Understanding Soil: Texture and Structure

The soul of your garden is the soil. A healthy soil translates directly into healthy plants, but soil itself is more complex than it first appears. Before amending your soil with compost, manure, or fertilizer, it's critical to understand what type of soil you have and how it behaves.

Soil Texture: What You Inherit

Soil texture is determined by the mineral particles it contains. Soil comprises air spaces and organic matter, but the majority consists of three types of mineral particles: sand, silt, and clay. The relative percentage of each determines the soil's texture, and texture is essentially permanent unless you excavate and replace the soil entirely.

Sand has the largest soil particles (0.4 to 0.01 inches in diameter) and the largest pores between particles. A soil with 50 percent sand tends to drain well and warms up quickly in spring, but it also tends to be less fertile and doesn't hold moisture well throughout the season.

Silt particle size falls between clay and sand (0.01 to 0.0001 inches in diameter). Silty soil characteristics are similar to clay but are moderated by the larger particle size.

Clay has the smallest soil particles (less than 0.0001 inches in diameter) and the least amount of water and air spaces between particles. A soil with at least 50 percent clay will have all the opposite characteristics of sand. It drains, dries out, and warms up slowly, but is very fertile and, once wet, holds water well.

The ideal soil texture is loam, composed of 40 percent sand, 40 percent clay, and 20 percent silt. This balance provides adequate drainage, water retention, aeration, and fertility.

Soil Structure: What You Make

While texture is inherited, structure is built. Soil structure refers to the way sand, silt, and clay particles are grouped together in aggregates. The size and arrangement of these aggregates influence nutrient availability and drainage.

Whether you have sandy, silty, or clay soil, adding organic matter in its many forms will improve soil structure. Microorganisms feed on organic matter and produce polysaccharides that help form humus, which enables small clay or silt particles to stick together to form larger aggregates. Larger aggregates create more pores for water and air to flow through. The result is better drainage, improved aeration, healthier plant roots, and more available nutrients.

The ideal amount of organic matter in most soils is between 5 and 10 percent. In sandy soils, humus acts like a sponge to catch and hold moisture and helps accumulate nutrients, making sandier soil more fertile. In clay soils, organic matter creates space and improves workability.

The ideal structure of topsoil (at least 10 to 12 inches deep) is granular with crumb-size groupings and plenty of pore spaces. Subsoil ideally has a blocky structure with cubes of soil and vertical openings that allow water and root penetration. Compacted soil has few air and water pore spaces and drains poorly.

pH: Acid or Alkaline?

Soil pH measures the sweetness (alkalinity) and sourness (acidity) of soil on a scale of 1 to 14. A pH below 7.0 is acidic; above 7.0 is alkaline. The correct pH for your plants matters because certain nutrients are only available to plant roots within specific pH ranges.

Generally, areas of high rainfall have low pH (acidic) soil, while areas of low rainfall have high pH (alkaline) soil. Most plants prefer a pH between 5.5 and 7.5, though some plants have specialized preferences.

Diagnosing Your Soil: Five Home Tests

You can conduct simple, inexpensive home tests to determine your soil's texture, structure, drainage, and pH. These tests provide a solid foundation for understanding your soil's needs before you invest in amendments.

Ribbon Test (Determines Clay Content)

  1. Take a handful of moist soil and roll it into a ball about the size of a ping-pong ball.
  2. Squeeze the soil ball between your thumb and fingers in your palm to form a ribbon.
  3. Stand the ribbon upright.
  4. Measure how far the ribbon extends before breaking:
    • No ribbon forms: At least 50 percent sand, very little clay
    • Less than 2 inches: About 25 percent clay
    • 2 to 3.5 inches: About 40 percent clay
    • Greater than 3.5 inches, doesn't break: At least 50 percent clay

Jar Test (Determines Sand, Silt, and Clay Percentages)

  1. Put 1 inch of dry, crushed garden soil in a tall quart jar.
  2. Fill the jar two-thirds with water and add 1 teaspoon of a dispersing agent (Calgon or table salt).
  3. Shake the jar thoroughly and let the contents settle.
  4. Sand settles to the bottom in about 1 minute. Measure this layer's depth.
  5. Silt settles in 4 to 5 hours. Look for a color and size difference between sand and silt layers.
  6. Clay takes days to settle. Determine its depth by subtraction. Some fine clay particles may remain suspended.
  7. Calculate percentages: if you have a 1/4-inch sand layer and the total soil depth is 1 inch, you have 25 percent sand. Repeat for silt and clay.

Percolation Test (Determines Drainage Rate)

  1. Dig holes 1 foot deep by 2 feet wide in various places in your garden.
  2. Cover the holes with plastic to let the soil dry out completely.
  3. Once dry, fill the hole to the top with water and time how long it takes to drain completely.

Interpretation:

  • Drains in less than 10 minutes: Soil will tend to dry out quickly in summer (sandy soil).
  • Drains in 10 to 30 minutes: Ideal drainage.
  • Drains in 30 minutes to 4 hours: Adequate drainage, but water slowly to avoid runoff.
  • Takes longer than 4 hours: Possible drainage problem (clay soil, hardpan, or high water table).

Note: Extremely dry soils, especially those with large clay content, tend to crack. Water may drain quickly through these cracks rather than because of good soil structure.

Compaction Test (Detects Hardpan Layers)

  1. Take a metal rod and walk around your property pushing it into the ground.
  2. If you can't easily push the rod at least 6 to 8 inches deep, you have compacted soil needing improved aeration.
  3. If you consistently meet resistance at a specific depth, a hardpan layer may be present.

Alternative: Dig a 1 to 2 foot deep hole in your garden and examine the soil profile. A hardpan appears as a dense horizontal layer with less crumbly particles than topsoil, few roots or earthworms, and possibly a darker color.

Check root health: Dig up a plant and examine its roots. White, vigorous, well-branched roots extending 6 to 8 inches deep indicate good soil structure. Roots that are shallow (1 to 2 inches), mushy, and gray indicate bacterial rot, likely from poor drainage. Shallow, brittle, black roots indicate fungal rot, also enhanced by drainage problems.

pH Test (Detects Alkalinity and Acidity)

To check for alkalinity:

  1. Take 1 tablespoon of dried garden soil.
  2. Add a few drops of vinegar.
  3. If the soil fizzes, the pH is above 7.5 (the free carbonates in alkaline soil react with acid).

To check for acidity:

  1. Take 1 tablespoon of wet soil.
  2. Add a pinch of baking soda.
  3. If the soil fizzes, the soil is probably very acidic (pH less than 5.0).

For extreme pH conditions on either end of the spectrum, take a professional soil test to determine the exact pH and the precise amendment rate needed.

The Living Soil: Earthworms and Mycorrhizae

Healthy soil is alive. Two of the most important players in this underground ecosystem are earthworms and mycorrhizal fungi, both of which create networks that benefit your plants.

Earthworms: Nature's Tillers

Earthworms are a sign of healthy soil. As they move through the soil in search of organic matter, they loosen it, create fertile soil aggregates, provide pathways for plant roots, redistribute organic matter, and improve drainage and aeration. But how you apply organic matter affects earthworm behavior and benefit.

Research at the USDA-ARS soil lab in St. Paul, Minnesota, shows that earthworms are creatures of habit. Once they locate their food source (organic matter), they remember where it is and return repeatedly. Crucially, the location of organic matter determines the pattern of their tunnels.

When organic matter is tilled 6 inches or more into the soil, earthworm tunnels tend to be horizontal as they patrol to find food. Horizontal tunnels allow some air and water penetration, but not optimally.

When organic matter is left on or near the soil surface (a 3 to 4 inch layer), earthworm tunnels tend to be vertical as they dig upward to their food source. Vertical tunnels provide superior air and water penetration into deeper soil layers and help remove chemical toxins. The soil in these burrows harbors more beneficial microbes, which can degrade pesticides and fertilizers.

Research at the National Soil Tilth Lab in Ames, Iowa, demonstrates that reducing tillage dramatically increases earthworm populations. Untilled soils have three to four times as many nightcrawlers (surface-feeding worms) per square yard as tilled soils, not because tillage destroys worms directly, but because it accelerates the breakdown of the organic matter worms need. Grass clippings and leaf mulches produce double the worm population compared to corn residue. In soils rich with worms, two inches of water can be absorbed in just 12 minutes, whereas worm-poor soils require 12 hours to absorb the same amount.

Mycorrhizal Fungi: The Root Partnership

The vast majority of land plants naturally form symbiotic relationships with mycorrhizal fungi. These fungi colonize plant roots and create masses of fine strands called hyphae that penetrate soil particles plant roots cannot reach. In exchange, the fungus feeds on sugars produced by the plant. The result is a dramatic increase in the plant's ability to absorb water and nutrients and fight off disease.

Mycorrhizal inoculation was first developed in the 1970s to help reforest disturbed lands such as strip mining sites. Today, these fungi are equally valuable for many urban and suburban soils where drought and compaction are common problems.

There are two main types of mycorrhizal fungi:

Ectomycorrhizae form mostly outside a plant's roots. They colonize many common trees and shrubs including alder, arborvitae, aspen, basswood, beech, birch, chestnut, fir, hazelnut, hemlock, hickory, larch, oak, pecan, pine, poplar, spruce, and willow. When present aboveground, they appear as small puffballs or mushrooms. Underground, you will find masses of white or orangish "roots" growing from the plant's roots.

Endomycorrhizae form mostly inside a plant's roots and are the most widespread type. They occur naturally in 90 percent of all vascular plants, including agricultural crops, grasses, desert plants, flowers, citrus, and shrubs. The special subgroup VAM (vesicular-arbuscular mycorrhizae) is particularly common. Endomycorrhizae are mostly invisible to the naked eye, though they can be seen with a microscope.

Key principle: Mycorrhizal fungi are more specific to soil than to plants. If your soil is poor quality (dry, compact, with little topsoil), adding mycorrhizal fungi will likely enhance your plants' ability to find and extract nutrients, even from soils where certain nutrients are chemically unavailable due to pH.

However, mycorrhizae provide the most benefit in less intensively maintained soils with long-lived plants. In heavily fertilized beds, mycorrhizae are of least benefit because nitrogen and phosphorus fertilizers inhibit their growth. Plants known to respond well to mycorrhizal inoculation include apple, artichoke, corn, fig, garlic, grapes, onion, potato, soybean, squash, strawberry, and tomato.

For established trees, shrubs, and perennials, lightly cultivate mycorrhizal inoculant into soil within the drip line. Alternatively, dig holes around the drip line and backfill with inoculant, or inject it into the root zone. Follow product recommendations for application rates.

Building Better Soil

Understanding your soil texture, testing its pH and drainage, and recognizing the living partnerships beneath the surface are the essential first steps toward gardening success. Once you know what you have, you can make informed decisions about amendments and techniques.

For guidance on improving your soil's structure and fertility through the addition of organic matter, see Healthy Soil: Building Structure, Life, and Fertility. For detailed information on specific soil amendments and products including mycorrhizal fungi, visit Soil Amendments: What to Add and Why.

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