The Soil on Which We Stand

(11/6) When I lived in Maryland, my garden produced a lot of food. In fact, my goal was to be able to eat at least one item from garden every day. I came pretty close, if you counted stored, canned and frozen vegetables. Then I moved to Adams County and started all over again with less impressive results. I knew there would be differences here, even before I put a shovel in the ground. I knew the soil was different.

Before we moved, I asked about the area’s history and learned it had been farmed, probably for corn and soybeans in the last few decades. That meant the soil would be somewhat compacted and likely devoid of nutrients. I observed that the lawn seemed to struggle. Where soil was visible, it certainly didn’t look like the clay loam I was used to. In fact, the soil survey indicated that it was clay loam. Once we moved in, I broke into the soil and found there was a lot of clay, and was pebbly as well. What made my Maryland soil better than Adams County soil? Let’s think about soil, what it is and how it’s formed.

Soil isn’t just dirt! Think about what you already know. When you were young and just learning about soils in grade school science, you probably learned that soil anchored plants, and that the soil contained nutrients the plants need to grow. This is a rather simplistic view of soil, because when we really start working with soil, looking at it and handling it, we find there is an entire soil ecosystem – both living and non living components – all working together and providing the above ground world with material it needs to exist as a result.

In any soil sample, about 95% is non-organic. Less than half of that, 45%, is the weathered, eroded material which had its origins in ‘rock.’ Soil is composed of rock particles which we differentiate by size – sand (the largest), silt and clay (the smallest). Soils are composed of varying amounts of all three. Loam is soil that is composed of roughly a third of each. We want some sand because it helps water move through soil, but not too much because sandy soil dries out too quickly. We also want some clay because it holds on to water, as well as attracting and holding many mineral nutrients plants need. Too much clay is problematic because clay particles can become compacted, thus preventing air and water from moving through soil to plant roots and the organisms that call soil home. Sand and silt help keep clay particles from packing too tightly. This year clayey soils were often too wet, too long, causing roots to struggle when they couldn’t get enough oxygen.

We garden in soil that was formed over long periods of geologic time from the bedrock beneath our feet. Soil fertility is due to the mineral content (iron, calcium, phosphorous, etc.) derived from the bedrock, the parent material that weathered into soil particles. Most of these mineral nutrients are held by clay particles, available to plant roots when they need them. Parent material varies in the amount of nutrient minerals it contains. There are several different kinds of rock underlying the soil in Adams County, and consequently, soil fertility varies throughout the county.

But if soil particles make up only 45% of soil and organic matter is 5%, can you guess what the other 50% is? Think about what the soil looks like – grains of mineral material of varying sizes. Does a handful of your wet soil compact easily or is it loose? You cannot pack sand tightly; there is a lot of space between the particles. On the other hand, clay compacts quite easily. That means, in loamy soil there is room between the particle of air and water, the other 50%. After a rain, water fills the spaces between the soil particles, where it eventually infiltrates, or moves downward into the soil, or is taken by plant roots. As water moves out of the spaces, air that plant roots and soil organisms need to live fills in.

Next week, I’ll get back to the plants, their roots and the soil organisms. For now, think about this: It takes 500 to 1000 years to build an inch of soil; that’s a lot of weathering. You can appreciate the ‘dirt’ beneath your feet.

A quick and dirty soil texture analysis:

Grab a one quart jar with a tight fitting lid; one with straight sides is preferred. Add soil (pebbles removed) to about half its volume. Fill the jar with water and add a teaspoon of calgon, if available. Let it sit undisturbed for about 24 hours. Measure the total height of the soil in the jar using a ruler (metric scale is easier). Measure the height of each of the three bands. The bottom is sand, the heaviest. The middle is silt, and top layer, clay, is the lightest. Calculate the % of each.

A rough analysis:

• Clay soil – 40-100% clay, 0-40% silt, 1-10% sand
• Loam soil – 7-27% clay, 28-50% silt, 1-15% sand
• Sandy soil – 1-10% clay, 1-15% silt, 85-100% sand