Welcome back. In this section, we'll be taking a closer look at soil composition. Let's see what this soil stuff that we've been talking about is made of. What do you think? What do you think is in soil? Don't say dirt. Is it minerals? Is it poop? Is it alive? Is it dead? Or was it never living in the first place? The answer is all of the above. Soil has an organic fraction. The organic fraction includes a living part. That's your earthworms, arthropods, bacteria, fungi, and so on. It includes decomposing organic matter. That's your carcasses, your excrement, dead plant materials and anything else you might consider dead. And finally, the fully decomposed or really dead part, which is decomposed organic matter, also sometimes called humus. Not to be confused with humus, which is a delicious spread made from garbanzo beans. Soil also has an inorganic or mineral fraction which was never alive in the first place and that inorganic fraction is described based on particle size. Here's a fun little challenge for you. Sand, silt, clay. I'd like you to try and rank these in order of particle size from largest to smallest. Ready, go! Okay, here's the answer. So what have we got. Ha, all right. This is the inorganic or mineral fraction of soil. You can see the relative particle sizes for gravel, sand, silt, and clay, which wouldn't even be visible at this scale, because it's microscopic. Just as an interesting side note, clay particles are flat and like a deck of wet playing cards, those particles can slide past one another, but they're very difficult to pull apart. It is this quality, the plasticity of clay, that makes it particularly good for making pottery and other products. The distribution of these different particle sizes in soil is described using something called the soil texture triangle. Sorry, wrong triangle. It's this one. On the bottom left corner, if your soil is 100% sand, it's sandy. On the bottom right, you have silt. On the top, you have clay and around the edge of the triangle, you can see the different percentages, how much is it silt, how much of it is clay, and how much of it is sand and so forth. If you have a roughly even distribution of sand, silt, and clay, your soil is called loamy. So what? Why does any of this matter? It matters because soil texture has major implications for what farmers are able to grow in that soil. For example, here's a quick quiz. Which of these soil textures do you think is the least permeable? The answer is clay. So when your soil is comprised of many small, flat, clay particles that are aligned close together, this is not going to allow water to readily infiltrate it. Hence, clay is the least permeable soil texture. Here's an interesting case study that relates to that. Baltimore's premiere urban farm, called Real Food Farm, is built on the site of a former reservoir. That reservoir was drained in the 1960s and filled in with soil that had a very high clay content. As a result, and the farmers just had a heck of a time with this, rainwater tends to pool and collect in that high clay soil effectively drowning the crops, if it's not dealt with properly. Now those farmers are really clever and they've been working to address this problem by building a network of drainage ditches and aerating the soil by planting deep rooted crops, like daikon radish, that bust holes and loosen up that soil. They're also bringing in some new soil of their own. There's been a number of interventions that farmers have had to do to deal with this clay, impermeable soil. Let's take a closer look at that organic fraction of soil. Far from lifeless dirt, fertile soil is teeming with organisms, many of which have important roles in agriculture. For example, bacteria living inside the roots of plants such as beans and clover extract nutrients from the atmosphere. Certain species of fungi that grow on the surfaces of roots pictured here also help deliver nutrients to plants. Our friendly earthworms, meanwhile, help decompose decaying leaves, excrete nutrient-rich castings, and loosen soil by burrowing through it. We'll talk more about these important ecosystem services in the next section. The soil ecosystem has been called the soil food web Illustrated here, in large part because everything in soil becomes food for something else. In the words of ecologists and conservationist, Aldo Leopold quote, land then is not merely soil, it is a fountain of energy flowing through a circuit of soils, plants, and animals. Organic matter, shown in the left half of this diagram, typically refers to that decomposing or decomposed material, a humus. And with a few exceptions, farmers generally strive to increase the percentage of organic matter in their soil and they usually do that by adding manure or compost or something of that nature. The reason farmers want more organic matter and living organisms associated with it, is because organic matter is largely responsible for soil's fertility and a range of essential ecosystem services, which as a nice segue, is exactly what we will be covering in the next section. So, we will see you again soon.
