Soil pH

Written by: Stanislas Fahy

Published on February 15, 2025

Time to read 6 min

Summary

Introduction Geological origins and soil nature Importance of soil pH What is soil pH? Why measure it? How to determine your soil composition? How to test soil pH Initial diagnosis: Is your soil acidic or alkaline? Does pH change depending on the soil profile? Explanatory table of pH and its effects How to adjust soil pH Frequently asked questions about soil pH (FAQ) Why trust Pouryère for your soil analysis? To learn more

Introduction

The term " soil profile " refers to all the horizons that make up a soil, from the surface down to the bedrock. Each horizon has specific physicochemical properties, including soil pH , which can vary with depth. Understanding the soil profile therefore also means understanding how the pH is distributed in these different layers. This information is essential for assessing fertility, nutrient availability, and crop suitability to the land.

Geological origins and soil type

Every soil originates from the parent rock , whether it be limestone, granite, schist, volcanic, or sedimentary. This rock determines from the outset not only the soil's pH , but also its texture (clay, sand, silt) and structure (aggregates, density, porosity).

The calcareous soil, resulting from marine deposits rich in calcium carbonate, tends to be alkaline, with a pH often between 7.5 and 8.5. It favors calcicolous plants but can block certain trace elements such as iron.
A granitic or schistous soil, inherited from acidic rocks, generally generates a low pH, often below 6. It is suitable for acid-loving plants but can limit the availability of phosphorus.
Volcanic soils, rich in silica and young minerals, often exhibit marked acidity, with a pH of 4.5 to 6.5. Their fertility is high but very dependent on organic management.
Sedimentary soils (alluvium, loess) exhibit varying pH levels depending on the nature of the deposits: neutral to slightly alkaline if they are calcareous, acidic if they come from siliceous sands.

These characteristics directly influence the bioavailability of nutrients (nitrogen, phosphorus, potassium), water retention, the activity of microorganisms and therefore the overall biological vitality of the soil.

Importance of soil pH

What is soil pH?

Soil pH (potential of hydrogen) measures the acidity or alkalinity of a soil on a scale from 0 to 14, where 7 means neutral. Below 7, the soil is acidic; above 7, it is basic.

Why measure it?

Knowing the soil pH is crucial:

Acidic soil (pH < 5) can retain too much aluminum and block phosphorus, making growth difficult except for calcifuge plants (e.g., rhododendrons, azaleas, camellias).
Basic soil (pH > 8) poses a problem for the absorption of iron and magnesium, causing in particular iron chlorosis (yellow leaves between the veins).
The ideal range (pH 6–7.5) is suitable for the majority of crops.

How can I determine the composition of my soil?

How to test the pH of the soil

Before making any adjustments, it is essential to accurately measure the pH . Several methods exist:

Quick tests in garden centers : indicator strips or colorimetric kits, simple to use but giving limited accuracy.
Electronic devices (pH meters) : commercially available, they provide a direct reading. A moist sample must be taken and the electrode immersed to obtain the value.
Laboratory analysis : The AGROKIT Pouryère is designed for home gardeners seeking a complete agronomic analysis. It allows you to measure fertility, soil structure, pH, and many other critical parameters. With this kit, the results include:
- soil pH
- C/N ratio (Carbon/Nitrogen), indicative of the decomposition potential of organic matter
- Total organic carbon, key to soil life
- Cation exchange capacity (CEC), which indicates the ability to retain nutrients
- Total carbonates, useful for determining the presence of limestone
- Contents of macro and trace elements (N, P, K, Ca, Mg, S, Iron, Zinc, Copper…)
- Texture and particle size (clay, silt, sand)
- The entire process is carried out in a COFRAC accredited laboratory (ISO 17025 standard), with results interpreted in a pedagogical manner.

First diagnosis: is your soil acidic or alkaline?

A simple preliminary observation can provide some guidance:

Presence of heather, ferns, mosses → soil probably acidic
Symptoms of chlorosis on certain susceptible plants → alkaline soil?
Alternatively, a measurement with the AGROKIT reliably indicates the actual state of pH and nutrients.

Does the pH change depending on the soil profile?

Yes. The pH can vary depending on:

The horizon (surface vs depth). For example, a soil may have a more acidic surface layer due to the decomposition of organic matter, while deeper horizons, rich in limestone, display a higher pH.
Seasonality (pH variability is a source of pH modification): In winter, with increased humidity, the pH tends to rise because water leaches out certain acids; in summer, microbial activity produces more organic acids, and drought sometimes exacerbates acidity. These variations generally remain between 0.5 and 1 unit but already influence nutrient availability.
The geological substrate : a deep limestone horizon can correct surface acidity through capillary action, while a granite or schist bedrock maintains a persistent acidic tendency. These contrasts explain why two neighboring soils can have very different pH profiles depending on their geology.

Table explaining pH and its effects

Here is a summary table to clarify:

pH range	Soil type	Possible effects / Compatible plants
<5	Very acidic	Toxicity (Al), nutrient blockage; only calcifuges (rhodos)
6-7.5	Optimal	Balanced fertility, favorable plant growth
>8	Alkaline	Iron/magnesium deficiency, iron chlorosis

This table allows you to quickly compare and guide your actions or planting choices according to pH.

How to adjust soil pH

Soil pH is not fixed: it can be adjusted to optimize growing conditions. Gardeners have several tools at their disposal:

To raise an overly acidic pH : add lime amendments (chalk, agricultural lime , dolomite). These products, available from agricultural cooperatives, garden centers, or specialized distributors, neutralize acidity and improve the availability of phosphorus and calcium. They are generally applied to the surface and then lightly tilled into the soil for lasting effectiveness.
To lower a pH that is too alkaline : sulfur, iron sulfate , or acidic organic matter (pine needles, oak leaf compost) can be used. These products can be found in garden centers, from agricultural suppliers, or through local composters. Their action is gradual: the sulfur oxidizes to sulfuric acid through the action of bacteria, lowering the pH over several months.
Organic matter plays a buffering role. The regular addition of compost, easily obtained through green waste collection centers, composting associations or made at home, helps to stabilize the pH in the ideal range for most plants.

Adjusting pH must be done carefully and based on precise analyses, as an excess can cause new imbalances. AGROKIT Pouryère is a valuable tool for determining the most suitable strategy before any amendment application.

The author of the article: Stanislas FAHY

Stanislas Fahy spent his entire youth in the Dordogne region, exploring its fields, forests, and vineyards. Passionate about nature, he quickly understood that our world must be cherished to preserve its beauty. After studying business, he specialized in sustainable development. Today, as co-founder of Pouryère, he dedicates himself daily to making soil knowledge accessible to everyone.

Frequently Asked Questions about Soil pH (FAQ)

Is your soil acidic or alkaline? Initial diagnosis

Answer: Observe spontaneous plants (e.g., heathers for acidity), or carry out a test with a kit (AGROKIT) or in a garden center to know the precise pH.

What is the purpose of knowing the soil pH?

Answer: This allows us to understand nutrient deficiencies or excesses, to adjust plantings, to avoid element blockages, and to amend correctly to optimize fertility.

Does the pH change depending on the soil profile?

Answer: Yes, depending on the depth, geological substrate and seasons, the pH varies and can influence plant health.

Why trust Pouryère for your soil analysis?

Comprehensive support, from sampling to interpretation

Pouryère supports you throughout the entire soil analysis process. Our sampling kits come with a comprehensive guide to walk you through the process. Once you've completed the analysis, simply send us your samples for full analysis and interpretation, which takes approximately ten days.

Solutions for individuals, farmers, communities and businesses

Each soil analysis kit is specialized and pursues a specific purpose:

AGROKIT for agronomic soil analysis.
POLLUKIT for pollutants.
PESTIKIT for pesticide detection.
PFASKIT for PFAS.
METKIT for soil metals.
SOLKIT for the complete health status of the soil (agronomic analysis, detection of pollutants and metals).
ISDIKIT is aimed at construction professionals.

Our field expertise and advanced diagnostic tools

Pouryère has advanced field expertise. We have developed the Soilscore, a rating that provides an environmental index. It is an overall soil quality score with indicators of fertility, pollution, and biodiversity, as well as practical advice for improving the quality of your land.