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examBoard: Cambridge
examType: IGCSE
lessonTitle: Soil pH and Plant Growth
Soil pH is one of the most important factors affecting plant growth. It influences which nutrients are available to plants and how easily they can be absorbed. Think of soil pH as the soil's personality - it determines how the soil interacts with plants and what kind of relationship they'll have!
Key Definitions:
The pH scale runs from 0 to 14:
Each unit on the pH scale represents a 10-fold change in acidity or alkalinity. This means pH 5 is 10 times more acidic than pH 6 and 100 times more acidic than pH 7!
Soil pH can be tested using:
Most plants prefer a pH between 6.0 and 7.5, but some plants have adapted to more extreme conditions.
Soil pH is like a gatekeeper for nutrients. It controls which nutrients are available to plants and which are locked away. This happens because pH affects how nutrients dissolve in soil water and how tightly they're bound to soil particles.
Different nutrients are available at different pH levels. This is one of the most important concepts to understand about soil pH and plant growth:
More available: Iron, manganese, zinc, copper, boron
Less available: Nitrogen, phosphorus, potassium, calcium, magnesium
Plants that thrive: Blueberries, rhododendrons, azaleas, potatoes
Balanced availability of most nutrients
Most nutrients are readily available in this range, making it ideal for most garden plants and crops
Plants that thrive: Most vegetables, grasses, many flowers
More available: Calcium, magnesium
Less available: Iron, manganese, phosphorus, zinc, copper, boron
Plants that thrive: Lilac, asparagus, cabbage family, some beans
When soil pH is too high or too low for the plants you're growing, several problems can appear. Learning to spot these signs can help you diagnose pH issues in your garden or farm.
Too acidic (low pH):
Too alkaline (high pH):
Several factors influence soil pH naturally:
If your soil's pH isn't ideal for the plants you want to grow, you can change it. However, it's often easier to choose plants that suit your soil than to dramatically change your soil pH.
To lower pH, add:
Note: Changes take time and may need to be repeated.
To raise pH, add:
Tip: Always follow recommended application rates and test soil again after a few months.
Blueberries are naturally woodland plants that require acidic soil (pH 4.5-5.5). When UK farmers began growing blueberries commercially, many faced challenges with the UK's naturally alkaline soils.
At Dorset Blueberries farm, the natural soil pH was around 7.2 - too alkaline for blueberries. The farmers used several strategies:
After three years of management, they maintained a soil pH of 5.0 in their blueberry beds, resulting in healthy plants and excellent yields. This shows how understanding and managing soil pH can make growing "challenging" crops possible even in naturally unsuitable conditions.
Over time, plants have evolved to thrive in specific soil conditions. Some plants have special adaptations that help them cope with extreme pH levels.
Plants like rhododendrons, azaleas and blueberries have adapted to acidic soils by:
Plants like lavender, lilac and many Mediterranean herbs have adapted to alkaline soils by:
While modifying soil pH can help grow certain plants, it's important to consider the wider environmental impacts:
Sustainable approaches include:
With your teacher's permission, collect soil samples from different areas around your school grounds. Test the pH of each sample and create a pH map of your school. Look for patterns - are some areas more acidic or alkaline than others? Can you explain why?
Observe what plants are growing well in different areas. Do they match what you would expect based on the pH readings? This hands-on experience will help you understand the real-world relationship between soil pH and plant growth.
Soil pH is a fundamental factor in plant growth and health. It affects which nutrients plants can access and how well they can grow. While most plants prefer a neutral to slightly acidic soil (pH 6-7), some have adapted to thrive in more extreme conditions.
Understanding soil pH helps us:
Remember that soil is a complex living system and pH is just one factor among many that influence plant growth. The best approach is to work with nature as much as possible, making minimal changes when needed and choosing plants that are naturally suited to your local conditions.
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