Background Information for Teachers: Soil

Compiled by Molly Nicholie

All soils are made up of a combination of mineral components (rock, sand, silt, clay) and organic components, including living organisms and humus (the decomposed residue of plant and animal tissues). The combination of these components effect the amount of moisture and air space within a soil, as well as its capacity to sustain plant growth. Soils are characterized by their components and can be a simple as sand or clay, that are dominated by one mineral component and as complex as a “loam” that has a fairly equal mix of sand, silt and clay. The mix of soil components will affect a soils ability to hold water and how fast water will infiltrate (flow through) through a soil. This activity will illustrate to students the differences in soil components and initiate discussion of what mix of soil components is best for growing plants. The following are some facts and information that may help you facilitate this discussion.

Sand – sand is the largest of the soil particles, and therefore has the largest pore space between particles. The large pores between grains of sand allow for water to infiltrate quickly. Sand is an important component of soil because it allows a soil to drain, but too much sand in the mix makes it difficult for the soil to hold water and nutrients and dissolved nutrients.

Silt – Although you will not find silt in your standard course of study objectives for 3 rd grade science it is good for you as the teacher to know that it exists. Silt is often found in and around rivers, streams and flood plains. Silt is a finer particle than sand, but not as tiny as clay. Many flood plains are used for growing crops because of the high soil fertility, due in part to the high percentage of silt in soils.

Clay – You can observe many of the properties of clay by observing it in nature. Mud puddles are a good illustration of clay’s ability to hold water, and inability to drain water through its particles. Clay is the tiniest of particles and rather than being round is in the shape of a flat platelets. The structure of a clay particle is what makes it so easy to pack together. Flat platelets fit tightly together, offering little space between particles for water to flow through or for air to be held for roots. Easily compacted, clays are good for building on, but soils too high in clay content are difficult to grow plants in. The tightly packed particles inhibit root growth, as well as offing little air for gas exchange. Clay has a very high water holding capacity, but because of its inability to drain this water typically drowns roots.

Loam – this term describes a soil that has a fairly equal mix of sand, silt and clay. A good balance of all three soil particles offers the best conditions for plant growth. Sand enables the soil to drain, clay holds water and micronutrients required for plant growth and silt adds the balance, offering structure to the soil and holding water and nutrients.

Humus - Mineral components offer the basic building blocks of soil, influencing the amount of water and air a soil can hold. Humus is part of the organic component of soil that offers the nutrients for plant growth. Consisting of decomposing plants and animals, the humus provides the food needed for plant growth. Don’t forget all of the other micro-organisms and soil creatures that play am important role in this decomposition. Without the help of fungus, bacteria and other organisms, organic materials would not decompose fast enough to provide nutrients for plant growth.