Understanding Soil Types and Improvements

Soil is the top layer of the earth’s surface that is composed of rock and mineral particles, organic matter, water, air and living organisms. The type and quality of soil plays a major role in determining which plants will thrive in an area. Understanding the different soil types and how to improve soil fertility are key for any gardener or landscaper.

The Main Soil Types

There are 12 major soil types that are defined by the United States Department of Agriculture. These are classified by the size of the soil particles and the composition of the soil. The main types of soil include:

Sandy Soil

• Made up of large particles with spaces between the particles
• Allows water to drain quickly
• Often very low in nutrients
• Found near beaches, riverbanks and deltas

Silt Soil

• Composed of medium sized particles
• Prone to compaction but holds nutrients better than sandy soils
• Often found near rivers and streams

Clay Soil

• Composed of very fine particles that stick together
• Slow to drain and hard to work when wet
• Nutrient rich but can become compacted
• Found in inland areas and prairies

Loam Soil

• Ideal soil for gardening and agriculture
• Made up of sand, silt and clay in relatively equal amounts
• Rich in nutrients and good drainage

Peat Soil

• Made up of decayed organic matter like sphagnum moss
• Absorbs a lot of water and rich in nutrients
• Often found in bogs and wetlands

Chalk Soil

• Alkaline soil high in lime
• Allows water to drain quickly
• Nutrients leach out of the soil easily
• Found in areas with limestone bedrock

Marl Soil

• Made up of clay and limestone
• Difficult to work when wet but cracks when dry
• Relatively nutrient rich soil good for agriculture

Soil Composition

In addition to particle size, soil composition and structure impact the properties of the soil. Some key elements that affect soil quality include:

• Organic matter – Decaying leaves, plants, animals and manure. Vital for nutrition and water retention.
• Air – The air found in the tiny spaces between soil particles allows oxygen circulation for roots and soil organisms.
• Water – Essential for transporting nutrients and for plants to uptake nutrients from the soil. Too much or too little limits plant growth.
• Minerals – Clay, silt and sand provide structure. The mineral types affect pH and ability to hold nutrients.
• Living organisms – Bacteria, fungi, worms, insects and other organisms help maintain soil fertility and form symbiotic relationships with plant roots.

Soil Texture Triangle

The soil texture triangle is used to determine soil type based on the percentages of sand, silt and clay particles.

To use the triangle, the percentages of sand, silt and clay are plotted on each axis. Where the points intersect shows the soil type. For example, a soil with 40% sand, 40% silt and 20% clay would be classified as a loam soil.

Soil Structure Types

In addition to texture, soil structure describes the arrangement and aggregation of soil particles. Good soil structure allows water, gases and nutrients to infiltrate the soil. The main types of soil structure include:

• Granular – Small, irregularly shaped aggregates that leave large pore spaces for water movement. Common in the topsoil layer.
• Platy – Flat, plate-like aggregates that can restrict water and root penetration. Common is clay soils.
• Blocky – Angular or sub-angular block shapes that allow some root penetration.
• Massive – Soil is one coherent mass with no distinct aggregates. Very dense with poor water infiltration.
• Prismatic – Tall column-like aggregates with flat tops and sides. Found in clay soils with expanding and contracting minerals.

Soil Horizons

A soil profile can be divided into layers known as horizons. Each horizon has distinct physical, chemical and biological characteristics. The main horizons include:

• O horizon – Top organic layer of decomposed plant and animal residue.
• A horizon – Mineral topsoil layer with most organic matter and microbial activity.
• E horizon – Zone of leaching, lighter color from loss of minerals and clays.
• B horizon – Subsoil with accumulation of clays and mineral deposits.
• C horizon – Weathered parent rock material.![Soil Profile][]

The horizons of a typical soil profile. Image source: NRCS

Soil pH

Soil pH measures the acidity or alkalinity of the soil on a scale of 1 to 14, with 7 being neutral. Most plants grow best in slightly acidic soil between 6.0 to 7.0. Some key points:

• Acidic soils (pH < 7) have more hydrogen ions which affect nutrient availability.
• Alkaline soils (pH > 7) have more sodium, calcium or magnesium ions. Can cause chlorosis.
• Extremes of 4 or 10 cause severe nutrient issues and no plant growth.
• Certain plants like azaleas prefer acidic soil, while vegetables need neutral.
• pH can be altered by adding sulfur (to lower) or lime (to raise).

How to Improve Soil Quality

Many factors like over-farming, erosion, pollution and development can degrade soil quality over time. Here are some tips to improve the health and fertility of your soil:

• Add organic material – Mixing in compost, manure, hay, leaves, cover crops or wood chips provides nutrients for microorganisms and improves soil structure.
• Use mulch – Covering soil with bark, leaves, straw or other organic mulch retains moisture and reduces compaction. As it decomposes it adds organic matter.
• Plant cover crops – Green manure crops like clover or rye add organic matter when tilled under and prevent erosion. Their roots improve aeration and drainage.
• Reduce tillage – Plowing and over-tilling destroys soil structure. No-till practices allow beneficial fungi and bacteria to thrive.
• Check soil pH – Test soil annually and add amendments like lime or sulfur to reach the optimal pH for your crops.
• Rotate crops – Changing plant locations each season prevents depletion of specific nutrients and reduces disease. Legumes in rotation add nitrogen.
• Control erosion – Use terracing on slopes, windbreaks and avoid leaving soil bare to prevent loss of topsoil. Plant on the contour.

Frequently Asked Questions

What are the three main components of soil?

The three main components of soil are sand, silt and clay particles with organic matter filling the spaces between them. The ratio of these affects the soil properties.

How can I tell what type of soil I have?

Simple tests to determine your garden soil type include shaking it in a jar to see how quickly it settles, ribbon testing, rolling a ball of wet soil in your hands, or using a soil texture triangle. Send off a sample to be tested professionally for in-depth analysis.

What does soil pH measure?

Soil pH measures the acidity or alkalinity of the soil. A neutral pH is around 7. Acidic soils have lower pH levels while alkaline soils test higher. Most plants grow best in the slightly acidic 6-7 pH range.

How often should soil pH be tested?

Test your garden soil pH annually to determine if amendments are needed. The ideal time is early in the growing season before planting crops. Field soil for agriculture should be tested every 2-3 years.

What can be added to soil to adjust the pH?

Common amendments used to raise pH in acidic soils include lime or wood ash. Lower soil pH can be reduced by mixing in sulfur, sawdust, pine needles or coffee grounds. Make adjustments gradually.

How can I improve drainage in clay soil?

To improve drainage in dense clay soil, add organic matter like compost or peat to loosen the soil structure. Planting deep rooted crops helps break up compacted soil layers. Avoid tilling wet clay soils.

A soil test and understanding your soil type are the first steps towards having a successful, productive garden. Improving your existing soil structure and fertility will provide better conditions for your plants to thrive.