What’s the Dirt on Soil?

People ask questions about their “dirt” all the time. For starters, please don’t call it dirt around a soil scientist! One of my professors used to say, “Soil is what you plant in and dirt is what you sweep up off the kitchen floor.” While dirt is a component of soil, soil is so much more.

Like buildings, plants require a foundation, which is typically a type of soil. Did you know there is a classification system for soil types? One system is centered around soil texture. There are twelve main classifications: sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay. While this is important information for building purposes, when it comes to gardening, it’s only part of the story.

Sand, silt, and clay are derived from weathered rocks and minerals. Sand is the coarsest particle, silt is medium sized, and clay is the finest particle. Varying the composition and source of these particles affects soil compaction, water holding capacity and drainage, and mineral or nutrient availability. Soil is also composed of organic matter or humus (decomposed plant material), air, water, and microorganisms.

Knowing soil texture tells us the strength of a soil: how it compacts, drains, and expands or shrinks. This is important for construction, but when it comes to growing plants, we need to know more. The USDA has a soil taxonomy system of classifying soil into 12 orders based on properties such as depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content. This gives us greater knowledge of what type of vegetation a soil can support.

Often, I get questions about why trees, shrubs, or lawns are doing poorly. People will ask if they need to fertilize. Compacted soil is usually the culprit in many of these situations. Compacted soils prevent good root growth, oxygen exchange, and nutrient uptake. These soils restrict the plant’s ability to grow.

Here in Livingston Parish, our soil varies from uplands to flatwoods to alluvial soils. These are predominately poorly drained soils comprised of loam over clay. The clay component in soil is important for nutrient exchange and water holding capacity. Unfortunately, too much clay leads to soil compaction. Often people think of adding sand to correct this, but if not done in the correct proportion, it can make the situation worse.

The addition of organic matter through compost and manures is usually a more effective and economical approach to improving soil structure. Organic matter also adds nutrients, beneficial soil microbes, and improves the air-water relationship in soil.

Organic matter is constantly being decomposed and thus needs to be maintained through additions of compost, manure, or cover crops. Tilling will speed up this decomposition as well. Maintaining the level of organic matter between 2-5% of total soil composition is a good target range.

Of course, soil texture is just one area of importance. The major component of soil comes from weathered rock and minerals. The source of which determines what nutrients are present in a soil and what will or will not grow. Soil testing analyses the soil for what minerals are available for plant use. The report generated by the LSU Soil Testing Lab gives you recommendations on what amendments are needed based on what plants or crops you want to grow.

As we transition from our spring gardens to our fall gardens, now is a good time to get your soil tested if you never done so or if it’s been over 3 years. Maintaining good soil health is critical to gardening success. Just please don’t ask to get your dirt tested!

8/1/2022 7:00:15 PM
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