From acidic to alkaline – the right substrate for every plant
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?
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.
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.
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:
The garden soil is too acidic
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.
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.
The garden soil is too acidic
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.
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.
This might also interest you