When dealing with the soil conditions of one’s own garden, one inevitably comes into contact with the topic of pH. The pH value has a great influence on the healthy growth of vegetables, fruit and herbs. For example, the availability of nutrients and the activity of the valuable soil life depend on it. If the pH value is in the wrong range, the plants can no longer absorb sufficient nutrients, even if the soil contains enough of them. In addition, not all garden plants need the same pH value: Some thrive better in acidic soil, others like it more alkaline.
How do I measure the pH value?
The pH value (Potentia Hydrogenii) is the unit of measurement for the acidic or alkaline character of the soil. The pH scale ranges from 1 to 14. The value 7 is considered neutral, what is above is alkaline (basic), what is below is considered acidic. However, plant growth is only possible between the values 4 to 8.
To determine which pH value the soil has, a measurement is taken. You can easily do this yourself, although there are several variants. The trade offers, for example, measuring devices that you simply stick into the soil, or special soil test sets. The cheapest method, however, is to measure with litmus paper. To do this, shake the soil in distilled water and measure with a pH test strip (approx. 10-15 g sample in 25 ml distilled water). Do not measure immediately, but wait 2-3 minutes until the soil has dissolved well. Then dip the measuring strip and compare the discoloured paper with the colour scale.
What is the significance of the pH value?
Most garden soils have a pH value between 6 and 7, and this is the range in which the vast majority of garden plants feel comfortable. The soil’s micro-organisms also feel particularly comfortable at these values and thus ensure optimum availability of plant nutrients. However, the “acid rain” has led to a drop in the pH value of our soils. Therefore, it makes sense to check the pH value annually and correct it if necessary.
A low pH value (below 5.5) reduces the activity of microorganisms in the soil and thus inhibits the decomposition of organic matter. This means that the more acidic the soil, the fewer nutrients are available to the plants. Germination behaviour is also negatively affected. At the same time, plants absorb more heavy metals in acidic soil. But an increased pH value (above 7.5) can also have negative consequences, because this leads to a reduced supply of the nutrients phosphorus, magnesium and iron.
Plants have different pH preferences
Most plants do very well when the pH of the garden is between 6 and 7. However, there are some candidates that prefer acidic soils with a pH of 4.5 to 6. For these plants, which include azaleas and rhododendrons, for example, the garden soil must be acidified. You can lower the pH of the soil with bark mulch or peat, for example. Peat, however, should be rejected from an ecological point of view, as peat bogs are an increasingly rare habitat for many plants and animals. In addition, peat extraction pollutes the climate through the CO2 that is released.
Then again, there are some plants that love alkaline soils with a value of 6.5 to 7.5. Such plants, like lavender, rosemary, black salsify or onions, are happy to receive lime fertiliser. You can raise the pH value of the soil with algal lime or limestone powder.
Below you will find a list of the most important acid-loving and lime-loving plants. It makes sense to plant those plants with the same pH-value preferences together in the garden, so that you can make the soil alkaline or acidic with the appropriate additives. Spread these substances over the desired area and work them into the soil. Alkaline substances include, for example, algal lime, dolomite lime, wood ash, primary rock flour and basalt flour. For example, to raise a medium garden soil with a pH value of 5 to a value of 6.5, you need about 200 g of lime per square metre. Light sandy soils are limed less than medium and heavy soils, so that the availability of minerals and trace elements is guaranteed in these rather nutrient-poor soils.
To acidify the soil, use bark humus, leaf compost, needle compost, granite meal and sulphur bloom. The soil can also be acidified with special substrates such as Rhodovital (200 g per square metre).
Which plant likes which pH value in the soil?
Lime-loving spice and medicinal plants (alkaline soils):
Anise, horehound, bear’s garlic, mountain cumin, mountain mint, borage, dill, dost, tarragon, fennel, St. John’s wort, coriander, mullein, caraway, lavender, lovage, laurel, marjoram, oregano, parsley, burnet, rose, rosemary, sage, chives, mustard, sweet umbel, thyme, woodruff, wormwood, hyssop, lemon balm.
Lime-loving vegetables (alkaline soils):
Cauliflower, lamb’s lettuce, leek, chard, salsify, celery, onions
Spice and medicinal plants for acid soils:
Arnica, bearberry, bearwort, angelica, heather, bilberry, cranberry, peppermint, perilla, thyme, rosewort, sorrel, silver lady’s mantle, sundew
Ornamental plants for acid soils:
Azaleas, ferns, camellias, hydrangeas, rhododendrons
Lime fertilisation: With lime against heavy metals
Lime fertilisation also has the advantage that fewer heavy metals (e.g. cadmium or lead) are released from the soil by the plants, which is a problem with some herbs and vegetables. St. John’s wort, linseed, yarrow, mustard and sunflowers, for example, are known to readily accumulate the toxic heavy metals. Among vegetables, it is above all celery, chard and rhubarb where accumulations are possible. With a pH-value of 7-7.5, heavy metal absorption is blocked. Elevated heavy metal levels are mainly found in big city gardens. You can also minimise a dreaded fungal disease, cabbage hernia, by liming the soil to pH 7.