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.