Soil is the medium in which most garden plants grow. This lesson explains what soil is, how its properties affect plants, and how a gardener can assess and modify it.
Assumed audience
- Reading level: general adult.
- Background: has completed the plant basics lesson and understands that roots absorb water and dissolved minerals from soil.
- Goal: understand the properties of soil that matter to plants and how to evaluate them.
What soil is
Soil is a mixture of four components:
- Mineral particles: broken-down rock, ranging from large (sand) to fine (clay). These make up the solid structure of soil.
- Organic matter: decomposed plant and animal material. It holds water, supplies nutrients as it breaks down further, and improves soil structure.
- Water: fills spaces between particles. Plant roots absorb water along with dissolved minerals.
- Air: fills spaces not occupied by water. Roots need air to function; waterlogged soil (where all air spaces are filled with water) suffocates roots.
Healthy garden soil is roughly 45% mineral particles, 5% organic matter, 25% water, and 25% air by volume. These proportions shift with rainfall, irrigation, and soil type.
Soil texture
Soil texture refers to the size of its mineral particles. There are three basic particle types:
- Sand: the largest particles (0.05–2 mm). Sand feels gritty. Sandy soil drains quickly because water passes easily through the large spaces between particles. It holds few nutrients because water washes them away.
- Silt: medium particles (0.002–0.05 mm). Silt feels smooth, like flour. Silty soil holds water better than sand but can compact easily.
- Clay: the smallest particles (less than 0.002 mm). Clay feels sticky when wet and hard when dry. Clay soil holds water and nutrients well but drains slowly. When saturated, it leaves no room for air.
Most soils are a mixture of all three. Loam is a roughly equal blend of sand, silt, and clay, and is generally considered ideal for gardening because it balances drainage, water retention, and nutrient availability.
How to test soil texture
Squeeze a handful of moist soil:
- If it feels gritty and falls apart, it is sandy.
- If it feels smooth and holds a shape but crumbles when poked, it is silty or loamy.
- If it feels sticky and holds a tight ribbon when pressed between thumb and finger, it is clay-heavy.
This is a rough test. For more precision, a cooperative extension service can perform a particle-size analysis.
Drainage
Drainage is how quickly water moves through soil. It depends on soil texture and structure:
- Well-drained soil: water passes through within a few hours. Most garden plants prefer this.
- Poorly drained soil: water sits for a day or more. Roots may suffocate. Only some plants tolerate this.
- Excessively drained soil: water passes through almost immediately. Plants may not absorb enough before it is gone.
How to test drainage
Dig a hole about 30 cm (12 inches) deep and wide. Fill it with water and let it drain completely. Fill it again and time how long it takes to empty:
- Less than one hour: excessively drained.
- One to four hours: well-drained.
- More than four hours: poorly drained.
Soil pH
pH is a scale from 0 to 14 that measures how acidic or alkaline a substance is. A pH of 7 is neutral; below 7 is acidic; above 7 is alkaline.
Soil pH matters because it controls which nutrients are available to plant roots. Most nutrients are most available between pH 6.0 and 7.0. Outside this range, certain nutrients become chemically locked in the soil and roots cannot absorb them, even if the nutrients are present.
Most garden plants grow best in slightly acidic to neutral soil (pH 6.0–7.0). Some plants have specific needs: blueberries prefer acidic soil (pH 4.5–5.5); asparagus prefers slightly alkaline soil (pH 7.0–8.0).
How to test pH
- Home test kit: an inexpensive kit from a garden center. You mix soil with a solution and compare the resulting color to a chart.
- Cooperative extension service: mail a soil sample for a professional analysis that includes pH, nutrient levels, and recommendations.
Organic matter
Organic matter is material that was once alive: fallen leaves, decomposed roots, animal waste, compost. In soil, it serves three functions:
- Water retention: organic matter absorbs and holds water like a sponge, making it available to roots between rains.
- Nutrient supply: as organic matter decomposes further, soil organisms release minerals that plants can absorb.
- Soil structure: organic matter binds mineral particles into clumps (aggregates) that create spaces for air and water, improving both drainage and water-holding capacity.
Gardeners increase organic matter by adding compost, aged manure, leaf mold, or other decomposed plant material to the soil surface or mixed into the top layer.
Soil amendment
A soil amendment is any material added to soil to change its properties. Common amendments include:
- Compost: improves structure, adds nutrients and organic matter. Works in any soil type.
- Sand: added to clay soil to improve drainage. Requires large quantities to be effective.
- Lime: raises pH (makes soil less acidic). Made from ground limestone.
- Sulfur: lowers pH (makes soil more acidic). Used for acid-loving plants.
- Peat moss or coconut coir: improves water retention in sandy soil. Adds organic matter.
Amendments are chosen based on the gap between the soil’s current condition and what the intended plant needs. A soil test identifies the gap; the amendment closes it.
Why soil matters for every later decision
The soil at a growing site determines which plants will thrive without intervention and which will require amendments or alternative methods (raised beds, containers). Understanding soil is therefore a prerequisite for assessing a site and choosing a growing method.