Soil and Nutrients: Videos & Practice Problems

Soil is a dynamic mixture of inorganic minerals, organic matter, and living organisms, essential for plant growth. Plants require 17 essential nutrients, categorized as macronutrients (like nitrogen, phosphorus, potassium) and micronutrients. Nutrient absorption occurs mainly through root hairs, utilizing cation exchange and selective permeability of the plasma membrane. Soil texture affects water retention and nutrient availability, while soil horizons indicate different layers. Understanding these concepts is crucial for effective agriculture and ecosystem management, emphasizing the importance of soil health in sustaining plant life.

Soil Nutrients

Hi. In this video, we're going to be talking about soil and the important nutrients found in it. Plants, as you know, produce their own food. They make sugars from photosynthesis, but they still have many nutritional requirements outside of this. So even though 95% of a plant's dry weight comes from carbon, hydrogen, and oxygen, which they can obtain from CO₂ and water, which of course are essential components to photosynthesis, most plants still need a bunch of stuff outside of this. In fact, vascular plants require 17 essential nutrients in order to live. Some nutrients are needed in greater quantities than others. Macronutrients are those that are needed in large quantities, and these include nitrogen, phosphorus, and potassium. In fact, these nutrients are so important we often call them limiting nutrients, because the availability of these limits the plant's ability to grow. And, you know, if you think about what these nutrients are used for, you know, they go into things like nucleic acids, proteins, phospholipids, you know, essential stuff that any cell needs to exist. Even though micronutrients are needed in smaller quantities, they're still just as essential to plants' life. Micronutrients include all of these elements you see here, and generally they're only found in trace amounts, very very small amounts. In fact, as the old saying goes, dosage makes the poison. These nutrients can be potentially toxic to plants in high concentrations. That is to say, if plants get too many of these nutrients, it can actually be very harmful for them. Some nutrients are considered mobile, in that they can be transported around the plant, while others are called immobile nutrients because they're kind of stuck where they are. So often when there are nutrient deficiencies for a plant, you'll see the old leaves die off, and they do this in order to sustain the young leaves. They're transporting their nutrients to the young leaves and dying off in the process, but this allows the young leaves to continue living in the hope that, you know, maybe they'll be able to get the nutrients they need. Now, young leaves tend to be the first to show nutrient deficiencies. That is, they're the most sensitive to nutrient deficiencies. And here in this image, if you're curious, you can see all of the different symptoms for the different types of nutrient deficiencies you might see in a plant, those that will become apparent in the old growth, and those that will become apparent in the new growth. And of course, over here, it's a nice little diagram of a plant, all the nutrients that are essential to it, and of course, the CO₂, H₂O, and sunlight that are part of photosynthesis. With that, let's turn the page.

Soil Composition

Forget everything you see on this page, guys. Soil is just dirt. It's just dirt. Soil is just dirt, it doesn't matter. That's actually not true. Soil is a highly dynamic composition of inorganic minerals, organic matter, trapped gases, liquids, mostly water, and many living organisms. Soil begins when weathered rock breaks up into gravel, sand, silt, and clay. This composes the base of soil. Soil is then enriched with organic matter. The good stuff is what we call humus. This is decaying organic matter, usually comes from dead cells and feces that organisms add to the soil, and this adds tons of nutrients.

Now, the texture of soil, that is the proportion of components, like gravel, sand, and clay, has a huge effect on plants. It affects the ability of plant roots to penetrate and absorb nutrients. It also affects the ability of soil to hold water and oxygen, and oxygen, and you might not realize what that oxygen is important for. Well, guess what? That oxygen is going to be the final electron acceptor in the electron transport chain of roots. So super important. Right? Obviously, it's key to _{cellular respiration.} Very fundamental process.

Now loam is a special type of soil. This is like, this is the good stuff. This is like the Dom Perignon of soil. Right? This has equal proportions of sand, silt, and clay with lots of humus. This is like this is la crème de la crème. This is that good good soil.

Now, when we talk about soil, what we're usually thinking of is topsoil because, you know, generally speaking, I don't know how much you dig around in the dirt, but I'm not going that deep. Topsoil is that outermost layer, and it has the highest concentration of humus and microorganisms. It's usually why it's a lot darker than the layers below it. It's composed of tons of different organisms, including bacteria, archaea, fungi, algae, nematodes, protists, insects, and worms. And those worms, those guys are super important to soil; they move the soils around, they cycle nutrients, and they break it up to make it better at retaining gases and water. Now those other organisms also do a ton to help maintain and enrich soil. In fact, we're going to really focus in on what bacteria, archaea, and fungi do in a later lesson when we talk about nitrogen fixation.

So anyways, we talked about topsoil. There are other layers to soil. We call these soil layers soil horizons. Kind of a funny name, but when you actually see a picture of it, it makes sense. Right? Have you ever seen a like a sunset off in the horizon, it's got layers of color? Right? Well, here we've got layers of color in the dirt. And this is, you know, this right here is an actual image. You can see that it's about 3 feet down from the surface. That's, you know, an actual picture. Here we have a diagram of some soil horizons that's going to go deeper than what we see here. This is really ending in the subsoil, but as you can see, the soil horizons go deeper and eventually hit what's known as bedrock. That's like rock bottom. Over here, you can see different types of soil compositions, of soil textures. This nifty little chart put out by the federal government actually, kind of cool little chart. Don't worry about memorizing any of the information there or even really, you know, trying to read too much into it. It's just to illustrate that there's a wide range of soil textures, and this stuff matters. Right? Our department of agriculture, that's who made this chart, you know, they care about this stuff because, well, if they don't get it right, you know, we don't eat.

So soil pH actually varies greatly, depending on where you are, and this can have an impact on nutrient absorption. Acidic soils, like you'll find in conifer forests, like forests with lots of pine trees, usually come from lots of decaying organic matter because this decaying organic matter produces organic acids. Now, alkaline soil, on the other hand, tends to be from limestone or calcium carbonate. This limestone, when it breaks down into the soil, will form bicarbonate, which is a weak base. And, just to be clear, acidic soil, we're talking about low pH, alkaline soil, high pH just to be crystal clear. Now, soil erosion is when wind and water carry soil away from a place. Roots actually help prevent soil erosion; they help lock the soil in there by, sort of, creating a matrix to hold it in place. Roots also tend to excrete acids, which in general is going to lower soil pH, and you'll see soon why this can be really important. So with that, let's flip the page.

Cation Exchange

Plants extract nutrients from the soil as ions. Most of this nutrient absorption is going to occur in the zone of maturation, which is behind the root tip, and where you'll find all those root hairs. The reason for this is that root hairs significantly increase the surface area available for water and nutrient absorption. In fact, a single stalk of a rye plant, which is kind of just like a wheat plant, very similar, can have a root system with the surface area equal to that of a basketball court. Now, that's pretty incredible, and it just goes to show how diffuse the network of roots and root hairs is, that it can create that much surface area, yet not take up that much volume.

Now, ions, you might recall, come in 2 flavors. We have our negatively charged anions and our positively charged cations. Anions are easier for plants. They're dissolved in water in the soil, and that makes them readily available for absorption. Unfortunately, ions dissolved in water are also easily leached from the soil. Leaching is the loss of nutrients through the movement of water.

Now there's one exception to all of this and that is phosphate, which is an anion. It's PO43-, a very negatively charged anion, but this is not dissolved in water and soil. It actually forms complexes with calcium and iron cations. Don't need to worry about that too much, just wanted to point out that not every single anion is going to be dissolved in water.

Now cations, though they do dissolve in water, in soil usually interact with clay anions or organic acids. And these remember are going to come from humus. These cations, rather, interact with clay and humus, making them harder for plants to extract. And here, in our example, you can see we have a clay particle with all of these cations, interacting with it, because this clay particle has lots of negative charges, but this could also be a particle of humus because it has all those organic acids, which once they deprotonate are, of course, going to have a negative charge, which is why these cations are also going to interact with those organic acids.

Now, plants do have a way of getting those cations from the soil. We call this cation exchange. Right? You don't get something for nothing. You have to give a little to get a little. So, basically, the way it works is soluble cations, like protons, and that's what plants are going to use, though cation exchange can occur with other cations. These soluble cations are going to bind to the negatively charged soil particles and cause cations like magnesium and, calcium, the ions that are nutrients that the plants want, to be released and allow the plants to absorb them. So, basically, what the plants are doing is exchanging one cation for another. Earlier we mentioned that plant roots will secrete protons. Well, that is to help with cation exchange. It should be noted that humus has what's known as a higher cation exchange capacity than does clay. Basically, that means humus will more readily exchange its cations, than will a clay particle.

Now, plants influence cation exchange by releasing CO₂, as well. And you might remember that CO₂ is a byproduct of cellular respiration. So the plant roots carry out cellular respiration, they're going to release their CO₂, and that CO₂ is going to form carbonic acid in water that is found in the soil. So this is going to lead to the release of protons and help facilitate cation exchange. And you can see that happening in this image here, the plant root is going to release that CO₂, which is going to turn into carbonic acid here. As you can see, it will deprotonate, here are those protons, and then those protons are going to trade places on this negatively charged soil particle with a cation. In this case, we have 2 protons, that's 2 plus charges, so we can take one calcium away. It's important to note there that it's not a direct exchange of particles. You don't just trade 1 proton for 1 magnesium. You have to balance charges. So it takes 2 protons, for example, to trade with magnesium.

Now, it should be noted that if soil gets too acidic, the rain can wash away cations. Those can be leached from the soil just like the anions, though it won't happen as readily unless the soil is very acidic. With that, let's turn the page.

Ion Exclusion

Nutrients like ions can easily pass through the cell wall, but the plasma membrane acts like a filter. Remember that term we talked about, selective permeability? The plasma membrane gets to decide what gets in and what doesn't. Plants use proton pumps to create electrochemical gradients, and these gradients allow ions to enter through transporters. These electrical, electrochemical gradients are actually strong enough in some cases to overpower counteracting forces. So through channels. Remember, that's a type of facilitated diffusion. Basically, you just need the channel there, and those ions will move through of their own accord. However, anions, like NO₃ here, have to use cotransporters. And those will often use a proton gradient, and they'll bring a proton into the cell as they bring in the desired anion. And remember that this is a form of secondary active transport. Now, ion exclusion is the idea that plants are able to filter harmful ions and poisonous metals, and prevent them from getting into the cells.

They can do this in two ways, what's called passive exclusion, and active exclusion. Passive exclusion basically doesn't require any sort of extra energy input. Basically, if the membrane lacks the necessary transporter to allow the ion to pass, it's not getting in. And you might also recall that the Casparian strip is going to force ions into those endodermal cells, because it's going to prevent them from moving all the way through the apoplast to the xylem. So it's going to force ions to cross a membrane, which gets to act as a filter. So basically here, these transporters, they're kind of like bouncers. Right? They're bouncers, and they get to decide who gets into club cell and who doesn't.

Now, active exclusion comes in the form of antiporters. In fact, usually we see these antiporters at the tonoplast, which is the membrane of the vacuole. In our diagram here, you can see we have a vacuole, this large purple structure inside the plant cell. And the tonoplast is going to be that membrane of the vacuole there. Now a great example of active exclusion is the sodium proton antiporters, or sodium hydrogen ion antiporters, whatever you want to call them. And these are going to help prevent sodium from poisoning plant cells. Plant cells are actually very sensitive to sodium, they have to carefully monitor their sodium concentrations. And if the sodium gets too high, they'll actually pull it into the vacuole to get it out of the way, to prevent it from poisoning the plant. The way they do this is they actually use proton pumps to create a proton gradient, so that the concentration of protons inside the vacuole is higher than the concentration of protons just in the cell, or in the cytoplasm. This gradient is going to be taken advantage of by these antiporters. These antiporters will move a sodium in, as they get rid of one of those protons. So they're going to take advantage of the proton gradient established by the pumps in order to get sodium into the vacuole. So this is a type of secondary active transport, and it involves antiporters at the tonoplast.

Plants also can help prevent poisoning through what are known as metallothioneins. These are cysteine-rich proteins that will actually bind to metals and prevent them from poisoning the organism. And these are not unique to plants either. You'll see metallothioneins in bacteria and fungi as well. And that's actually all I have for this lesson, so I'll see you guys next time.

Here’s what students ask on this topic:

What are the essential macronutrients for plant growth and why are they important?

Macronutrients essential for plant growth include nitrogen (N), phosphorus (P), and potassium (K). These nutrients are crucial because they play vital roles in various cellular functions. Nitrogen is a key component of amino acids, proteins, and nucleic acids, which are essential for plant growth and development. Phosphorus is involved in energy transfer through ATP, nucleic acids, and phospholipids, which are critical for cell membrane integrity and function. Potassium regulates osmotic balance, enzyme activation, and photosynthesis. These macronutrients are often termed 'limiting nutrients' because their availability directly impacts plant growth and productivity.

How does soil texture affect plant growth?

Soil texture, determined by the proportion of sand, silt, and clay, significantly affects plant growth. It influences the soil's ability to retain water and nutrients, as well as root penetration. Sandy soils drain quickly but may not hold nutrients well, while clay soils retain water and nutrients but may impede root growth due to their density. Loam, a balanced mixture of sand, silt, and clay, is ideal for most plants as it provides good drainage, nutrient retention, and root penetration. The texture also affects soil aeration, which is crucial for root respiration and overall plant health.

What is cation exchange capacity (CEC) and why is it important for soil fertility?

Cation exchange capacity (CEC) is a measure of a soil's ability to hold and exchange cations (positively charged ions). It is crucial for soil fertility because it determines how well soil can retain essential nutrients like calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺). Soils with high CEC can hold more nutrients and release them to plant roots, enhancing nutrient availability and uptake. Humus, a component of organic matter, has a higher CEC than clay, making it particularly effective in nutrient retention and exchange. Understanding CEC helps in managing soil amendments and fertilizers for optimal plant growth.

How do plants absorb nutrients from the soil?

Plants absorb nutrients from the soil primarily through their root hairs, which increase the surface area for absorption. Nutrients are taken up as ions, with anions (negatively charged ions) being more readily available in water, while cations (positively charged ions) often bind to soil particles. Plants use cation exchange to release cations from soil particles by secreting protons (H⁺). This process allows essential nutrients like calcium (Ca²⁺) and magnesium (Mg²⁺) to be absorbed. Additionally, plants utilize selective permeability of the plasma membrane and active transport mechanisms to regulate nutrient uptake and prevent toxicity.

What role do soil organisms play in nutrient cycling?

Soil organisms, including bacteria, fungi, nematodes, and earthworms, play a crucial role in nutrient cycling. They decompose organic matter, releasing nutrients back into the soil in forms that plants can absorb. Bacteria and fungi are particularly important in breaking down complex organic compounds into simpler molecules. Earthworms aerate the soil and mix organic matter, enhancing nutrient availability. These organisms also contribute to the formation of humus, which improves soil structure, water retention, and cation exchange capacity. Understanding the role of soil organisms is essential for maintaining soil health and fertility.