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From acidic to alkaline – the right substrate for every plant

Measuring the pH in the soil

Hydrangeas favour acidic soils and liming the lawn makes life harder for moss – certain things are now common knowledge. But should you simply lime the garden every year without first measuring the pH? Not a good idea! To meaningfully affect the relationship between acids and bases in the garden soil, a little background knowledge is necessary. This article explains what role the pH plays in the garden soil and how you can measure and influence it.


What does the pH do to our garden soil?

Acid or alkaline soil – so, what happens then?

The optimal pH in garden soil is between 6.5 and 7.5. The ratio of acids and bases in the soil is balanced and the vast majority of garden plants and soil organisms feel good in this condition. If the pH is out of balance, two things are basically affected.

1. The availability of nutrients in the soil depends on the pH.

Nutrients can exist in the soil as dissolved salt compounds, enabling plants to absorb them easily. But depending on the pH, individual main and micronutrients can bind to soil particles. If the pH is sub-optimal, then it is possible that although the soil contains an abundant supply of all the nutrients, the plants cannot absorb them.
It is reasonable to suspect this if you only have stunted or deformed plants after fertilising extensively. If the soil pH is too low, plants will not have enough nitrogen, potassium and calcium, for example. On the other hand, if the pH is too high, then the supply of these same nutrients is abundant but iron, manganese or boron are lacking.
However, the excessive availability of certain nutrients can also be problematic for your plants in extreme cases. Large quantities of aluminium are available to plants in soils that are high in acidity, for example. You may already be familiar with this effect in a scaled down form from the blue colouring of French hydrangeas that grow in acidic soils containing aluminium. But too much aluminium poses a health threat, which means that hardly any garden plants will survive in the long term.

By ensuring that the soil pH is balanced, you are helping your soil to help itself. Only then can it provide the plants with all the nutrients available. As you can see from our table, extremes are never healthy for ultimately there's always an undersupply or oversupply of individual main and micronutrients.

2. Soil organism activity depends on the pH

The activity of soil organisms is also dependent on the pH. In acidic soil with a low pH, soil organism activity is extremely low. To better understand the consequences of this in the garden, let's take a brief look at the tasks of our small soil helpers in four steps:

  1. The first step sees larger soil organisms like earthworms convert organic substances in the soil (e.g. leaves and sticks) into valuable humus. Much smaller microorganisms then decompose this and nutrients such as nitrogen are produced, which are then available to plants. 
  2. The two (the larger soil organisms and the smaller microorganisms) thus combine to form a kind of recycling chain. The structure of the soil is effected if the soil organisms are no longer active, i.e. it isn't as fine and crumbly and leaves or sticks are not decomposed and converted into humus.
  3. This in turn effects the microorganisms, which do not find enough humus for processing (mineralisation).
  4. At the end of the chain, the plants in your garden soil also lack the very minerals that are organically bonded, i.e. only released by the activity of the microorganisms. These include primarily nitrogen, but also potassium and phosphorus.
PH value

Low pH – what happens when your soil is too acidic?

  • The soil is said to be acidic when the pH is less than 6.5. Bog-soil plants thrive in such conditions. Rhododendron, blueberries or hydrangeas favour a pH of 4.5 to 5. Only very few plants are able to tolerate soil conditions if the pH drops even lower than that. 
  • Microorganisms are less active. This affects the structure of the soil and humus mineralisation, i.e. the transformation of organic soil material into essential nutrients that the plants need to survive.
  • Certain heavy metals that are present in the soil (e.g. cadmium or mercury) or even a light metal such as aluminium are more easily absorbed by plants because they are no longer fixed. This can cause damage to plant health.
  • Also inhibiting the growth of plants, large quantities of iron and manganese are available to plants in acidic soils, while the concentrations of calcium, potassium, magnesium or boron, for example, are far too low.
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The garden soil is too acidic

How do you increase the pH in soil?

Fortunately, raising the pH in soil isn't that difficult. Lime neutralises the excess acids in acidic soil, increasing the pH in the process. In this way, lime also ensures that nutrients that have accumulated in the soil, such as calcium, potassium and magnesium, become available to plants again. Liming can also improve the water and air supply in soil in the long term – soil organisms are given a better habitat and become more active.
But liming doesn't work overnight. After you spread garden lime, it will take a few months and plenty of water for it to have any effect. That's why September or October is the best time to increase the pH in garden soil by liming. There is enough rainfall during the winter months to dissolve the lime granules and wash them into the soil, which saves a lot of work and watering. However, if the pH in your garden soil is badly out of balance, it may take several years of alternately measuring and liming until the situation normalises and plant growth recovers.


Granulated lime is much easier and more precise to apply than ground (very fine) lime, as the wind blows it away quite quickly.

High pH – what happens when your soil is alkaline?

The soil is said to be alkaline (or basic) when the pH is more than 7.5. Alkaline means that the soil contains a lot of lime. Hardly any plants can tolerate extreme levels of pH (about 9). Such an unusually high pH in the soil can also be caused by very frequent liming or many years of watering with calcareous (hard) water.

  • Nutrients such as iron, manganese, copper, zinc and boron are only available to plants in small quantities, which can lead to stunted growth and deformed plant parts.
  • At the same time, soil life is stimulated – earthworms and microorganisms are especially active, causing organic soil material to decompose into humus and in turn humus into minerals to such an extent that the soil structure suffers as a result. The soil has hardly any structure left to absorb water or air.
  • On the other hand, the active work of the soil organisms leads to an oversupply of nitrogen, potassium and phosphorus in the soil – all substances that were previously organically bonded and are now released through the decomposition processes.
PH value

The garden soil is too acidic

Too much lime in the soil – how can you lower the pH in the soil?

Lowering the pH in your garden soil is much more complicated than raising it. Basically, you have to put acid and organic material – ideally organic material that contains a lot of acid – back into the soil, which contains a lot of lime and rocks. Pine needles or peat are especially suitable for this. But you can also try to restore the balance to lime content with common garden waste, such as lawn clippings or autumn foliage. Ensuring the soil is only watered with lime-free tap water or rainwater is another option. The fertilisers used should also contain as little lime as possible and ideally have a physiologically acidic effect, which is the case with fertilisers containing ferrous sulphate or high levels of ammonium, for example. COMPO 'Iron-Dünger' fertiliser, for example, contains ferrous sulphate and therefore has an acidic effect. In the event of uncertainty, it's advisable to ask the local garden centre for a suitable product.
Nevertheless, it often takes many years for the pH in the soil to start to lower gradually. Fortunately, soils that are especially high in alkaline are very rare in our part of the world, so this is a problem that the vast majority of gardeners do not have to deal with.

How do you measure the soil pH?

As a rule, you don't need an overly accurate result to determine which way you need to go with your garden soil. That's why simple self-tests at home are usually sufficient. You can use test strips similar to those used in swimming pools or liquids that – depending on reading – turn a different colour. You could consider buying a special measuring device if you want to take measurements frequently and easily.
On the other hand, you can also have a soil sample analysed if you want more precise information. The advantage here is that since the measurement is extremely accurate, precise conclusions can be drawn about which plants will grow particularly well in your soil. Such soil analyses also deliver information about the concentration of other nutrients in the soil, so you can establish whether an entirely different problem may prevail.



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