Soil Composition and Fertility

Soil is the loose material covering much of Earth's land surface in which plants can grow. It is much more than broken rock. Healthy soil is a complex mixture of mineral particles, organic matter, water, air, and living organisms.

Soil supports terrestrial ecosystems by:

• providing plants with water and mineral nutrients
• anchoring plant roots
• storing and filtering water
• providing habitats for organisms
• recycling nutrients
• storing carbon

The properties of soil strongly influence which plants can grow successfully in an area.

What Is Soil Made Of?

Most soils contain five major components:

• mineral particles
• organic matter
• water
• air
• living organisms

The proportions vary greatly between different soils.

A fertile agricultural soil, a sandy beach soil, and a forest soil may therefore have very different properties even though they contain the same basic components.

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Mineral Particles

Most of the solid material in soil comes from the weathering of rocks.

Weathering gradually breaks rocks into smaller mineral particles.

These particles are commonly classified by size as:

sand → silt → clay

The proportions of sand, silt, and clay determine a soil's texture.

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Sand

Sand contains relatively large mineral particles.

Sandy soils usually:

• have large spaces between particles
• allow water to drain quickly
• contain plenty of air
• warm relatively quickly
• hold fewer nutrients than many other soils

Because water drains rapidly, sandy soil may dry out quickly.

Plants growing in sandy soils may therefore require more frequent watering.

Clay

Clay contains extremely small mineral particles.

Clay soils usually:

• have very small spaces between particles
• hold water well
• retain many nutrients
• drain slowly
• can become waterlogged
• may contain less air when saturated

Clay soils can also become compacted.

When wet, clay can be sticky. When dry, some clay soils become hard and difficult for roots to penetrate.

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Silt

Silt particles are intermediate in size between sand and clay.

Silty soils often:

• hold more water than sandy soils
• drain better than heavy clay soils
• feel smooth
• retain nutrients relatively well

However, soils containing large amounts of silt may be vulnerable to erosion.

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Comparing Sand, Silt, and Clay

No soil type is automatically "best."

The suitability of a soil depends on:

• plant species
• climate
• rainfall
• drainage
• management

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Loam

Loam is soil containing a useful mixture of:

• sand
• silt
• clay

Loamy soils are often well suited to agriculture because they can combine:

• good drainage
• good water retention
• nutrient retention
• sufficient air spaces
• workable soil structure

Loam is not an equal mixture of the three particle types. Different types of loam can contain different proportions.

Soil Texture

Soil texture describes the relative amounts of sand, silt, and clay.

Texture affects:

• drainage
• water retention
• aeration
• nutrient retention
• root growth
• ease of cultivation

Texture is mainly determined by the mineral particles present and is difficult to change substantially over a large area.

Farmers can, however, improve many soil properties by adding organic matter.

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Soil Structure

Soil structure describes how individual soil particles are arranged into larger clusters called aggregates.

Good soil structure creates spaces of different sizes.

These spaces allow:

• water movement
• air movement
• root growth
• movement of soil organisms

A soil can therefore have suitable texture but poor structure if it has become heavily compacted.

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Pore Spaces

The spaces between soil particles are called pores.

Some pores contain:

water

while others contain:

air

The balance is important.

Roots need water, but they also require oxygen for cellular respiration.

A healthy soil therefore needs both:

water + air

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Water in Soil

Soil water supplies plants with:

• water needed for photosynthesis
• dissolved mineral ions
• materials required for cellular processes

Plants absorb much of their water through their roots.

However:

too little water → wilting and reduced growth

while:

too much water → reduced soil oxygen and possible root damage

Good soil should usually hold water while allowing excess water to drain.

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Soil Air

Plant roots carry out cellular respiration.

They therefore require oxygen.

Soil organisms also require oxygen.

If all the pore spaces become filled with water, oxygen availability can decrease.

This explains why some plants grow poorly in permanently waterlogged soils.

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Organic Matter

Organic matter consists of material derived from living or once-living organisms.

Examples include:

• dead leaves
• dead roots
• animal remains
• waste material
• decomposing organisms

Soil organisms gradually break this material down.

Organic matter is one of the most important contributors to healthy soil.

Humus

Humus is dark, relatively stable organic material formed after extensive decomposition.

Humus can improve:

• nutrient availability
• nutrient retention
• water retention
• soil structure
• aeration

It can help sandy soils retain more water and help clay-rich soils develop better structure.

This makes organic matter valuable across many different soil types.

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Decomposers

Soil contains enormous communities of living organisms.

Important decomposers include:

• bacteria
• fungi
• earthworms
• insects
• other invertebrates

These organisms break down dead material.

During decomposition, nutrients contained in organisms can eventually become available for plants again.

A simplified nutrient cycle is:

plant → dead organic matter → decomposers → mineral nutrients in soil → plant

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Earthworms and Soil

Earthworms can influence soil by:

• breaking down organic material
• mixing soil
• producing nutrient-rich casts
• creating channels
• improving aeration
• improving water movement

Earthworms are therefore often associated with biologically active soils.

However, soil ecosystems contain many organisms besides earthworms, and their importance varies among ecosystems.

What Is Soil Fertility?

Soil fertility describes the ability of soil to provide conditions and nutrients that support plant growth.

Fertility depends on several interacting factors.

These include:

• nutrient availability
• organic matter
• pH
• water availability
• drainage
• aeration
• soil structure
• biological activity

A soil can contain many nutrients but still support poor plant growth if other conditions are unsuitable.

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Essential Plant Nutrients

Plants require several mineral nutrients.

Three important nutrients frequently discussed in agriculture are:

nitrogen (N)

phosphorus (P)

potassium (K)

Commercial fertilizers often display these as an N-P-K ratio.

Different nutrients perform different roles in plant growth.

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Nitrogen

Nitrogen is needed to produce important biological molecules including:

• amino acids
• proteins
• nucleic acids
• chlorophyll

Nitrogen deficiency may lead to:

• poor growth
• pale or yellowing leaves

Nitrogen compounds in soil can come from:

• decomposition
• fertilizers
• animal waste
• nitrogen fixation

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Phosphorus

Phosphorus is important in:

• ATP and energy transfer
• DNA and RNA
• cell membranes
• root development
• reproduction

A phosphorus deficiency can reduce plant development.

However, excessive phosphorus entering waterways can contribute to eutrophication.

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Potassium

Potassium contributes to processes including:

• enzyme function
• water regulation
• stomatal function
• plant stress responses

Adequate potassium can help plants maintain healthy physiological processes.

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Nutrients Are Not the Whole Story

Adding more fertilizer does not automatically make soil healthier.

Plants can only benefit from nutrients if other conditions are appropriate.

For example, poor growth could result from:

• unsuitable pH
• waterlogging
• drought
• soil compaction
• disease
• poor soil structure

rather than nutrient deficiency.

Good soil management therefore requires identifying the actual limiting factor.

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Soil pH

Soil pH measures how acidic or alkaline the soil is.

The pH scale is commonly represented from:

0 to 14

with:

pH 7 = neutral

below 7 = acidic

above 7 = alkaline

Many plants grow well in slightly acidic to approximately neutral soil, but the preferred range depends on the species.

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Why Soil pH Matters

pH influences the chemical forms and availability of nutrients.

At unsuitable pH values:

• some nutrients become less available
• some substances may become more soluble
• microbial activity can change
• plant growth can decline

Therefore, soil may contain a nutrient but plants may have difficulty absorbing it.

This is one reason soil testing can be useful before adding fertilizer.

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Soil Testing

Farmers, gardeners, and scientists can test soil for characteristics such as:

• pH
• nutrient concentrations
• organic matter
• moisture
• texture

Testing provides evidence that can guide management decisions.

Instead of simply adding fertilizer, a farmer can determine which nutrients are actually deficient.

Water-Holding Capacity

Water-holding capacity describes how effectively soil retains water.

Sandy soils generally have:

lower water-holding capacity

Clay-rich and organic-rich soils generally have:

higher water-holding capacity

However, holding the maximum possible amount of water is not always desirable.

Plants need a balance between:

water retention + drainage + aeration

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Drainage

Drainage describes how easily excess water moves through and away from soil.

Poor drainage can cause:

• waterlogging
• reduced oxygen
• root stress
• increased risk of some plant diseases

Extremely rapid drainage can also be problematic because soil may dry too quickly.

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Soil Compaction

Soil compaction occurs when soil particles are pressed closely together.

It can result from:

• heavy machinery
• vehicles
• repeated foot traffic
• livestock
• working wet soils

Compaction reduces pore space.

This can cause:

less pore space → less air and water movement → more difficult root growth

Comparing Plant Growth in Different Soils

Imagine identical plants are grown under the same conditions.

Soil A

• mostly sand
• low organic matter
• drains rapidly

Likely result:

Water and nutrients may be lost quickly.

Soil B

• heavy clay
• strongly compacted
• poor drainage

Likely result:

Roots may receive insufficient oxygen.

Soil C

• loamy
• rich in organic matter
• good structure

Likely result:

The soil may provide a more favorable balance of water, nutrients, drainage, and aeration.

This demonstrates why plant growth depends on several soil properties interacting together.

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Soil Profiles

Soil often develops in layers called horizons.

Together these layers form a soil profile.

A simplified profile may contain:

O horizon — surface organic material

A horizon — topsoil

B horizon — subsoil

C horizon — partly weathered parent material

Below these may be solid bedrock.

Topsoil

Topsoil is usually the upper mineral soil layer.

It often contains:

• mineral particles
• organic matter
• plant roots
• microorganisms

Topsoil is particularly important for plant growth.

Loss of topsoil through erosion can greatly reduce agricultural productivity.

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Factors Affecting Soil Formation

Soils develop over long periods.

Important factors include:

• parent rock or material
• climate
• organisms
• topography
• time

For example, rainfall influences:

• weathering
• leaching
• plant growth
• decomposition

Different combinations of these factors produce different soils.

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Climate and Soil Fertility

Climate influences soil through:

• rainfall
• temperature
• weathering
• decomposition
• vegetation

Warm, wet conditions can promote rapid decomposition but may also increase leaching.

Dry conditions may slow biological activity and limit plant growth.

Climate therefore affects both soil formation and nutrient cycling.

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Leaching

Leaching occurs when water moving through soil carries dissolved substances downward.

Heavy rainfall can increase leaching.

Some nutrients may therefore move below the depth reached by plant roots.

Sandy soils can be particularly vulnerable because water moves through them relatively quickly.

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Soil Erosion

Soil erosion is the movement and removal of soil by agents such as:

• water
• wind

Human activities can increase erosion by removing vegetation.

Bare soil is particularly vulnerable because plant roots and surface vegetation normally help hold soil in place.

Why Erosion Reduces Fertility

Erosion often removes nutrient-rich topsoil.

This can reduce:

• organic matter
• nutrients
• water-holding capacity
• soil depth
• biological activity

Severe erosion can make productive land increasingly difficult to farm.

Protecting soil is therefore an important part of sustainable agriculture.

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Improving Soil Fertility

Farmers and gardeners can improve soil health using several approaches.

These include:

• adding compost
• adding appropriate fertilizers
• planting cover crops
• rotating crops
• reducing erosion
• reducing compaction
• maintaining vegetation
• managing irrigation
• monitoring soil pH

The best strategy depends on the soil problem.

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Compost

Compost is partially decomposed organic material.

Adding compost can:

• add organic matter
• improve soil structure
• increase water retention
• supply nutrients
• support soil organisms

Compost therefore improves more than simply nutrient concentration.

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Fertilizers

Fertilizers supply plant nutrients.

They may be:

• organic
• mineral or synthetic

Fertilizers can improve crop growth when nutrients are limiting.

However, excessive fertilizer use can cause:

• nutrient runoff
• groundwater contamination
• eutrophication
• unnecessary expense

The sustainable approach is:

right nutrient + right amount + right place + right time

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Crop Rotation

Crop rotation involves growing different crops in a planned sequence.

This can help:

• manage nutrient demand
• reduce some pests and diseases
• improve soil structure
• reduce erosion

Some rotations include legumes, which form associations with nitrogen-fixing bacteria.

These bacteria can increase biologically available nitrogen within the system.

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Cover Crops

Cover crops are grown partly to protect or improve soil rather than primarily for harvest.

They can:

• protect soil from erosion
• add organic material
• reduce nutrient loss
• improve structure
• suppress some weeds

Roots also help hold soil in place.

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Evaluating Soil for Plant Growth

When evaluating soil, do not look at only one characteristic.

Consider:

Texture — How much sand, silt, and clay?

Structure — Are particles arranged into healthy aggregates?

Organic matter — Is there material supporting nutrient cycling and soil structure?

pH — Are nutrients available to the plants?

Water retention — Can the soil store sufficient water?

Drainage — Can excess water escape?

Aeration — Can roots obtain oxygen?

Nutrients — Are essential mineral nutrients available?

Biological activity — Is there a functioning soil community?

Together, these determine how effectively soil can support plants.

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Example: Choosing Soil for Tomatoes

Suppose three soils are available.

Soil C would probably provide the most balanced conditions.

However, Soil A could potentially be improved by adding organic matter, while Soil B could benefit from management that improves structure and drainage.

Soil properties can often be managed, even when the basic texture cannot easily be changed.

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Example: Investigating Soil Fertility

Students could investigate how soil type affects plant growth.

Use identical seedlings and grow them in:

• sandy soil
• clay-rich soil
• loamy soil

Keep constant:

• plant species
• light
• temperature
• pot size
• initial plant size

Measurements might include:

• plant height
• leaf number
• biomass
• soil moisture

The investigation could then connect observed plant growth to soil properties.

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Common Mistakes

Thinking Soil Is Just Dirt

Soil is a complex ecosystem containing minerals, water, air, organic matter, and organisms.

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Assuming Clay Soil Is Always Bad

Clay can hold water and nutrients very effectively.

Problems arise when drainage or structure is poor.

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Assuming Sandy Soil Is Always Bad

Sandy soils often have excellent drainage and aeration.

Their main limitations may be low water and nutrient retention.

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Thinking Fertility Means Only Fertilizer

Fertility also depends on:

• pH
• organic matter
• water
• aeration
• structure
• biological activity

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Assuming More Fertilizer Means More Growth

Once nutrient needs are satisfied, additional fertilizer may provide little benefit and can cause environmental damage.

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Confusing Soil Texture and Structure

Texture = proportions of sand, silt, and clay

Structure = arrangement of soil particles into aggregates

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Thinking Waterlogged Soil Gives Plants More Water

Waterlogged soil may contain plenty of water but insufficient oxygen for healthy roots.

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Assuming All Plants Prefer the Same Soil

Different plants are adapted to different:

• pH levels
• moisture conditions
• nutrient levels
• textures

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Ignoring Organic Matter

Organic matter influences nutrients, water retention, soil structure, and biological activity.

It is a central component of soil health.

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Key Terms

Soil — A mixture of mineral particles, organic matter, water, air, and organisms that develops at Earth's surface.

Soil fertility — The ability of soil to provide suitable conditions and nutrients for plant growth.

Soil texture — The relative proportions of sand, silt, and clay.

Sand — The largest of the three major soil particle-size categories.

Silt — Soil particles intermediate in size between sand and clay.

Clay — The smallest major soil particle-size category.

Loam — Soil containing a useful mixture of sand, silt, and clay.

Soil structure — The arrangement of soil particles into larger aggregates.

Pore space — Spaces between soil particles that can contain air or water.

Organic matter — Material in soil derived from living or once-living organisms.

Humus — Dark, relatively stable organic material produced through decomposition.

Decomposer — An organism that breaks down dead organic material and contributes to nutrient recycling.

Water-holding capacity — The ability of soil to retain water.

Drainage — Movement of excess water through and away from soil.

Aeration — The presence and movement of air through soil.

Soil compaction — Compression of soil that reduces pore space.

Soil horizon — A distinct layer within a soil profile.

Topsoil — Upper mineral soil layer usually containing considerable biological activity and organic matter.

Leaching — Movement of dissolved substances downward through soil with water.

Soil erosion — Removal and transport of soil by water, wind, or other processes.

Compost — Decomposed organic material added to soil.

Crop rotation — Planned sequence of different crops grown on the same land.

Cover crop — Crop planted partly to protect or improve soil.

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Key Takeaways

• Soil contains mineral particles, organic matter, water, air, and living organisms.
• Mineral soil particles are commonly classified as sand, silt, and clay.
• Soil texture depends on the proportions of sand, silt, and clay.
• Sandy soils generally drain rapidly but retain less water and nutrients.
• Clay-rich soils retain more water and nutrients but may drain poorly.
• Loamy soils often provide a useful balance of drainage, water retention, and aeration.
• Soil structure describes how particles form aggregates.
• Pore spaces provide both water and air for plant roots.
• Organic matter is extremely important for soil health.
• Humus can improve nutrient retention, water retention, and soil structure.
• Decomposers recycle nutrients from dead organisms.
• Soil fertility depends on more than simply the amount of fertilizer present.
• Nitrogen, phosphorus, and potassium are important plant nutrients.
• Soil pH influences nutrient availability.
• Compaction reduces pore space and can restrict root growth.
• Excess water can reduce oxygen available to roots.
• Erosion can remove fertile topsoil.
• Compost, crop rotation, cover crops, and careful fertilizer use can help maintain soil fertility.
• Soil properties should be evaluated together when predicting plant growth.

A useful way to think about healthy soil is:

MINERALS + ORGANIC MATTER + WATER + AIR + ORGANISMS → HEALTHY SOIL SYSTEM

And successful plant growth requires a balance of:

NUTRIENTS + WATER + OXYGEN + ROOT SPACE + SUITABLE pH.