What is pH?

Photo Credit:

Soil pH is an indicator of how acidic, neutral or alkaline (basic) a soil is, based on the hydrogen ion concentration – where pH is reported as a negative logarithm and ranges from 0-14. Common classes of soil pH include: extremely acid (3.5-4.4), extremely to slightly acid (4.5 to 6.5), neutral (6.6 to 7.3), and slightly to very alkaline (7.4 to 9.0).  A pH range of 6.0 to 7.0 favors growth of most crops, although some crops have specific acid or alkaline soil requirements to be productive.  Understanding soil pH will help to understand the influences of soil chemistry and biology and how nutrient availability and soil organic carbon are impacted. To guide you in selection of an article, the Soil Nexus team suggests that you choose an article based on soil types of interest, which are often typical of a geographical region.

Do you have an excellent resource that isn’t listed here? Submit it for consideration.

Regional Educator ResourcesOther Educator ResourcesTechnical Resources
Title Source Resource type and Date Short Summary / Preview
Liming Missouri Soils University of Missouri Extension Webpage

Feb 2000

Just about everything people and nature do to the soil tends to create acidity. Liming acid soils increases yields and provides a number of other soil health benefits.
Missouri Limestone Quality: What Is ENM? University of Missouri Extension Webpage

Oct 1993

The effectiveness of limestone refers to its ability to neutralize soil acidity. To measure the ability to reduce acidity, a rating system was developed. This rating system is called effective neutralizing material (ENM).
The Meaning and Importance of Soil pH Iowa State University Extension Blog Post

May  2002

Garden soils are often described as acidic (sour) or alkaline (sweet). The relative acidity or alkalinity of soil is indicated by its pH.
Soil pH North Dakota State University Extension Webpage

Not Dated

The pH of a soil influences nutrient availability, toxicity, microbial populations, and activity of certain pesticides. Soils with a pH less than 7 referred to as acidic and pH levels greater than 7 are considered basic or alkaline.
Soil Acidity SARE Governmental / Extension Webpage

Not Dated

Several problems may cause poor growth of acid-sensitive plants in low pH soils.
Soil pH – NRCS & others Webpage


Soil pH affects the soil’s physical, chemical, and biological properties and processes, as well as plant growth. The nutrition, growth, and yields of most crops decrease where pH is low and increase as pH rises to an optimum level.
Soil phosphorus availability and lime: more than just pH? Soil Science Society of America Researcher Blog Post

Not Dated

One influence on phosphorus availability is the soil’s pH level. If soils are too acidic, phosphorus reacts with iron and aluminum. That makes it unavailable to plants. But if soils are too alkaline, phosphorus reacts with calcium and also becomes inaccessible.
Soil Quality Indicators: Chemical Indicators and Soil Functions USDA NRCS Fact Sheet

Feb 2015

Fact Sheet. Different chemical indicators widely used are related to the respective basic functions they measure.
Soil Quality Indicators: Soil pH USDA NRCS Fact Sheet

Apr 2011

Fact Sheet. Soil pH generally refers to the degree of soil acidity or alkalinity.
Soil pH USDA – NRCS NRCS Fact Sheet and Instructor Guide

Not Dated

Soil pH is a measure of soil acidity or alkalinity. It is an important indicator of soil health. It affects crop yields, crop suitability, plant nutrient availability, and soil micro-organism activity which influence key soil processes. Soil pH can be managed by measures such as applying the proper amount of nitrogen fertilizer, liming, and cropping practices that improve soil organic matter and overall soil health.
Commercial Greenhouse Production: pH and Electrical Conductivity Measurements in Soilless Substrates Purdue University Extension Publication

Jun 2009

Monitoring the pH and electrical conductivity (EC) of growing substrates is a way to correct issues before they become problems that damage crops. This publication recommends best practices for establishing a pH and EC testing program in soilless substrates, and outlines the procedures, advantages, and disadvantages of three accepted methods for testing media solutions.
Some basic concepts of liming Kansas State University Extension Blog

Jul 2014

This short article provides an overview of when liming is needed and for which field crops this is especially important.
Soil pH and Liming Kansas State University Extension Training Manual

Not Dated

This is a section from an extension handbook. This section is on pH and liming requirements for common row crops.
Lime and soil pH Iowa State University List of University Resources

Not Dated

This is a list of extension publications and conference proceedings on lime and soil pH. The breadth of topics covered is from basic to very specific on this topic
Title Source Resource type and date Short Summary
Soil pH and Organic Matter Montana State University Extension Fact Sheet

Mar 2017

After reading this module, the reader should: (1) Know what soil pH is and how it relates to soil acidity, (2) Understand how soil pH affects nutrient availability in the soil, (3) Learn techniques for managing soil pH, (4) Know the soil organic matter cycle, and (5) Understand the role of soil organic matter in nutrient and soil organic carbon management.
Soil pH: What it Means State University of New York Webpage

Not Dated

From the SUNY College of Environmental Science and Forestry: Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units. Soil pH is defined as the negative logarithm of the hydrogen ion concentration. The pH scale goes from 0 to 14 with pH 7 as the neutral point.
Soil pH – The Master Variable University of Connecticut Extension Blog Post

Dec 2018

Monitoring your soil pH is essential to ensure that it is falling within the range best suited for the vegetation you are growing. The Standard Nutrient Analysis performed at our lab gives you a pH value, a buffer pH value, a lime/sulfur recommendation, available micro & macro nutrient levels, and a fertilizer recommendation.
Soil Acidity, pH, and Liming UMass Amherst University Webpage

Not Dated

One of the most important aspects of nutrient management is maintaining proper soil pH, a measure of soil acidity.
Soil Acidity: It’s not just pH Washington State University Presentation

Not Dated

General information on soil acidity.
Changing the pH of your soil Clemson University Extension Webpage

Oct 2012

To raise the pH of soil, the common practice is to apply a material that contains some form of lime. Two materials commonly used for lowering the soil pH are aluminum sulfate and sulfur.
Soil Acidity and pH Cornell University Presentation

Not Dated

General information on soil acidity.
Water and Soil Characterization – pH and Electrical Conductivity Montana State University University Webpage

Not Dated

pH is a measure of the acidity of the water or soil based on its hydrogen ion concentration and is mathematically defined as the negative logarithm of the hydrogen ion concentration.
Soil Acidity and Liming: Basic Information for Farmers and Gardeners NC State Extension Extension Webpage

Dec 2018

Nearly all North Carolina soils are naturally acidic and need lime, which neutralizes the acidity, for optimum growth of crops, forages, turf, trees, and many ornamentals. Even though most of these soils have been limed in the past, periodic additions of lime based on soil tests are still needed.
Soil Acidity and Liming for Agricultural Soils NC State Extension Extension Webpage

Mar 2017

Nearly all North Carolina soils are naturally acidic and need lime, which neutralizes the acidity, for optimum growth of crops, forages, turf, trees, and many ornamentals.
Soil Acidity and Aglime Penn State Extension Webpage

Nov 2014

Soil acidity is among the important environmental factors which can influence plant growth, and can seriously limit crop production. The soil pH is a general indicator of whether aglime is needed to reduce the acidity.
Soil pH: What it Means State University of New York University Webpage

Not Dated

As the amount of hydrogen ions in the soil increases the soil pH decreases thus becoming more acidic.
Title Source Resource type and Date Short Summary
Agronomic Services — Mehlich Buffer Acidity North Carolina Department of Agriculture and Human Services Governmental Webpage

Apr 2013

The Mehlich buffer method for determining exchangeable acidity (Ac) was developed under the premise that an increase or decrease in exchangeable acidity can be quantified by measuring the corresponding increase or decrease in the pH of a buffered reagent.
Soil pH Michigan State University, LTER Extension/Research Protocol

Not Dated

Protocol to measure soil pH. Soil pH is a measure of the hydrogen ion activity in soil solution; high hydrogen activity (low pH) is an index of soil acidity which can affect nutrient availability and plant growth.
Exchangeable Ions, pH, and Cation Exchange Capacity (Chapter 6 of Standard Soil Methods for Long-term Ecological Research) Oxford University Press Book chapter


This book chapter provides the definition of Exchangeable Ions, pH and Cation Exchange Capacity. It also lists materials needed and describes the procedure, calculations and special considerations in determining these soil properties.

Technical review: March 18, 2020