Soils for Plant Growth » Air and Water in Soil

What you'll learn this session

Study time: 30 minutes

The importance of air and water in soil for plant growth
Soil porosity and how it affects water and air movement
Different types of water in soil and their availability to plants
How soil texture and structure influence air and water content
Drainage and aeration problems in soils
Management techniques to improve soil air and water conditions

Air and Water in Soil: Essential Elements for Plant Growth

Plants might look like they only need the soil to hold them upright, but there's much more going on beneath the surface! Healthy soil isn't just dirt it's a complex mixture where air and water play crucial roles in supporting plant life.

Key Definitions:

Soil porosity: The percentage of soil volume that consists of pore spaces (gaps between soil particles).
Soil aeration: The process by which air from the atmosphere is exchanged with the air in soil pores.
Field capacity: The amount of water a soil can hold against gravity after excess water has drained away.
Wilting point: The moisture level in soil where plants can no longer extract enough water to meet their needs.

💧 Why Plants Need Water in Soil

Water in soil is essential for plants because it:

Dissolves nutrients, making them available for root uptake
Provides hydrogen and oxygen for photosynthesis
Maintains cell turgidity (firmness) and structure
Transports nutrients and sugars throughout the plant
Cools plants through transpiration

🌬 Why Plants Need Air in Soil

Air in soil is vital because it:

Provides oxygen for root respiration
Enables aerobic microorganisms to break down organic matter
Prevents toxic build-up of carbon dioxide
Allows nitrogen-fixing bacteria to function
Prevents waterlogging and root rot

Understanding Soil Porosity

Soil isn't solid it's full of tiny spaces called pores. These pores make up about 40-60% of the total soil volume in healthy soils. These spaces are like tiny rooms that can be filled with either air or water and the balance between the two is critical for plant health.

Types of Soil Pores

Not all pores are created equal! The size of soil pores determines how they function:

🔎 Macropores (>0.08mm)

These large pores allow for quick drainage and good aeration. They're common in sandy soils and help prevent waterlogging, but don't hold water well for plants.

🔍 Mesopores (0.03-0.08mm)

Medium-sized pores that hold water against gravity but still allow plants to extract it easily. These are the ideal pores for plant growth as they balance water retention and availability.

🧠 Micropores (<0.03mm)

Tiny pores that hold water so tightly that plants struggle to extract it. Clay soils have many micropores, which is why they can hold lots of water but plants may still wilt.

Water in Soil: Not All Water is Available to Plants

When it rains or when you water your garden, the water doesn't just sit there in a puddle underground. Soil water exists in different forms and not all of it can be used by plants.

📈 The Water Availability Spectrum

From most to least available to plants:

Gravitational water: Excess water that drains away due to gravity. It fills all pores temporarily after heavy rain but quickly drains through macropores.
Available water: Water held in mesopores that plants can easily extract. This is the ideal water for plant growth.
Unavailable water: Water held so tightly in micropores that plant roots cannot overcome the soil's hold on it. Plants will wilt even though this water is present.

📊 Important Water Thresholds

Understanding these key points helps manage irrigation:

Saturation point: All pores filled with water (100% water, 0% air)
Field capacity: Gravitational water has drained, leaving ideal growing conditions (approximately 60% water, 40% air in pores)
Permanent wilting point: Only unavailable water remains; plants cannot recover without additional water
Available water capacity: The difference between field capacity and wilting point this is what plants can actually use

How Soil Texture Affects Air and Water

The size of soil particles (texture) dramatically influences how air and water behave in soil. This is why some soils drain quickly while others stay waterlogged for days.

🍟 Sandy Soils

Particle size: 0.05-2.0mm

Air content: High

Water retention: Low

Drainage: Rapid

Sandy soils have mostly macropores, allowing excellent aeration but poor water retention. Plants may suffer from drought stress even after regular watering.

🧱 Loamy Soils

Particle size: Mixed

Air content: Balanced

Water retention: Moderate

Drainage: Good

Loamy soils have an ideal mix of pore sizes, providing good aeration while retaining sufficient water for plant growth. They're considered the "Goldilocks" of soils.

🧳 Clay Soils

Particle size: <0.002mm

Air content: Low

Water retention: High

Drainage: Poor

Clay soils have mostly micropores, resulting in poor aeration but excellent water retention. Plants may suffer from oxygen deficiency in wet conditions.

Soil Structure: The Arrangement Matters

While texture refers to particle size, structure refers to how these particles arrange themselves. Good soil structure creates pathways for both air and water movement, regardless of the soil's texture.

Healthy vs. Damaged Soil Structure

Soil structure can be improved or damaged by our management practices:

✅ Practices That Improve Soil Structure

Adding organic matter (compost, manure)
Minimising tillage and soil disturbance
Using cover crops to protect soil surface
Encouraging earthworms and soil organisms
Rotating crops with different root systems

❌ Practices That Damage Soil Structure

Compaction from heavy machinery or foot traffic
Working soil when it's too wet
Excessive tillage that breaks soil aggregates
Leaving soil bare and exposed to rainfall impact
Overuse of chemical fertilisers that reduce soil biology

Case Study Focus: The Dust Bowl

During the 1930s in the American Great Plains, poor soil management led to one of history's worst ecological disasters. Farmers had removed native grasses and repeatedly ploughed the soil, destroying its structure. When drought struck, the soil lacking both structure and moisture simply blew away in massive dust storms.

Over 100 million acres of farmland were affected, forcing thousands of families to abandon their farms. This catastrophe taught us that soil structure is crucial for both water retention during drought and soil stability during wind events.

Today, conservation practices like no-till farming, cover cropping and maintaining organic matter are used to prevent similar disasters by preserving soil structure and its ability to manage both air and water.

Common Soil Air and Water Problems

When the balance of air and water in soil is disrupted, plants suffer. Recognising these problems is the first step to solving them:

🛑 Waterlogging

Symptoms in plants: Yellowing leaves, wilting despite wet soil, shallow root systems, susceptibility to disease

Causes:

Poor drainage due to compaction or clay soils
High water table or impermeable layer below soil
Excessive irrigation or rainfall

Solutions:

Install drainage systems (tiles, ditches)
Create raised beds for better drainage
Add organic matter to improve structure
Use deep-rooted plants to create channels

🛑 Compaction

Symptoms in soil: Hard surface, standing water, poor plant growth, difficulty inserting a spade

Causes:

Heavy machinery or foot traffic, especially when soil is wet
Lack of organic matter to cushion compaction forces
Continuous cultivation at the same depth

Solutions:

Mechanical aeration (core aeration, subsoiling)
Establish permanent pathways to confine traffic
Add organic matter to rebuild structure
Use cover crops with strong tap roots (like radishes)

Managing Soil for Optimal Air and Water Balance

Creating the perfect environment for plant roots requires thoughtful management of both air and water in the soil. Here are some practical approaches:

Irrigation Best Practices

Smart watering helps maintain the ideal balance:

Water deeply but infrequently to encourage deep root growth
Monitor soil moisture rather than following a rigid schedule
Use drip irrigation to deliver water directly to the root zone
Water in the morning to reduce evaporation losses
Avoid overwatering, which displaces air from soil pores

Mulching for Moisture and Structure

A layer of mulch on the soil surface can:

Reduce evaporation, keeping more water available to plants
Protect soil structure from rainfall impact
Moderate soil temperature extremes
Add organic matter as it breaks down
Suppress weeds that compete for soil moisture

Quick Test: Assessing Your Soil's Air-Water Balance

You can get a rough idea of your soil's condition with these simple tests:

The squeeze test: Take a handful of moist soil and squeeze it. If it forms a ball that breaks easily when poked, it has good structure. If it stays in a tight ball, it may have poor aeration.
The infiltration test: Remove the bottom from a coffee can, push it 3cm into the soil and fill with water. Time how long it takes to drain. Faster than 3 minutes = good drainage; slower than 30 minutes = poor drainage.
The earthworm count: Dig up a 30cm cube of soil and count the earthworms. More than 10 indicates good soil health with adequate air and water balance.

Summary: The Delicate Balance

Healthy soil for plant growth requires a delicate balance between air and water. Too much water drives out air, causing root suffocation. Too little water leaves plants unable to take up nutrients. By understanding how soil texture, structure and management practices affect this balance, we can create optimal growing conditions for plants while conserving our precious soil resources.

Remember that what happens beneath the soil surface is just as important as what happens above it. Plants may reach for the sky, but their success begins with the right balance of air and water in the soil around their roots.

🧠 Test Your Knowledge!