How Does Soil pH Affect Nutrient Availability?

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How does soil pH affect nutrient availability? It changes the chemical form of many nutrients, and that form decides whether a root can take them up. Plants obtain nearly all their nutrients from the soil, but not all the nutrients present in the soil are accessible to plants. This article explains the mechanism, using a Purdue Extension publication on soil pH and a Mississippi State University Extension publication on soil pH and plant suitability.

Quick answer

• Soil pH affects the form and, therefore, the solubility of nutrients, which determines how much of each nutrient a plant can access.
• Most nutrients are at their optimal availability to plants when the soil pH is between 6 and 7, according to Purdue.
• When pH is high, micronutrients such as iron and manganese can become deficient, while low pH can cause toxicities such as aluminum.
• Soil pH is a measure of acidity or alkalinity, and it is measured on a logarithmic scale.

What soil pH measures

Soil pH is a measure of soil acidity or alkalinity. The scale runs from 0 to 14, and it is a measure of the hydrogen ion concentration of a solution. Mississippi State describes the same scale: acidic values are those less than 7.0, neutral values are 7.0, and basic values are those greater than 7.0.

The scale is logarithmic, which is why small numbers of pH units represent large differences in acidity. Purdue explains that soil with a pH value of 6 is 10 times more acidic than a soil with a pH value of 7, and 100 times more acidic than a soil with a pH value of 8.

Purdue also distinguishes the pH of the soil solution from a second measure. Soil pH is a measure of the concentration of hydrogen ions in the soil solution, and it describes the acidity the roots experience. Buffer pH, which is also referred to as reserve or stored acidity, is a different value that reflects acid bound to soil particles.

Why the soil’s reserve acidity matters

Buffer pH is the reason one soil can need more amendment than another to change its pH. Purdue states that soils that have a lower buffer pH value require more lime to neutralize the acidity than soils with a higher buffer pH value. The soil solution pH is only part of the picture, because the particles hold a reserve of acidic ions that must also be neutralized.

How nutrients become available or unavailable

Plants obtain nearly all their nutrients from the soil, but not all the nutrients present in the soil are accessible to plants. The reason is that soil pH affects the form and, therefore, the solubility of nutrients. A nutrient that is present in the soil but in a form the root cannot absorb is effectively unavailable to the plant.

Plants absorb most nutrients from the soil through their roots. Purdue notes that plants require 17 different nutrients to grow. Three of them, nitrogen (N), phosphorus (P), and potassium (K), are required in comparatively large amounts, and they are frequently applied as fertilizers. The rest are micronutrients, which are needed in smaller amounts but are just as vital.

Purdue states the consequence directly: when the supply of any nutrient is less than the plant requires, plant growth is inhibited, regardless of the abundance of other nutrients. This is why a soil that is rich in one nutrient can still produce a weak plant if another nutrient is locked up by the wrong pH.

Why high pH causes deficiencies

High soil pH is the classic case of nutrients becoming unavailable. Purdue states that high soil pH will lead to deficiencies of micronutrients such as iron and manganese. If plants do not have access to the micronutrients they need because the soil pH is high, their growth will be suppressed, and nutrient deficiency symptoms such as chlorosis, or yellowing, may appear.

Mississippi State gives a related pattern for forest soils. At higher pH levels, above 7.5, calcium and potassium are overabundant. Its summary is that in these situations many plants take up too many of some nutrients but not enough of others, and that imbalance causes toxic conditions.

Why low pH causes toxicity

Low soil pH works in the opposite direction. Purdue states that low soil pH can lead to toxicities of certain elements such as aluminum. When soils are acidic, some nutrients are available in excess, and plants take up more than they need, often with toxic results.

Mississippi State gives threshold values for forest soils. At lower pH levels, below 5.0, aluminum, iron, and manganese are readily available for plant uptake. The publication also states that soil pH values at the extremes, below 4.0 and above 8.5, can create toxic nutrients and make necessary nutrients unavailable to plants.

The optimal range for most nutrients

Purdue gives the range that matters most for garden soils. Most nutrients are at their optimal availability to plants when the soil pH is between 6 and 7. Many plants grow best in soils in this pH range.

Mississippi State uses a slightly different framing for trees. Soils with a pH of 6.5 to 7.0 generally provide the best growing conditions, and in this pH range most nutrients are readily available. The publication adds that the vast majority of commercially important tree species can live in a broad soil pH range if the proper balance of required nutrients is maintained.

Plants vary in their preference. Purdue notes that there are some plants, such as azaleas and blueberries, that are adapted to growing in more acidic soils. Mississippi State states that normally, pines and hardwoods prefer acidic to neutral soils, although some species grow well over a broad range.

How soil microbes link pH to nutrients

Soil pH also changes nutrient availability through the soil’s living community. Plants cannot take up nutrients that are stored in organic matter until the organic complexes are broken down by microorganisms that help make more nutrients available in a form that plants can take up. Purdue states that many of these microorganisms are most active when the soil pH is around 8, and that when soil pH is less than 6, their activity is severely depressed.

Purdue adds that the natural decomposition of organic matter is a reaction that produces acid, and that the reaction can be accelerated by tilling the soil.

What pushes soil pH up or down

Parent material is one cause. Purdue states that soils developed from basic rocks, such as basalt or limestone, generally have higher pH values than those formed from acidic rocks, such as granite or shale. Climate is another. Rain can leach basic nutrients such as calcium and magnesium from the soil, leaving behind acidic ions such as aluminum and hydrogen in greater concentrations. For this reason, soils formed under high rainfall are more acidic than soils formed under arid conditions.

Management also matters. Purdue states that in the soil, substances such as ammonium- and urea-based fertilizers, organic matter, and elemental sulfur undergo acid-producing reactions, which will lower the soil pH. Adding lime, an alkaline material, will raise soil pH.

Position in the landscape matters too. Purdue notes that soil pH can change with the horizontal location in the landscape and soil depth.

Changing pH or choosing plants

Before modifying the pH of any soil, Purdue advises that the optimum pH of the plants being grown, the current pH of the soil, and any effects the potential amendments could have beyond modifying the pH should all be considered. Purdue also notes that in some cases it may be more practical to select plants that grow well in the existing soil pH than trying to change the soil pH.

For a gardener, this is the practical decision point. The pH tells you which nutrients are likely to be available and which may be locked up. If the pH is far from the 6 to 7 range, and the plants you want do not tolerate it, changing the soil or changing the plant choice are the two options the sources describe. For the first step, the guide on how to test garden soil pH at home explains how to get a reading, and the guide on how to make garden soil acidic covers the acidifying side of the change.

How we chose

Both sources were selected from university extension publications that describe soil pH and its effect on plant nutrition. Each was opened on 2026-10-27 and checked for an HTTP 200 response before its text was used. Both are PDFs, and the text was extracted from the fetched files.

The Purdue publication was used for the mechanism, the buffer concept, and the management points. The Mississippi State publication was used for the pH thresholds and the nutrient-availability ranges, with the note that it is written for forest soils and tree species.

What we could not verify

The explanation above is limited to what these two sources state. Several related points could not be verified from the text available, and they are listed here so readers can see where the article stops.

What the sources do not say

The sources do not give a single pH value at which each individual nutrient becomes unavailable for every garden crop. The Mississippi State table gives availability bands for forest tree conditions, and the Purdue figure is described as a general pattern for mineral soils, not a crop-by-crop table.

The sources do not give an amount of lime or sulfur needed to move a given soil from one pH to another. Purdue points to a separate publication for lowering soil pH and another for liming, and the figures there were not part of the text available for this article.

Frequently asked questions

What does soil pH measure?

Soil pH is a measure of soil acidity or alkalinity. The scale runs from 0 to 14, and most soils have a pH range of 4 to 8.5, according to Purdue.

Why do nutrients become unavailable at high pH?

Purdue states that soil pH affects the form and solubility of nutrients. High soil pH leads to deficiencies of micronutrients such as iron and manganese, which can cause chlorosis, or yellowing, in the leaves.

Why is low pH linked to aluminum toxicity?

Purdue names aluminum among the elements that can become toxic when soil pH is low. Mississippi State adds that at pH levels below 5.0, aluminum, iron, and manganese are readily available for plant uptake.

Which pH range gives the best nutrient availability?

Purdue states that most nutrients are at their optimal availability to plants when the soil pH is between 6 and 7. Mississippi State gives the same 6.5 to 7.0 range for the best growing conditions for trees.

Should I change the soil pH or choose different plants?

Purdue states that in some cases it may be more practical to select plants that grow well in the existing soil pH than trying to change the soil pH. The choice depends on the optimum pH of the plants and the current pH of the soil.

Sources

• Purdue Extension, “Soil pH,” HO-240-W, by Michael V. Mickelbart, Kelly M. Stanton, James J. Camberato and Brad D. Lee. https://www.extension.purdue.edu/extmedia/HO/HO-240-W.pdf (opened 2026-10-27)
• Mississippi State University Extension Service, “Soil pH and Tree Species Suitability in Mississippi,” Publication 2311 (POD-09-22), revised 2022. https://extension.msstate.edu/sites/default/files/publications/P2311_2022b_web.pdf (opened 2026-10-27)