Soils Soils are made up of solid, liquid, and gas components. Solids Minerals which have been eroded from rock. Soil scientists call this the parent material. Some of the mineral components of a soil might have been imported from long distances by way of wind or water. Organic matter- Living things and the products of living things such as dead leaves, dead bodies, urine, feces, dropped fruits, etc. The living components of soils include plant roots, fungi, bacteria, single celled organisms and lots of different kinds of invertebrate animals. Liquid The spaces in between the solids in soils contain some amount of watery solution; it’s never “pure” H2O. The nutrients that a plant needs to take up are present in the water as well. There are also other chemical elements present in the solution that can affect a plant’s health and well being…and yours! Gas Soil contains air spaces that allow plant roots to take in oxygen and release carbon dioxide- roots need to breathe!

Soil Texture- The relative amounts of different sized particles of parent material will determine how closely they will fit together and the size of the air spaces present. Sand- largest particles Silt- medium particles Clay- tiny, charged particles, suspended in solution Soil texture influences how well the plant roots can breathe, how well the soil can hold water, and how well it can hold the nutrients that plants need.

Loam is the term used to describe a soil that has a mix of sand, silt and clay. As shown in the diagram below, there are lots of different textures that soils can have! Here’s an accurate but simple test you can do to find out what kind of soil you’re working with. A real quickie that can be used in the field is the squeeze test.

Sandy soils tend to have very good drainage and so lots of air spaces (good aeration, roots can breathe), but that means that they can’t hold on to water or nutrients very well. Clay soils have very good water and nutrient holding capacity but poor aeration; roots can suffocate in poorly drained soils. Silty soils have texture somewhere in between that of sand and clay soils. A loam that has a mix of sand, silt, and clay is most likely to have good water and nutrient holding capacity and good aeration. The texture of soil can almost always be improved for plant growth by adding organic matter! In fact, the importance of organic matter and biodynamics will be covered in the unit on composting a whole lot more!

The image above show how particle size matters. In the sand, large particles mean large spaces between them. Water will run through quickly. In the silt, medium sized particles means medium spaces. Water runs through more slowly than in sand and the soil becomes saturated more quickly. In a clay, the tiny particles mean tiny spaces between them. Clays are quickly saturated, hold water very tightly, and additional water will run off of the surface.

The middle image above shows how having a mix of particle sizes affects water holding; small spaces are mostly full, large ones mostly empty. More importantly, it shows what happens over time with water in soil. At the saturation point, there are no air spaces at all. Eventually plant roots would suffocate in saturated soil. Fortunately, gravity allows soils to drain. A soil at field capacity is a well watered one that has drained. There’s plenty of air and plenty of water for plant roots. When the amount of water in soil gets very low, soil particles will hold on to it so tightly that plants can’t take it up, so they wilt and may die.

You may have heard that opposites attract You may have heard that opposites attract. That’s very true in chemistry. Clay particles tend to carry negative charges and so attract and hold on to chemical with positive charges (called cations). Many important plant nutrients carry positive charges.

Cation exchange capacity (CEC) is a measure of a soil’s ability to hold nutrients. High CEC means good nutrient holding ability. The CEC of your soil will depend on many things and it generally can be improved a lot by adding organic matter.

pH is a term that many of us can associate with the idea of something being “acidic” or the opposite of acidic, “basic” (or alkaline). But what does that have to do with soils? pH is the measure of how many hydrogen ions (H+) are present in a watery solution. You may already know that the chemical symbol for water is H2O. The H in that symbol stands for hydrogen. There are always some H+ ions (H atoms that carry a positive charge) present in water, even in pure water. Hydrogen is present in all organic molecules, and charged H+ ions play many important roles in living things. They also have a big influence on chemical reactions. Soil pH is most important in terms of the chemical reactions that take place there. It influences plants’ ability to take up nutrients.

Low pH (less than 7) mean that lots of H+ ions are present. Hi pH (over 7) means that few H+ are present. Often that means that many OH- ions are present. They’re what’s left after H2O has lost an H+. At low pH, H+ ions compete for binding sites on clay particles. At high pH, OH- ions can bind to nutrient cations, making them unavailable. The “best” soil pH varies by crop, but is usually not very far from neutral (pH 7) The diagram above shows the availability of various nutrients at different soil pH. A thick line means high availability, so Aluminum (Al) is available at low pH but not at neutral or high pH. Nitrogen (N) is most available between pH’s of 6 and 8.

Soils vary a lot over time and space due to climate, parent material, and organic matter inputs. Over time, soil profiles develop.

The part of the world you’re in will influence the soils there.

What about the soil at your site? Here’s a super tool! USDA WEB SOIL SURVEY The basics of how to use this are included on the site. (Detailed instructions to follow). Note that this site offers things like information about soil pH and CEC. Those factors vary a lot over time and these surveys were done long ago. They still have great value and it’s important to remember that soils can be very variable, even over relatively small spaces.