Hey everybody, it's Mr. Smeeds and today we'll be talking about topic 4.3, which is soil composition and properties. So in video 4.2, we focused on what soil is. Here we'll focus on the different properties that give soil its fertility or its ability to support plant growth.
So let's take a look at our objectives here. Our objective today is to be able to describe the similarities and differences between properties of different soil types. So the knowledge we need in order to do that is that all soils are made up of sand, silt, and clay. and the different sizes of these particles determine how the soil holds water and nutrients. We'll learn how to classify different soils based on their texture or the percentage of sand, silt, and clay.
We also need to understand the different factors that affect water holding capacity of a soil. And finally, we'll learn about some of the basic tests that can be done to measure the fertility of a soil or how well it's able to support plant growth. Our suggested science skill for the day is to be able to describe an aspect of a research method or an investigation.
So in video 4.2, we talked about how soil is made up of lots of different things. Rock fragments like sand, silt, and clay, humus, organic matter, and even water or air. So here today we'll be focusing on the geological or the rock-based portions of the soil.
So when rock gets weathered, it gets broken down into tiny little particles, and we call those particles sand, silt, and clay. Now these particles have different sizes, and their size determines a lot of the characteristics of a soil. So when we talk about a soil's texture or what it feels like, what we're really talking about is the percentage of sand, silt, and clay. Because individual sand particles are so big, the space between them or the pores are also really big.
So we can see that if we look over at this diagram here of the sand, we can see how large these individual sand particles are, which means that the space in between them or the pores is also going to be very large. Clay, on the other hand, is a very small particle that packs together very densely. And so the pores or the spaces in between clay are very very small when we compare them to those large pores within sand. The amount of pore space in a soil is referred to as its porosity.
So the more sand in a soil the more porous it is. This makes it easier for air and water to enter the soil, which is vital for plants since the roots need to take in both water and oxygen in order to survive. So now we'll talk about something called a soil texture chart and how to use it. So all soils are made up of sand, silt, and clay, but pretty much no soil is just pure sand, silt, or clay. All soil contains some mixture of the three.
So because the percentage of sand, silt, and clay determines the texture and the porosity, it's the main way that we classify different types of soil. So if we wanted to calculate the texture of a soil sample, we could get a sample of the soil and then set it in a jar of water and let it settle out overnight. Because the different particles...
have different densities, what we'll be left with in the morning is a layer of sand, silt, and clay that we can then measure to determine the percentage of each. So if we look at this example we have here in the mason jar on the screen, this is going to be 45 percent sand, 35 percent silt, and 20 percent clay. So let's take a look on our soil texture chart and we would see that that lines up here in the loam category and that's because we have 45% sand and if we follow this out here 35% silt and then if we were to follow this over we'd have 20% clay.
So this soil would be categorized as a loam soil. Now I made that look pretty easy but here are some tips for how you can use a soil texture chart to determine the classification of a soil. So the first tip is that we will always start on the bottom with the percentage of sand.
Then what we want to do is move out to a point where the sand and the silt meet. So in this example, we'd move from 40% sand to the point where it meets with silt, and that's 40% silt. And then we're going to go straight over to determine the percentage of clay, and that's 20% clay.
You don't want to go from 40% sand and then straight over to silt. Because 40 plus 80 is 120. We can't have 120% of anything in a soil. So that's the key there.
And then that last rule is make sure that it adds up to 100. If it doesn't add up to 100, you didn't do it right. If it adds up to 100, then you probably did. So I want you to practice now by trying to find the percentage of sand, silt, and clay of this blue circle on the screen here. All right, let's see how you did.
So starting at the sand. we can see that this is 30% sand. And remember, we want to go out to the percentage of silt on a diagonal, which would be 20%.
And then we want to go straight over to find the percentage of clay. And that's, oops, that's 50%. So there we have 30, 20, 50. 30% sand, 20% silt, 50% clay. Now we'll take a look at how porosity relates to water movement through soil in a little more depth. So remember from our first slide in this video that porosity is the pore space between soil particles.
So the more porous a soil is, the more easily water enters it. Well if a soil is more porous, then it's also more permeable. Permeability just refers to water's ability to drain through a soil. So that means that water can drain through really easily if there's large porcines or high porosity.
On the other hand, if a soil is more permeable, that means it doesn't hold water as well. So we would say it has a lower water holding capacity. So porosity and permeability have an inverse relationship with holding capacity, meaning the more porous or sandier soil is, the less water it can hold on to. So what does this mean for plants?
Well, plants need a balance between permeability and water holding capacity. They need some degree of porosity so that oxygen and water can enter the soil and get to their roots. But if the soil is too sandy or too porous, then it has really high permeability and water just drains straight through it before the roots have a chance to access it.
If the soil is too clay dominated though, then the soil doesn't really drain water at all and the roots either won't get access because the water can't even penetrate the soil Or the water will penetrate the soil, but then it will stay there, and that will actually suffocate the plant's roots because they won't get access to the oxygen that they need. So the ideal state for plants is a nice medium balance between the permeability that comes with sand and the water holding capacity that comes with clay. And that's loam, which is roughly a 40-40-20 mix of sand, silt, and clay.
This is really an ideal soil for many plants because it allows water and oxygen to enter the soil and drain through at a moderate pace. that prevents water logging but doesn't dry the soil out too quickly. So if we look at this diagram here we can see that the sandy loam, which is again a nice mixture of sand and clay with some silt as well, is going to allow water to move in, it's going to allow the water to be held, moderate water holding capacity, but it's also going to allow it to drain through. If we're looking at a more clay dominated soil the drainage is very slow and it becomes waterlogged which can suffocate plant roots. And if we have a sand dominated soil, the drainage is too rapid so the porosity is too high and the holding capacity is too low.
So again a sandy loam is a nice happy medium that really allows plants the proper drainage but also the proper access to water. Now we'll look at some soil properties that affect soil fertility or the ability of soil to support plant growth. Since plants need nutrients and water from the soil we'll look at the factors that affect each of these.
So starting out with nutrients. Some of the most important plant nutrients that are found in the soil are nitrogen, phosphorus, magnesium, calcium, and potassium. Factors that increase soil nutrients include more organic matter, which gets broken down by decomposers and turned into humus, which releases these nutrients for plants. Humus also improves the ability of soil to hold water though, so it's an especially critical component of soil.
Then we have clay, which is negatively charged. Because many soil nutrients are positively charged, they bind to clay particles, which helps them remain in the soil and available for the plants. And finally, there are bases like calcium carbonate, which is given off by limestone.
And this helps buffer or neutralize the pH of soil so that it doesn't become too acidic. One of the main factors that decreases soil nutrients is acidity. So soil that is too acidic leaches nutrients or loses them very easily. when rain or other water washes through the soil. Excessive farming of the same crops from year to year can also deplete the soil because the plants use up the nutrients and then if no organic matter is returned to the soil and if the topsoil is eroded heavily from repeated tilling or compaction by farm equipment then nutrients will be lost from this soil at an even faster rate.
Now we'll look at water. So soil that is well aerated meaning it has a lot of channels through it that come with organisms like earthworms burying through it. That will allow the soil to hold more water and more air. The addition of organic matter or compost or humus can also act like a moisturizing layer that prevents evaporation loss from the soil.
So that will help it retain more water. And then finally, we have root structure in soil. So especially if there are native plants that have deep roots, this will really help retain moisture in the soil and prevent it from drying out. Factors that dry out soil include the compaction of soil since there are fewer pores to hold moisture. as well as topsoil erosion and the loss of root structure.
And the last thing we'll discuss today is soil quality tests. So we'll be doing many of these in class, but I want you to have an idea of what they are before we get to that. So soil texture, remember, is just a measure of the percentage of sand, silt, and clay that can be found just by letting a soil sample settle out in a jar of water overnight and then measuring the depths of the different layers of sand, silt, and clay.
This can give you an idea of what kind of soil you have so you can classify it as a loam or a sandy loam and it can also tell you how porous or permeable the soil may be. However, if you want to measure the permeability directly That's a relatively easy thing to do by just setting up a plastic column, filling it with your soil sample, and then measuring the amount of time it takes for a given volume of water to drain through your sample. Now you'd want to do this relative to a control. This can indicate if the soil is too permeable or if it retains water for a more optimal amount of time, which will allow plants roots to access it.
Then we have pH, which is the most basic chemical test you can do on a soil. So you can just mix up a soil sample with some water, dip in a pH test strip, and this will give you an idea of the measure of the H plus or hydrogen ion concentrations in the sample. So the more acidic a soil is, generally the less optimal it is for plant growth since acidic soils leach nutrients away very easily.
Then we have color, which is a really simple test. You can just take a sample of soil and you can compare that to a color chart that you can find in something called a soil book. This will give you an idea of how much humus or decomposed organic matter there is in the soil. Remember that the darker the soil, the more humus, and generally the better that that soil will hold nutrients and water that's needed for plant growth.
And then finally, we can measure nutrient levels directly with various chemical tests that will measure nitrogen sources like ammonium or nitrates and phosphorus sources like phosphates. Since these are key limiting nutrients for plant growth, The more of these nutrients, generally the better a soil can support plant growth. Alright, and our practice FRQ today for topic 4.3 is to describe an aspect of a research method or an experiment. So what I want you to do is try to identify and describe one test that can be conducted on a soil sample and then explain how the results of that test could allow you to give advice to a farmer who's trying to grow crops in that soil. Alright everybody, thanks for tuning in today.
Don't forget to like this video if it was helpful, subscribe for future apes video updates, and check out other notes over here to the side. And as always, think like a mountain, write like a scholar.