What is an alkaline soil? Soils that have a base rock of limestone – often found in coastal areas – are always alkaline. Textbooks define this soil property as pH 7 or higher, but in my experience, problems become really pronounced at around pH 8. When your fruit trees, roses, or palms don’t thrive despite watering and fertilising, it’s worthwhile getting a soil pH meter or a pH test kit to take some readings, especially if you live close to the sea.
So why are alkaline soils so difficult to garden in? For one, alkaline soils are typically very low in organics. And secondly, many nutrients, especially nitrogen, phosphorus, iron, manganese, and boron are not available to plants in high alkaline soils. For example, one common pH-related condition is ‘lime-induced chlorosis’, an iron deficiency caused by high pH levels. And of course, nutrient deficiencies weaken plants and make them more vulnerable to pest and disease attacks.
- Top up compost and fertiliser more regularly. The Black Marvel plant food range is high in nitrogen and in fast acting, rapidly absorbed high tech iron, which is ideal to keep leaves green and avoid chlorosis.
- Work on lowering the pH a bit by using fertilisers containing sulphur, like Richgro All Purpose Azalea, Gardenia & Camellia Fertiliser. Specially formulated to lower normal soil pH to acidic levels for plants like azaleas that need a low pH environment, these fertilisers will to exactly the same in a high alkaline garden: lower the soil pH around your plants over time, if you use them regularly.
- Grow plants that like high alkaline environments. Obviously, choosing local native plants makes sense, as they have evolved in the local soils and won’t mind the high pH one bit. Many Mediterranean plants also grow well in high alkaline soils.
- Or, if you just need certain plants to keep you happy, you can always grow them in pots
Birgit Daller is a horticulturist and garden designer, and the owner and creative head of Art of Green Landscapes & Design.