Many flower garden favourites do well in acid soil. Rhododendrons, azaleas, camellias, magnolias, gardenia and daphne all fit into this category, as do orchids, which naturally grow in decaying bark and leaf litter. Fungal soils of the forest tend to be more acidic, so these flowering lovelies are worth considering for gardens that edge native bush for some contrasting colour.
The science of pH
On the pH scale where 1 is most acidic and 14 is most alkaline, most New Zealand soils lie within the range of pH 5-7.
Soil pH of 4.8-5.2 is considered strongly acidic, and 6.6-7.5 is considered neutral. Many plant species are less tolerant of strongly acidic soils. Nutrients are less available to these species if the soil solution is too acid.
The relationship of plants to soil is a complex beast, and there is a new school of thought when it comes to understanding soil. The authors of Teaming With Microbes state that each plant has an optimal soil pH, but say this has more to do with the plants' relationship to certain fungi and bacteria, which thrive at that particular pH, than the soil chemistry.
Luckily for me, strawberries, blackberries, potatoes and kumara are fine with acid soil of pH5, as are apple trees. But veges from the brassica family, onions, beets and lettuce prefer more neutral-to-alkaline soil from pH 6-7.5. That begs the question: how do we grow a wide range of plants and vegetables in the same garden if they have such different pH requirements?
One solution would be to rotate crops with similar preferences together. Add garden lime before planting brassicas, but leave it out for several years during your subsequent rotations. Plant primarily for your natural soil type, particularly when choosing the larger trees and shrubs in your garden.
Adding lime, dolomite, wood ash or crushed oyster shell to your compost heap will gently elevate your soil pH when dug into your garden and buffer plants from pH extremes.
Test the pH of your soil to get an overview and do some research to find out what plants will grow happily there. If your soil is more or less neutral, you'll be able to grow most things.
If plants are growing in less than their ideal pH you'll find they won't thrive, despite your good efforts. In very acid soils, aluminium becomes more soluble and can have a toxic effect on plant roots. You can request a soil test for this, too.
In the past I've dug in dolomite lime (for soils low in magnesium) or garden lime (calcium carbonate) in vege gardens, and often alternate both. But we will now be working on a larger scale, so I thought I'd do the research first to check out other options.
Many of Environmental Fertilisers' range are organically certified. Its "EF Recharge" has calcium and other soil goodies and is up to 10 times more biologically active than agricultural limestone.
We'll also be applying "EF Activated Carbon" which helps absorb aluminium and improve soil structure and moisture-holding capacity. I contacted its soil fertility adviser, who also recommended "EF Soil Force", which combines calcium, phosphorus, paramagnetic rock dust, fish protein, beneficial soil bacteria and fungi to help our fruit trees along.
Testing your soil
Kits to test soil pH are available at most garden centres. A fun DIY test for kids (or adults) is to dig a handful of soil, mix with distilled water to form a slurry, and add a ¼ cup vinegar. If it bubbles the soil is alkaline. To test for acid soil, add baking soda instead. If neither bubble, your soil is more neutral.
For a detailed soil test, check out a range of recommendations on the Environmental Fertilisers site, which includes Hills Labs (tests mineral reserves in soil), and Reams (which measures plant available nutrients). ef.net.nz
