Understanding Acid And Alkaline Soil
The pH scale (from 1 to 14) is defined in very scientific terms as “the negative logarithm of the concentration of hydrogen ions in a solution” – in gardening and landscaping terms, the measure of acidity or alkalinity in soil.
A measure of “1” on the scale indicates an acid strong enough to dissolve anything it comes in contact with; one of “14” an alkali of similar destructive power; “7” is perfectly neutral. Pure water for example has a pH of “7” – being totally neutral.
The scale refers not only to soil (or soil moisture to be exact), but to any solution. Cosmetic companies for example develop their products to the pH equivalent of the skin.
Each step in the scale is 10 times that of the one before. A soil reading of pH5 is ten times more acid than a soil of pH6 and so on. Most soils fall between pH5.5 and pH8.6. Neutral or slightly acid soil in a range around pH7.5 to pH6 is considered ideal for most plants.
It’s important to know the pH of your soil before planting – kits to test your soil are readily available from most garden centres. Understanding the difference between acid and alkaline soil is critical to gardening success
Acid Soils contain very little lime (compounds of calcium). Alkaline soils have larger quantities of lime, in neutral soils the lime is in balance with other elements.
Calcium affects the availability to plants of other nutrient elements such as iron, which can be in extreme shortage in extremely alkaline soil. Plants that cope poorly with this will show signs of Chlorosis and need to be grown in acid soils. Other plants prefer lime and will like the soil to be alkaline.
The ideal is for the soil to be slightly on the acid side of neutral. This will suit most plants and those that need it can be given a small dose of lime to improve their health. It’s always best to group these plants together so a specific area of the garden can be treated.
It’s more difficult to make an alkaline soil acid though it can be achieved by adding an organic mulch – pine needles are quite acid and have proven efficient as an organic mulch for this purpose.
Adding sulphur can help also – as it weathers it makes the soil water acid and locks up some of the free calcium as insoluble calcium sulphate. The down side – sulphur will kill off microorganisms in the soil.
If your garden soil is alkaline and you want to grow acid loving plants it’s best to do so in containers or raised beds of lime-free soil. Water only with rainwater, as where the soil is alkaline the tap water is likely to be alkaline as well.
It’s a general (though by no means hard and fast) rule that soils in humid coastal climates tend to be acid. Soils in drier inland climates are typically alkaline.
The acidity or alkalinity of your soil can be easily quantified on the pH scale. Anything (on a scale of 1 to 14) less than 7 is acid and above is alkaline. Hydrangeas are nature’s indicator, flowering blue in acid soil and pink in alkaline soil. If they can’t seem to make up their mind and flower both ways your soil is likely neutral.
If you can’t be bothered to purchase a testing kit try the method your grandmother likely used – put a little soil on the tip of your tongue. If it is slightly sweet to the taste, your soil is alkaline, slightly sour and your soil is acid.
Want to know what to plant in acid or alkaline soil? Send us a message and I will be happy to offer some options.