Alkaline soil has a pH of above 7 and is most commonly found in chalk and clay soils.
Soils that have a base rock of chalk and limestone are always alkaline. So, for example, the gardens near the White Cliffs of Dover are likely to have alkaline soil in their gardens. One of the downsides of a chalky soil is that it is often a shallow soil, which increases problems of low fertility and gives insufficient root depth for larger shrubs and trees.
Over a period of time most soils become more acidic because: –
- Alkaline nutrients such as calcium are leached from the soil
- Alkaline nutrients such as calcium are used up by plants
- Rainfall tends to be acidic
- Micro-organism activity tends to release acids
Adding organic matter to an alkaline soil increase the nutrients and improves the moisture retentive properties. Wood ash can also be used, and is particularly good for soil around fruit trees such as apples.
The pH of a soil can be increased by adding lime in the autumn and winter. This is commonly done by gardeners who want to grow brassicas such as cabbage the following spring. Raising the pH reduces the risk of clubroot, Plasmodiophora brassicae, which is a soil-borne fungal disease. Liming the soil should be done carefully as adding too much lime can create imbalances which affect the ability of plants to take up nutrients from the soil.
Ornamental flowers and plants that prefer or tolerate an alkaline soil include:
- Ceanothus, Californian lilac, a wall shrub
- Dianthus, also known as cottage garden pinks
- Geranium, ie hardy border geraniums
Edible plants that prefer or tolerate an alkaline soil include the vegetables:
- Asparagus, which delights in soil pH7 and above
- Brassicas such as cabbages, broccoli, sprouts
Edible plants that prefer or tolerate an alkaline soil include the fruit: –
- Citrus fruit, oranges and lemons