Sand

We’ve all played with sand at some point. There were probably waves crashing in the distance, the smell of tanning lotion and sunscreen mixing with salty air. Sand gets everywhere and it can be used to make some amazing temporary castles and other works of art. It is also a component of soil.

Sand from Pismo Beach (Mark A. Wilson) Public Domain

What makes soil?

Soil is a combination of minerals, organic stuff (living and dead), liquids, and gases. The liquids and gasses, mostly air and water, move through large and small spaces called macropores and micropores, respectively. Soil can be mostly clay, mostly silt, mostly sand, or somewhere in between. Clay, silt, and sand classifications are more about particle size than actual material, but here’s the typical breakdown:

• clay - less than 2 μm; flat plates of feldspar that hold together using electrostatic forces
• silt - boxy or spherical particles of quartz or feldspar that are 2 to 50 μm (0.05mm)
• sand - quartz, shells, obsidian and other minerals, larger than 50 μm and smaller than gravel

Electron micrograph of sand grains (NASA)

​Electron microscopic of clay (USGS) Public Domain

[Note: μm stands for micrometer, or micron. One micron equals one one-millionth of a meter.]

Soil texture and nutrient availability

If you live in Florida, you know all too well how difficult it is to keep nutrients and water in your sandy soil. This is because the spaces between the grains of sand are so big. At the other end of the spectrum, clay is made up of flat plates that tend to stick together, holding tightly to water and nutrients and making it difficult for plant roots to move through it.  It very few macropores and micropores, which means drainage and aeration are common problems. This is also why it makes such nice pottery.

The Sand-Clay Myth

What is a gardener to do? Our intuition tells us that we can lighten heavy clay soil by adding sand. It sounds right. Sand has plenty of spaces! Putting the two together should give us a nice, happy medium, right? Wrong. Instead, the tiny clay particles fill in all the spaces around the sand grains, creating a soil that is even heavier than before!

Organic mulch to the rescue!

When I say ‘organic mulch’, I am not necessarily saying organic in the OMRI sense, although that is what I use. Organic mulch refers to mulch composed of materials that were or are alive: plants, animals, bugs, manures, that sort of thing. It does not include ground up plastics or other manufactured materials. When you incorporate organic mulch into sandy soil, you provide materials that can bind nutrients and water to the planting bed. The macropores become partially filled with water- and nutrient-retaining compost. When you top dress heavy clay soil with an organic mulch, earthworms, microorganisms, irrigation, and other actions will slowly incorporate chunks of non-clay material below the soil line, creating macropores and micropores for air, water, and plant roots to move through. Top dressing means you just leave the material on top of the soil, rather than digging it in. Digging clay soil is generally not helpful because of the smooth surface left behind by the shovel. When that surface dries, it can be impenetrable.

So, leave the sand at the beach or in your egg timer. If you have clay soil, organic mulch is what you want to use. You may be surprised to learn that sand is a non-renewable resource in high demand, due to our penchant for concrete. Apparently, creating sand take eons and we use a lot of it.