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4th Feb 2023

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Understanding where your soil pH is on the acid/alkaline scale is an essential measure if you wish to obtain the very best results and yields from your plot. GrowMad's Claire is here to explain all you need to know about soil pH and how to optimise it...

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What is Soil pH and Why is it Important

| Introduction

Every grower understands that some soils are more acidic or alkaline than others, but to get the very best fruit and vegetable crops from your plot or allotment, having the correct pH levels is essential.

pH is a measure of how acidic, neutral or alkaline your soil is, and the wrong pH can cause issues with producing the very best fruit and veg.

If you are interested in precisely what pH is, how it may affect your yields and how you can manage it, then this is a good place to start.

There's a lot to cover, so have your soil test kit at the ready and let's dip in...

| What is pH and why is it so important?

Every atom (the basic bit of every element/compound/material/liquid etc.) contains a nucleus of smaller particles, surrounded by electrons spinning around it in virtual orbits

Atom Structure2

If an atom loses or gains an electron, it alters its characteristics and becomes an Ion. In its natural state, an atom has no overall charge, but if it loses an electron it becomes positively charged, called a Cation. If the atom gains an electron, it becomes negatively charged- an Anion. This can be illustrated when you rub a balloon on your jumper and then it sticks to a wall, the balloon has been given a negative charge by gaining electrons from your jumper and attaches to the positively charged wall.

pH is short for 'potential of hydrogen' or ‘power of hydrogen’ and measures the hydrogen ion concentration in a substance. pH levels range from acidic and neutral to alkaline. The measurement was first used by Danish biochemist SPL Sorensen at the Carlsberg laboratory in 1909.

Water is a neutral chemical, each molecule of it is made of two hydrogen and one Oxygen atom, H2O. A small number can separate and become Hydrogen cations H+ and Hydroxide anions OH-. However, the number of these remains equal so it sits at pH7 which is neutral.

Acids, however, have a higher proportion of H+ than OH- and alkaline substances have more OH- than H+.

The pH scale that you may be familiar with is this one -

Untitled design 13

The pH scale is used to measure the acidity or basicity (alkalinity) of an aqueous solution. The scale runs from pH1 (highly acidic) to pH14 (highly alkaline) with pH7 being neutral.

The colour spectrum is gained by using a Universal Indicator- as a liquid or impregnated paper strip. This is a dye obtained from a species of lichen.

Digital pH meters can also be used to give a numerical value, but for testing soil samples a paper strip indicator is usually sufficient.

"For those interested, the pH scale is logarithmic in nature. The concentration of hydrogen ions in a solution is calculated as the negative logarithm of the number of hydrogen ions. -log(H+)

Although it sounds complex, (it is) it represents a factor of 10 change with each incremented number eg. Each decrease in number means ten times as much H+, so pH 2 is ten times more acidic than pH 3, and conversely, pH 14 is ten times more alkaline than pH 13"

Having the right soil pH is key to growing a healthy crop but is often not considered, however, knowing and understanding your soil pH level is essential and probably the single most important chemical measurement you should be aware of.

Ornamental gardeners may appreciate that some heathers need acidic soils, but vegetable growers may not factor it as much as their nutrient levels or soil structure.

While these are obviously very important, the pH of the soil plays a major role in how well your plants can absorb nutrients that may be present in your soil, but not necessarily available.

Although soil pH levels don't directly affect plants, they will also determine which bacteria and fungi will thrive and which nutrients will be available to your plants. Fungi favour a slightly acidic environment and bacteria more alkaline. Soil pH will also affect earthworm activity and populations.

It should also be noted, that plants give out substances via their roots, called exudates.

These carbon-rich excretions are used to feed bacteria and fungi in the soil, which in turn feed the plants with nutrients. These exudates, however, can produce acids which can help to alter pH levels in the area around the root tips, more suited to the particular plant.

Therefore, the plant may well be able to control its environment better than the gardener can!

| Testing Your Soil pH

For the home gardener, there are a number of ways to test your soil pH and remember, it may vary across different parts of your allotment or growing beds. Taking independent tests from various areas of your plot may give more accurate results. Also, samples taken from the rhizosphere (areas adjacent to the plant root zone) can be very different to the surrounding area.

You can discover more about the various soil test kits available here.

If you would prefer to use a 3rd party company to analyse your soil, you can find a useful list of labs here.

As shown in the diagram above, the ideal range for most veg is between 5.5 and 7.5, with a soil pH of 6.5 considered optimum. A Universal Indicator scale can show whether your soil is 6, 7 or 8, which is a rough guide.

pH Paper 2

However, you can purchase indicator papers with a more detailed range. The wrong pH often won’t kill plants outright, but it can affect their growth and result in below-standard yields, although many plants are able to adapt to a range of pH levels.

Extremes of pH decrease the availability of most nutrients. Acidic soils reduce the availability of macro- and secondary nutrients, while alkaline soils can reduce the availability of most micronutrients.

Indicator paper showing the pH of tap water.

For your fruit or vegetables to obtain exactly the right amounts of nutrients, you can see why having the correct pH levels is so important for each individual crop type.

In the table below, the availability of different nutrients is shown by the width of the white band at differing pH values. The wider the bar means more nutrients will be available. (PDA.org)

Each plant will have an optimum pH range dependent on which nutrients it needs, although the normal range is between 5.5 and 6.5. This can be adjusted in order to achieve the correct pH to ensure maximum nutrient availability.

| The Influence of Soil pH on Nutrient Availability

The availability of different nutrients is shown by the width of the white band at differing pH values. The wider the bar indicates more nutrients will be available. (PDA.org)

Each plant will have an optimum pH range dependent on which nutrients it needs, although the normal range is between 5.5 and 6.5. This can be adjusted to achieve the correct pH to ensure maximum nutrient availability.

Most brassicas for instance such as Brussels, cauliflower, cabbage and kale, will prefer higher pH levels (6.5 - 7.5) whereas potatoes will tolerate far lower levels (5.8 - 6.5).

| How to Correct Your Soil pH Levels

After you have either tested your soil or had it tested for you, you will have a far more enlightened idea of where your soil pH levels currently reside.

Soil too Alkaline?

If your soil pH is too alkaline, you will need to increase the acidity of a soil sample (lower the pH level).

The easiest method is to apply either aluminium sulphate, iron sulphate or sulphur.

Beware of lowering pH too much though, as an acidic pH can increase the solubility of metal ions such as Aluminium (Al), Manganese (Mn), and Iron (Fe), which are toxic to plants in excess. Too much soluble Al can cause the slowing or stopping of root growth.

Follow individual product guidelines for application rates.

Soil too acidic?

If your soil pH is too low, this can be amended by introducing alkaline products such as lime (calcium/magnesium carbonate).

Again, follow individual product guidelines for application rates.

| Takeaway & Conclusion

So there you have it, hopefully, this article has given you some insight into soil pH and just how important it is to have the correct levels for each individual crop you are planning on growing. Most soils are quite forgiving and a pH range of between 5.5 and 7 will suffice for the majority of crops but there are exceptions.

Also, soil pH can be useful in helping to manage disease problems such as clubroot in brassicas and many soil micronutrient deficiencies. You can discover more about clubroot in this useful GrowMad clubroot article.