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Soil Chemistry Fundamentals, Part 1 – Understanding Soil pH and How it Affects Plant Nutrient Availability

Angelo (admin)
soil pH scale

What is soil pH? How does it affect soil microorganisms and nutrient availability to plants? What is the optimum soil pH for plants? In this article we’ll answer all those questions and more, explaining soil chemistry in a practical way that is useful to gardeners.

What is Soil pH?

In chemistry, the pH scale is used as a measure of how acidic or alkaline a substance is, which in this case is soil. The pH scale ranges from 0 to 14, where a pH of 7 is neutral. The lower the pH value, the more acidic a substance is, while the higher the pH value is on the scale, the more alkaline (basic) it is. By comparison, pure water has a neutral pH of 7, and sits right in the middle of the pH scale.

As a useful guide to gardeners, the United States Department of Agriculture classifies soil pH ranges as follows:

pH < 3.5 Ultra acidic

pH 3.5 – 4.4 Extremely acidic

pH 4.5 – 5.0 Very strongly acidic

pH 5.1 – 5.5 Strongly acidic

pH 5.6 – 6.0 Moderately acidic

pH 6.1 – 6.5 Slightly acidic

pH 6.6 – 7.3 Neutral

pH 7.4 – 7.8 Slightly alkaline

pH 7.9 – 8.4 Moderately alkaline

pH 8.5 – 9.0 Strongly alkaline

pH > 9.0 Very strongly alkaline

Technical detail: In case you’re wondering, pH stands for power of hydrogen and the technical definition of pH is the negative log of hydrogen ion (H+) concentration in a water-based solution. Put more simply, pH is a measure of the availability of hydrogen ions (H+) in a solution.

The technical definition of pH may not mean much to most people, but what is important to note is that the pH scale is logarithmic, meaning that each pH value represents an increase or decrease of not one, but ten times!

To illustrate this point with some examples:

Understanding The Real Extent of Small pH Changes on Soil

As we can see, a change of a single point of pH represents a huge tenfold change in soil acidity or alkalinity, so it’s quite amusing when gardeners complain that they only managed to change their soil pH by ‘one’!

What about pH changes of less than one? What degree of change in acidity or alkalinity do they correspond to?

Technical detail: The formula to calculate the the x (times) change in acidity or alkalinity = 10^(change in pH points)

To save you the maths, I’ve created the graph below, which is really easy to follow:

Changing soil pH by less than a single point is still a significant change, because the pH scale is logarithmic, not linear

If we follow the horizontal (Change in pH points) axes of the graph all the way to the far right hand side of the graph, and go up to the blue line of the graph, we can see that a 1 point decrease or increase in pH creates a x10 change in acidity or alkalinity respectively.

Similarly, from the middle of the graph, a 0.5 point decrease or increase in pH creates a x3.16 change in acidity or alkalinity respectively. A decrease of 0.5 on the pH scale will make the soil a little over 3 times more acidic, while a increase of 0.5 pH will make the soil a little over 3 times more alkaline.

Soil pH and Buffer Capacity of Soils

All living things in nature have the capacity to self regulate to maintain a stable state, known as homeostasis. Living organisms maintain homeostatic states within their bodies, and on a larger scale, ecosystems also do the same to maintain constant conditions to support life.

Soil, being a living ecosystem, has mechanisms which allow it to maintain a stable state and resist extreme fluctuations in soil conditions, including changes soil pH.

When discussing the chemistry of acidity and alkalinity, we need to introduce another concept, and which is that of a buffer, or buffering agent. A buffer is an aqueous solution (a substance dissolved in water) that has a highly stable pH, so if you add an acid or a base (alkali) to a buffered solution, its pH will not change significantly.

By definition, buffering agents are a weak acid or weak base that helps maintain the pH of an aqueous solution after adding another acid or base.

Similarly, soil also resists changes in pH in order to maintain stable conditions. The buffer capacity of soil is defined as a soil’s ability to maintain a constant pH level when an acidifier or alkalizer is added to it.

Technical detail: How does soil do this? Soil is comprised of a mixture of buffered systems which can neutralize acids by bonding hydrogen (H+) ions, and neutralize bases (alkalis) by the release of hydrogen (H+) ions. Their effectiveness is dependent on numerous physical, chemical, and biological properties of soils.

The Composition and Properties of Different Soils

Soils are composed of a mixture of sand, silt and clay.

We’ve covered sandy, clay and silty soils, but what is loam?

A loamy soil is one that combines sand, silt and clay particles in relatively equal amounts, so it can retain moisture while draining well, and also allow sufficient air to penetrate to reach the roots, making it ideal for most garden plants.

Soils with a loam texture can contain different proportions of sand, silt and clay, so there are sandy loams, silty loams, loamy sand, and clay loams.

What the Buffer Capacity of Different Soil Types Means to Gardeners

Sandy soils have the lowest buffer capacity, so they can acidify faster because they don’t resist the change in pH very well, but for the same reason they can also be corrected or recovered the easiest. Less lime is needed to increase the pH of acidic sandy soils compared to clay soils for example.

Clay soils, and soils which contain lots of organic matter, have the highest buffer capacity and do resist changes in pH more strongly.

Technical detail: soils high in clay or organic matter are able to resist a becoming acidic because they have a larger number of surface sites which are able to bind hydrogen (H+) ions, which are responsible for acidity. Due to their high buffering capacity though, once they become acidified, they are also able to resist attempts to increase the pH to make them less acidic. Adding lime will neutralise the hydrogen (H+) ions in the soil solution, but a well buffered acidified soil will release bound hydrogen (H+) ions from the soil surface to maintain equilibrium and resist increase in pH.

All soils with a high buffer capacity will acidify more slowly, but require more lime to raise the pH when they do acidify.

Clays are generally better buffered than loams, which in turn are better buffered than sands.

To put this information into practical terms, the following table below shows the expected pH change that will result from applying 1 tonne per hectare (t/ha) or 0.1 kilograms per square metre (kg/m2) of garden lime (pure calcium carbonate) to various soil types with with an acidic starting pH.

Soil type pH change

Sand 0.5 – 0.7

Loam 0.3 – 0.5

Clay 0.2 – 0.3

What is the Optimum Soil pH for Plants?

Most plants prefer a neutral soil around pH 6.5 to 7.5, and will grow favourably in the broader pH range of 5.5 to 8*.

(*measured in a 0.01M CaCl2 solution instead of water for greater accuracy, and denoted as pHCa – see section ‘How to Test Soil pH’ for further information)

When the soil pH is above or below this optimum range, it changes the soil chemistry and affects the soil microbiology, which adversely impacts plant processes to reduce growth and yields.

All plants are affected by extremes of pH. but they vary widely in their tolerance of acidity and alkalinity. Some plants can grow well over a fairly wide pH range, while others have very specific soil requirements and may be very sensitive to small variations in acidity or alkalinity.

Some plants may prefer more acidic or alkaline soils, or tolerate them quite well, and these plants are listed below:

Plants which prefer acidic soils with a pH of 4.5 to 6.0:

Ornamentals

Fruit

Vegetables

Plants which prefer alkaline soils with a pH of 7.0 to 8.0:

Ornamentals

Fruit

Vegetables

Note: When looking up reference material to find the recommended soil pH for a plant, keep in mind that the stated preferred pH may vary from one source to another, depending on where you read the information. Some sources may cite wider or narrower ranges of pH that a specific plant will tolerate, and the ranges stated may start at a higher or lower pH value, but the recommended pH ranges will always be roughly similar with enough overlap to provide useful guidance as to what a plant requires for optimum growth.

Some sources state the soil pH range which plants will tolerate, which is a wider range than the preferred soil pH range. This can be seen by the vegetables listed in the Old Farmer’s Almanac lists as suitable for alkaline soils:

Only two listed are listed as being able to tolerate pH 8, which is defined as slightly to moderately alkaline.

The rest are listed as being able to tolerate pH 7.5, which is defined as only slightly alkaline.

A point worth noting is that all these alkaline-soil tolerant vegetables listed above have a stated lower pH range rating of pH 5.5 – 6.0, which is defined as strongly to moderately acidic!

Most vegetable plants will perform best when the soil pH is between 6.5-6.8, which is defined as slightly acidic to neutral.

As a general rule, most fruit trees and berries prefer the soil to be neutral to slightly acidic.

We can see this in the optimum pH ranges for berries listed below:

There are some exceptions though. Goji berries natively grow in slightly alkaline soil (pH of 7-8), and do not grow well in acidic soils.

It’s safe to say that most horticultural plants grow best in soils with a pH between 6.0 (slightly acid) and 7.5 (slightly alkaline). Generally speaking, soil nutrients are most available to plants when the soil pH is between 6.5 to 7.5, which explains why many garden plants grow best when the soil pH is around 6.5 (slightly acidic). It’s no coincidence that organic matter breaking down in the soil makes it slightly acidic…

The soil pH affects which soil nutrients are available to plants and which are not. To better manage nutrient availability, we first need to understand what function the various nutrients perform, and how each one is affected by soil pH.

Understanding Plant Nutrition

For plants to grow, they derive some of their food through photosynthesis, they take carbon dioxide (CO2) from the air, water from their roots, and use the energy from sunlight to produce sugars and carbohydrates.

Plants also take up various minerals from their roots which they require to grow and function. Some nutrients, such as those present in balanced fertilizers, which are required in larger amounts, are the macronutrients, while others, even though quite essential, are only required in very small amounts, so we call these trace elements.

Macronutrients

Plants draw specific nutrients from the soil in order to grow, and some of these nutrients are required in greater amounts than others. The main key nutrients which plants require to grow are referred to as macronutrients, because they’re required in large quantities.

Some macronutrients are more important than others, and are termed primary macronutrients, while the remaining macronutrients which are required in lower quantities are known as secondary macronutrients.

Primary macronutrients

The three main macronutrients which plants obtain from the soil are:

All fertiliser labels list an NPK ratio such as ‘NPK analysis: 3.7 – 2 – 1.8’ which indicates the proportions of nitrogen, phosphorus and potassium in the fertilizer.

Secondary macronutrients

The secondary macronutrients, which are required in lower quantities than the primary macronutrients, but are still very important, are:

What do these macronutrients do?

Rather than go into complex plant chemistry which is only of concern to plant scientists and means almost nothing to the majority of gardeners, it is simpler to explain the functions in general terms that are relevant to practical gardening.

Plants use the primary and secondary macronutrients as follows:

 

These are the main nutrients which plants require in large quantities, but some nutrients which are still essential for plant health and vigour are required in very small amounts.

Micronutrients

Micronutrients are nutrients which the plant requires in trace amounts, such as:

In fertilizers, these micronutrients are referred to as trace elements.

Most balanced fertilisers contain at least some of the more common trace elements, which are listed on the label. Trace elements can also be purchased as separate product form fertilisers, often as a mixture of trace elements combined together, but in some cases, as individual elements, such as Iron in the form of Iron chelate.

Now that we’re familiar with the nutrients that plants require, we can now look at what happens to them in the soil when the pH changes.

How Soil pH Affects Plant Nutrient Availability

Soil is one of nature’s most complex ecosystems, and soil chemistry is exceedingly complicated, so it should be no surprise that changing the soil pH will have many flow-on effects on plants, both positive and negative.

In the diagram below, we can see how the primary and secondary macronutrients, which plants require in the greatest quantities, and the micronutrients or trace elements, which are required in smaller quantities, are most available to plants when the soil pH is between 6.5 to 7.0.

The macronutrients are most available to plants when the soil pH is between 6.5 to 7.5, a slightly wider range than the micronutrients,

At either extreme, when soils become extremely acidic or alkaline, many nutrients become locked up and less available to plants, which leads to nutrient deficiencies and negative impacts plant growth and productivity.

From the left-hand side of the diagram, we can see that at a low pH, where the soil becomes more acidic:

Please note that aluminium (Al) is not a plant nutrient, and is therefore not shown in the diagram, but is present in soils and the effects of a very low soil pH (very high acidity) can lead to aluminium toxicity in plants, it can be extremely toxic to plant roots.

From the right-hand side of the diagram, at a higher pH, where the soil becomes more alkaline:

How Soil pH Affects Soil Microorganisms

Soil pH doesn’t only affect nutrient availability, it also affects soil microbe activity and the mobility of heavy metal (including pollutants such as lead, mercury and cadmium).

Remember that soil is a complex living ecosystem, the ‘soil-food web’ which breaks down organic matter to make nutrients available to plants.

For soil organisms to function, they need:

Keeping the soil pH close to neutral is usually the best strategy, and if you’re gardening organically and not using synthetic fertilisers, composting and building the soil organic matter levels, then soils will usually look after themselves without the need to mess around with them.

In arid environments soils tend to be more alkaline, and in humid environments they tend to be more acidic, limestone soils will naturally be more alkaline, peat soils will be more acidic.

In the following articles in this series we;ll look at how to accurately test soil pH, and how to change the soil pH safely without causing further problems.

See the next article in this series here – Soil Chemistry Fundamentals, Part 2 – How to Change Soil pH in Organic Gardening

References:

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