We know different plants require different conditions. Sun or shade. Water-holding or well-draining soil. Lots of nutrients or a key few. But for each complex plant personality there’s a basic starting point to giving it the best chance to thrive: pH.
Measuring pH – which stands for “potential of hydrogen” – has become an important tool in a wide variety of industries, from medicine, biology and agriculture, to food science and oceanography. Using the pH scale we’ve learned our bodies are slightly alkaline (a higher ranking on the scale), water sits around neutral (the middle), and beer is more acidic (lower ranking).
The pH scale is a relatively new measurement tool and its invention is thanks to beer. In 1875, Carlsberg founder J.C. Jacobsen set up the Carlsberg Laboratory in Copenhagen, to aid his tinkering with concoctions of fermented grains, malts, hops and yeast, in a pursuit of brewing beer at the highest quality. But it wasn’t until 1909 the brewery’s chemist, Søren Peder Lauritz Sørensen, introduced the concept of a scale to measure the acidity or alkalinity of a substance containing water. The Carlsberg brewers found they were able to use this “pH scale” to better understand the chemistry of beer and further fine-tune its flavour and taste, and in doing so, revolutionised modern brewing. After the scale was embraced by the wider scientific community, Sørensen was nominated for a Nobel Prize for his efforts.
How pH works in soil
Angelo Eliades is a Melbourne-based gardener. He works part-time at Bulleen Art & Garden as a produce and permaculture specialist and runs workshops in pruning, natural pest control and small-space gardening. He also advises on edible produce and medicinal herbs.
Eliades says if soil in a garden is too acidic or too alkaline for a particular plant’s taste it can’t access key nutrients. “Nutrients can be present in your soil but they might not be available to the plant because of the soil’s pH,” says Eliades. “But [by altering the pH] you can adjust conditions to create the environment the plant naturally grows in.”
The pH scale runs from 0 to 14, where 0 is extremely acidic and 14 is extremely alkaline. Neutral pH is around 7 which is the general conditions needed to allow the soil’s macronutrients (such as nitrogen, phosphorus and potassium) to thrive.
“Most plants are usually comfortable around a neutral pH of 7, give or take,” says Eliades. “So 6.5 to 7.5 is considered neutral. The majority of plants will grow best in that range. If your soil is more alkaline, you’re going to have iron deficiencies. In highly acidic soil, phosphorous availability usually disappears, as does calcium which is used in the structure of the plant, and magnesium, which affects the greening of the leaves. So [depending on the plant], if you have soil that is swinging dramatically in either direction, the plants will start showing symptoms of nutrient deficiencies.”
How to test pH at home
There are two methods for testing soil pH: a fancy chemical lab test – accurate but expensive – and a simple, if less stable, home-testing kit.
Such kits will only set you back around $20 and are available from most nurseries and hardware stores. Eliades says it’s important to approach the results only as a guideline. “You can come to a reasonable indication but it’s never 100 per cent,” he says. “People make the mistake of taking them as gospel and getting too preoccupied with the exact pH.”
So what can you grow?
While it’s true most fruits, vegetables and flowers will grow well around that neutral pH range of 6.5 to 7.5, some plants will perform better in more acidic or alkaline conditions.
For example, you’ll likely harvest higher yields of strawberries, blueberries and currants in slightly more acidic soil (5.5 to 6.5), while sunflowers and brassicas prefer soil that’s slightly alkaline (7.5).
“Potatoes will take quite an acidic soil, while asparagus will take quite an alkaline soil,” says Eliades. “Most berries and fruit trees prefer things slightly on the acidic side. But cranberries are ridiculous, they grow in 4.5.” Azaleas, camellias, rhododendrons can grow alongside those cranberries in very acidic soil.
How to alter your soil’s pH
Shifting your soil’s acidity can be achieved with compost (which is also excellent for building up those health-giving nutrients and encouraging essential microorganisms) or adding pine needles or oak leaves as mulch.
You can also add a handful of store-bought sulphur (for increased acidity) or dolomite (for alkalinity). With these methods it's vital to understand a little goes a long way. And you’ll need patience – the shift will be gradual.
“It takes about six months to take effect,” says Eliades. “If you’re adding sulphur, after six months, the pH will only shift by 0.2, 0.5 or 1 pH level. What most people don’t realise is the pH scale is logarithmic, so a shift from pH 7 to pH 6 is 10 times more acidic. So if you shift your soil 0.1, 0.2, 0.8 that’s a lot. Nature prefers a steady balance, and there are mechanisms in place to regulate change. You can’t rush chemistry.”
This article is produced by Broadsheet in partnership with Carlsberg. The pH scale may have been developed in a humble beer factory, but its use and importance to the natural world is infinite. Watch how climate researcher Katherine Richardson applies it to charting the presence of C02 in the world’s oceans in the video below.