Why does soil pH matter for plants?

Soil pH controls the chemical form and solubility of nutrients and toxic elements; near neutral pH most nutrients are readily available, but in strongly acid soils phosphorus is tied up and toxic aluminium is released, while in very alkaline soils micronutrients such as iron and zinc become unavailable.

How does liming raise soil pH?

Liming materials such as ground limestone supply carbonate that reacts with and neutralizes soil acidity, consuming hydrogen ions and displacing exchangeable aluminium from colloids, which raises the pH and reduces aluminium toxicity.

Soil pH and Acidity

Soil pH measures the acidity or alkalinity of the soil solution and is a master variable that controls nutrient availability, the toxicity of elements such as aluminium, and microbial activity.

Definition

Soil pH is the negative logarithm of the hydrogen-ion activity in the soil solution, a measure of soil acidity or alkalinity; soil acidity comprises the active acidity in solution and the much larger reserve acidity held on the exchange complex.

Scope

This topic covers the sources and forms of soil acidity, active versus reserve acidity and pH buffering, the strong influence of pH on nutrient and metal availability, and the management of acidity by liming. It treats soil reaction as a central chemical property linking colloid chemistry to fertility.

Core questions

What causes soils to become acidic or alkaline?
What is the difference between active and reserve acidity?
How does pH control nutrient availability and aluminium toxicity?
How is soil acidity corrected by liming?

Key concepts

Soil pH and soil reaction
Active and reserve acidity
pH buffering capacity
Aluminium and manganese toxicity
Nutrient availability versus pH
Liming and lime requirement

Key theories

Active and reserve acidity: Soil acidity exists as a small active pool of hydrogen ions in solution buffered by a much larger reserve of exchangeable aluminium and hydrogen on colloids, so changing pH requires neutralizing the reserve, which determines lime requirement.
pH control of availability and toxicity: Soil pH governs the solubility and chemical form of nutrients and toxic elements; most macronutrients are most available near neutral pH, while strong acidity solubilizes toxic aluminium and manganese and ties up phosphorus.

Mechanisms

Acidity develops as basic cations are leached and replaced by hydrogen and aluminium, as roots and microbes release acids, and as ammonium fertilizers and acid deposition add hydrogen ions. Most of the acidity is held in reserve on the exchange complex as exchangeable aluminium, which hydrolyzes to release hydrogen ions, so the soil resists pH change. Liming adds carbonate that neutralizes this acidity, raising pH, displacing aluminium, and restoring availability of many nutrients.

Clinical relevance

Soil pH is one of the most informative and routinely measured soil properties: it guides the choice of crops, the need for and amount of lime, and the diagnosis of nutrient deficiencies and metal toxicities, and it strongly influences the fate of nutrients and contaminants in the environment.

History

The recognition that exchangeable aluminium, rather than hydrogen alone, controls most soil acidity emerged in the mid-20th century and reshaped understanding of liming, while the strong dependence of nutrient and metal availability on pH became a cornerstone of soil fertility and environmental soil chemistry.

Key figures

Donald L. Sparks
Nyle C. Brady
Ray R. Weil

Seminal works

brady2016
sparks2003

Frequently asked questions

Why does soil pH matter for plants?: Soil pH controls the chemical form and solubility of nutrients and toxic elements; near neutral pH most nutrients are readily available, but in strongly acid soils phosphorus is tied up and toxic aluminium is released, while in very alkaline soils micronutrients such as iron and zinc become unavailable.
How does liming raise soil pH?: Liming materials such as ground limestone supply carbonate that reacts with and neutralizes soil acidity, consuming hydrogen ions and displacing exchangeable aluminium from colloids, which raises the pH and reduces aluminium toxicity.