Cation Exchange Capacity (CEC)
Cation Exchange Capacity (CEC) is a key measure of soil health and fertility. It describes a soil’s ability to hold and exchange positively charged nutrient ions, known as cations, including calcium (Ca), potassium (K) and magnesium (Mg).
These nutrients are held on the negatively charged surfaces of clay particles and organic matter.
High CEC soils (such as clay-rich or organic soils) retain nutrients more effectively, reducing nutrient loss through leaching
Low CEC soils (typically sandy soils) lose nutrients quickly and often require more frequent feeding
CEC is therefore a useful indicator of a soil’s ability to act as a nutrient reservoir, supplying plants steadily over time and supporting strong, healthy growth.
Improving organic matter content is one of the most effective ways to increase CEC in garden soils.
Soil pH – Acidity and Alkalinity
Soil pH measures how acidic or alkaline a soil is. The term pH refers to the “power of hydrogen”, describing the concentration of hydrogen ions (H⁺) in the soil.
The pH scale ranges from:
pH 0–6: acidic
pH 7: neutral
pH 8–14: alkaline
Most garden plants prefer a slightly acidic to neutral soil, typically between pH 6 and pH 7.
Plant labels, reference books and online sources usually indicate the preferred pH for individual plant species.
How Soil pH Affects Nutrient Availability
Soil pH plays a major role in determining whether nutrients are available to plants.
Acidic soils
In acidic soils, higher levels of hydrogen ions increase the solubility of certain nutrients, making them more readily available to plants. These include:
Iron (Fe)
Manganese (Mn)
Zinc (Zn)
Copper (Cu)
Alkaline soils
In alkaline soils, lower hydrogen ion levels cause many trace elements to become tightly bound to soil particles. As a result, nutrients may be present in the soil but unavailable to plants, leading to deficiency symptoms.
A soil pH above 7.5 can significantly reduce the availability of:
Iron (Fe)
Manganese (Mn)
Zinc (Zn)
Copper (Cu)
Cobalt (Co)
Boron (B)
Phosphorus (P)
Working With Your Natural Soil pH
It is usually more effective to work with your existing soil pH rather than trying to change it dramatically.
Acidic soils are best suited to ericaceous plants such as rhododendrons, camellias and blueberries
Alkaline soils suit lime-loving plants such as many Mediterranean herbs and certain perennials
Plants can influence their local soil environment through root exudates, which slightly modify pH and nutrient availability. This process is most effective when plants are established from smaller sizes and allowed to adapt gradually.
Adjusting Soil pH (When Necessary)
Where adjustment is required:
Lime can be added to raise soil pH (reduce acidity)
Sulphur can be added to lower soil pH (increase acidity)
Changes should always be gradual and based on soil testing to avoid disrupting soil biology.
Testing Soil pH
Most digital soil pH meters available from garden centres provide sufficient accuracy for ornamental garden use. Regular testing helps guide plant choice, soil improvement and nutrient management.
Understanding soil pH alongside CEC allows gardeners to make informed decisions that support healthy plants, resilient soils and long-term garden success.