A Beginner's Guide to Backyard Soil
In An Introduction to Backyard Soil, I introduced a very basic understanding of what soil is, how it forms, and how it is organized. In this generalization, I discussed how soil can be understood as clumps and voids, how it forms as a result of weathering, and how it forms horizons. I also discussed some of the reasons why something so basic as soil is commonly misunderstood, due both to its complexity and the fact that it is studied from so many perspectives.
This post focuses on the clumps, the physical properties of soil. I am assuming that readers have read the introduction, or are already familiar with the complexities involved in understanding soil properties and are looking for a digestible explanation.
Recall that according to science, soil is:
“The surface mineral and/or organic layer of the earth that has experienced some degree of physical, biological and chemical weathering.”
Summarily, this is a medium composed of solid particles composed of mineral and organic contents, and pores filled of air and water. The mineral component is what informs soil texture.
Texture
Texture is the most basic way we discuss soil, specifically its range between a coarse texture, called sand, and a fine texture, called clay. In between is an intermediary texture known as **silt**. These categories are determined purely based on particle size and nothing else.
Sand is any mineral particle between 2mm and 1/16mm, small.
Silt is any mineral particle between 1/16 and 1/256mm, really small.
Clay is any mineral particle less than 1/256mm, stupid small.
Sandy soils are coarse-textured, have good drainage and plenty of oxygen but retain little water. Clay soils are the finest-textured and have plenty of water, but drainage and aeration are poor. Silt has a texture akin to dust or flour, and will become suspended in water, rather than water passing through it. The properties of a mineral will generally inform what kind of particle it forms, but this is less important for a basic understanding.
In common garden parlance, loam is a happy mixture of the three, typically described as a proportion of 40-40-20/sand-silt-clay, that is ideal for plant growth. Percentages of these particle types together form various intuitive descriptions of soil types with regard to texture. Soil scientists have created a useful triangle by isolating each particle type.
A compass developed by the USDA and partnered soil scientists. Follow the indices along the edges to determine soil texture.
Using the graphic above, we can see that loam, the most desirable soil type when discussing texture, falls halfway between silt and sand, but with a lower concentration of clay.
Infiltration
Soil's ability to accept and retain water is important for both plant growth and a household irrigation budget. If the soil is too dense, excess water will sheet off and into the adjacent watershed. If it is too coarse, water will drain through without being adequately captured. Plants use water to maintain their tissue structure, so they require soil with adequate retention properties. But they also need the soil to drain properly drainage so it can replenish its gases between waterings and the roots don't rot.
Sandy soils have the highest infiltration rate, almost 1" per hour. By contrast, the infiltration rate for clay soils is 4 to 20 times as long. If curious, you can a very formal soil infiltration test, to determine what you have in your yard. Or you can go outside and pour a cup of water onto some bare earth and note how quickly it seems to disappear.
Determining Texture - The Pedon
When a soil scientist describes a soil type's texture, they generally refer to the "pedon." This is a fancy word for clump.
A pedon, or clump, or handful of soil.
The pedon is the basic unit for describing soil. Examining backyard soil is as simple grabbing a handful of it and getting a sense for how it feels, course or fine, wet or dry, and noting its color. You can also look a little bit closer for bits of dead plant material. The ideal texture is slightly gritty (silt) with noticeable roughness (sand), but still feels moist and will form a ribbon when squeezed between the thumb and finger (clay).
Amending Soil
While loam is the ideal medium, it rarely occurs as a result of natural processes inside a residential parcel. Most likely your soil will need to be amended in order to find the appropriate texture and infiltration properties to your site. Before amending, I recommend considering whether you’d prefer to just adapt your plant selection to the existing soil, as this often saves money and effort over time at the expense of conventional aesthetic sensibilities. This is a useful approach for areas of your yard that need to tolerate high traffic from pets and kids, or will not receive much attention. For areas with more specific requirements, such as a vegetable garden, it is necessary to modify the soil so it can meet the basic needs of the plant.
Soil texture will determine the type of amendment you choose, so it is important to have at least a good guess. Most garden centers will be able to recommend something based on texture alone. However, you don't need to purchase a packaged product. There are lots of materials that are freely available that will improve soil texture, such as sawdust, animal manure, home compost, or fallen leaves. Selecting free and sustainable resources are preferable to distributed merchandise (and cheaper!), but it does require clarity about what you are trying to achieve and matching the material to the goal.
Conclusion
There's a common sense notion that improving soil means improving quality of life for plants. This is mostly true. Soil stores the nutrients plants need to promote tissue growth and flowering, and dictates the amount of air and water available to them. But it's important not to get carried away in discussing "good" soil from the fact that some plants adapt to poor soil, or even no soil at all. Simply getting "good" soil won't guarantee a positive outcome, nor will it prevent an undesired one. Plants will grow where they will.
Plants require very little when it comes to soil
