil structure is the best performing. This type of soil structure has a lot of pores and it is found in very great and gardening. Why do you think that this kind of soil has a lot of pores? Because, look at the photo in front of us. Look at all the pores between our different particles and our different aggregates that we have. This is why it has so much air flow and pores that go through because of this sort of structure. Due to this ,this is the most desirable soil to have on the soil surface and a lot of times this also has the appearance of almost like a cookie crumb and you can think of a cookie crumb or that Parmesan cheese again and is the best kind of soil structure that we want to have because it has the group the most amount of aeration and water retention through this because of the pore spaces.

Now we have blocky soil structure. Now these shapes are cube -like and they either have very sharp or smooth edges. They're commonly found in the subsoil and you can remember this because it looks a lot like Italian sausage and here in that cartoon illustration we don't have a lot of air spaces here compared to the granular. We do have some but it's not as much because now we have a lot more. That's what I get is a lot thicker than the cranium. The granular was very small and that's why we had so many pores here.

The aggregates are a lot bigger and then stacked on top of each other now we have columnar or prismatic structure and this looks a lot like breadsticks. Okay! These peds are very tall and vertical shaped and they have less pores than that blocky structure and of course they have less pores than the granular and why do they have less force? Is it because of how long it is? How those vertical heads are in a very columnar we don't have those in blocks we had different blocks that had a shorter length and here we have a lot longer length so that decreases the amount of pore space we have and with a decrease in the amount of pore space we have there's a decrease in the aeration and the water that's going through here as well.

Next we have platy structure and this looks a lot like a stack of pepperoni. Oh I'm vegetarian, so that's disgusting but it does look like a stack of pepperoni. These are flat, horizontal plates and the water can move slower through these plates in comparison to the blocks, especially if there were a lot of clay. So here the length is very short, the width is a lot longer. So now, we have even more of a decrease in those pores so even more of a decrease in water penetration so that drainage is going to be a lot lower as well and now we have massive structure and this looks like uncooked pizza dough and here we don't really have a lot going on we really have no structure whatsoever. We don't have those breakups that we had in the rest of the different types of soil structure. It's just kind of one piece which if it's one piece where the pores at and that kind of answers your question for the drainage as well the drain is it's really gonna be very low, slow draining because there are no pores here. There is no kind of structure so that is called massive. This kind of soil is not very useful for farmland whatsoever and this massive structure occurs if a clay soil is repeatedly squished and cramped together due to compaction until they're very little or no pores. The water cannot move through this layer because there are zero pores and it usually means soil is in really bad shape. This can sometimes be found in construction sites because of the severe compaction. Now we have the single grain structure and it's like a pile of salt. This soil has no peds and it looks like a sandbox or piles of salt. All particles usually stay separated and this can be found at a local beach or sand dunes so there's not a lot of aggregates here. The PEDS are non-existent. So here we definitely have a lot of pore space but my question is how is it going to retain a lot of water and that's going to be lacking here. So this is pretty much saying, you know, how much humans have impacted on the soils and how much damage this will cause, especially the soil structure. Yes, it is very difficult to change a soil texture but if you think that's difficult it's way more difficult to change the soil structure after it's been destroyed by compaction and we're gonna have a whole presentation on compaction because it is very serious and everyone needs to know about it but yes, so soil structure once it is destroyed you cannot bring it back to its original soil structure. It is very difficult. I will not say you cannot and it's impossible but it is very difficult. I love this: good structure equals healthy soils. Yes! That is perfect and that is definitely a great quote to live by and then another one is "keep it covered!" What do you think is meant by "keep it covered"? A big portion of keep it covered is ground covers and cover crops. The benefits that is provided here is erosion control. There are a lot more roots to hold the soil down so now rain will not strike the surface of the soil and carrying it with its loose soil particles all over the place. It also gives extra plant roots. This not only provides roots to hold soil in place when they die but plant roots contribute to the creation of organic matter and this organic matter has enzymes that function as a glue to hold soil structure together. I mean we saw all of the different peds which are created by these glues that keep all of those together and adding organic matter that's going to be our number one key to keeping that structure the way we want and keeping a good structure that means we have healthy soil so I'm gonna ask you a question: why would you not keep them covered? Why do people not keep them covered? Well there's two parts to that. In some parts of the country it causes the soil to be too wet in the spring especially under clay soils and to and in some parts of the country it's too cold and so it takes a long time to warm up to get a decent crop in. So we have those two things that we have to kind of play to but the main point that we want to get across is we want that organic matter in there to provide you know us with some good structure and that good structure leads to healthy soils. Next, I wonder what happened here but the main point here is texture and structure work together to form a behavior. We look at soil and we can decide where the best places are to build houses and avoid building houses or roads in areas that are likely to slide like we have and the unfortunate picture that we see here.

Soil pH

This refers to the acidity or alkalinity of the soil or the concentration of hydrogen ions (H+). This alkalinity can be represented on a scale ranging from 0 to 14.

Importance of Soil pH

Soil pH affects nutrient availability, soil organism, soil organic matter and therefore affects soil fertility.

Acid makes the soil sour and this adversely affects plant growth. If there is too much alkaline in the soil, plants will not grow either.

Soil that is too acidic can be corrected if limestone is added to it. Soils that are too alkaline can be treated with organic matter to improve their acidity.

However, additives should be used with caution as well as knowledge of their total effect on plants. Some crops prefer slightly alkaline or slightly acidic soils; most crops prefer a pH of 6 to 7.5.

Video Transcript

Topic: pH value.

Why is pH value important to a farmer?

What is happening to the tree?

Is it merely because of pH?

Yes, you are right.

pH is the measure of acidity or basicity of a substance.

It is expressed by positive numbers ranging from 0 to 14 where 7 indicates a neutral value.

Neither acidic nor basic.

When the pH value of a substance is below 7.

It is termed as acidic and when it is above 7, it is termed as basic.

The pH value of ideal soil is usually close to neutral.

It ranges from about 6.5 to 7.5.

If we don't maintain this pH, growth of most of the plants is adversely affected.

Are you shocked?

Did the white marble just turn yellowish brown due to acid rain?

Yes, you are right.

The pH of rainwater is about 5.6.

This means that rainwater is slightly acidic.

This slightly acidic nature of rainwater does not really cause any damage.

But, in last few years, many harmful substances are being released into the atmosphere.

Addition of these substances drops the pH of rainwater.

In certain cities, the proportion of these harmful substances is so high in the atmosphere.

That it drops the pH of rainwater to a much lower level making it strongly acidic.

When this strongly acidic rainwater falls down, it is termed as acid rain.

This acid rain not only discolors white marbles, but also harms human beings and animals in several ways.

Did the plant just turn yellow?

Why do you think this happened?

The plant turned yellow due to increase in pH value of the soil.

A pigment named chlorophyll gives a plant its green color.

The plant prepares chlorophyll with the help of iron.

When the plant absorbs water from soil.

It also takes in iron present in that soil and prepares chlorophyll.

However, when the pH of soil increases, that is, when it becomes more basic, the availability of iron decreases.

As a result, the plant does not get the required amount of iron to make chlorophyll and hence, it turns yellow.

Also, besides the availability of iron.

pH value of soil also affects the level of toxic substances, growth of bacteria and roots, etc.

Thus, when a farmer has to grow any crop, he has to know the value of pH of his soil for a good yield.

Otherwise, his crop will face undesirable consequences.

Ultimately, he may suffer a great loss.

Quiz

Complete the following quiz in your portfolio.

1. Draw a pie chart showing the percentage composition of the soil. (8)

Draw each of the following soil structures:

2. blocky (1)

3. platy (1)

4. granular (1)

5. Draw and label a soil profile. Include all possible soil horizons. (7)

6. Which of the following is NOT a type of soil water?

a. Hygroscopic water

b. Microscopic Water

c. Capillary water

d. gravitational water

7. Which of the following water is available to plants?

a. gravitational water

b. capillary water

c. microscopic water

d. all of the above

8. Water that flows freely under the flow of gravity is called

a. Hygroscopic water

b. Microscopic Water

c. Capillary water

d. gravitational water

9. Water held in the pore spaces of the soil due to surface tension is called

a. Hygroscopic water

b. Microscopic Water

c. Capillary water

d. gravitational water

10. Water held in a thin film around soil particles is referred to as

a. Hygroscopic water

b. Microscopic Water

c. Capillary water

d. gravitational water

11. Wilting point occurs when this type of water is available in the soil...

a. Hygroscopic water

b. Microscopic Water

c. Capillary water

d. gravitational water

12. Field capcaity occurs when this type of water is available in the soil

a. Hygroscopic water

b. Microscopic Water

c. Capillary water

d. gravitational water

13. Soil aeration is necessary for all of the following except

a. respiration of plant roots

b. respiration of organism

c. transpiration of water through plants

d. germination of seeds

14. Which of the following do NOT affect soil aeration?

a. temperature change

b. number of organisms in soil

c. cultivation practices

d. soil structure

15. Match the correct mineral fraction with its correct size...

a.gravel less than 0.02mm

b. clay between 0.2 to 0.02 mm

c. sand between 0.2 mm to 2 mm

d. silt greater than 2 mm

16. Which of the following is NOT a function of organic matter

a. it changes soil texture.

b. it improves soil structure.

c. it supplies nutrients.

d. it increases water retention in soils.

17. Which of the following refers to the O Horizon?

a. a layer of partially decayed organic matter

b. a layer known as surface soil

c. a compact layer of soil where leached nutrients accumulate

d. the bedrock layer

18. Which of the following refers to the A Horizon?

a. a layer of partially decayed organic matter

b. a layer known as surface soil

c. a compact layer of soil where leached nutrients accumulate

d. the bedrock layer

19. 17. Which of the following refers to the B Horizon?

a. a layer of partially decayed organic matter

b. a layer known as surface soil

c. a compact layer of soil where leached nutrients accumulate

d. the bedrock layer

20. Which of the following refers to the R Horizon?

a. a layer of partially decayed organic matter

b. a layer known as surface soil

c. a compact layer of soil where leached nutrients accumulate

d. the bedrock layer

True or False

Write T for true and F for false.

21.Soil compaction can change soil composition.

22.Soil aeration is the replacement of stagnant soil air with fresh air.

23. Soil aeration is driven by the movement of water through the soil, by mass flow of air and by diffusion.

24. Soil aeration limits the groth or new roots.

25. Organic matter refers to the sustance formed due to the decomposition of plant and animal waste.

26. Soil pH refers to the concentration of Hydrogen ions.

27. The pH of a soil is measured on a scale from 0-15.

28. pH 0-6 is referred to as acidic.

29. pH 8-14 is referred to as basic

30. pH affects the availability of nutrients in the soil.

31. pH can be decreased by adding soil organic matter and increased by adding limestone.

Total: 45 marks

Discussion

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