What is an alkaline soil? Alkalinity and acidity are opposites on the pH scale. An alkaline soil has a pH measurement that is above 7 or neutral, while acid soil is below 7. pH influences the ability of plants to access the mineral nutrients that are contained in the soil. For instance, citrus trees are less able to take up the iron that they need when the soil is alkaline, and this shows up as yellowing between the veins of their leaves. Most fruit and vegetable plants thrive in a near-neutral soil (pH 6-7), so if your soil is more like 8.0 or 9.0, the pH needs some amendment before the other steps you take to improve your soil are likely to be effective.
Places where soil is likely to be alkaline include coastal areas with underlying limestone and building or renovation sites where cement, lime, rubble or other building waste may have been dumped and then hidden with a thin layer of soil or mulch prior to sale – probably one of the reasons why standard new-house landscaping often starts to look shabby so soon after sale! These materials increase the pH of the soil and are best removed if possible.
How to test – pH can be tested with an inexpensive soil pH test kit, readily available from hardware and garden centres. There are simple instructions in the pack and a test only takes a few minutes. These kits are more accurate than pH ‘probes’ that are poked into the soil, not as precise as a comprehensive test from a soil lab, but should be within half a point of the result from a lab test. After mixing a soil sample with the indicator dye and dusting with the powder provided, it only takes a minute for the colour to change so that it can be matched against the pH chart in the kit. Anything in the purple range (or the grey of 7.5) is alkaline.
How to treat – although pH determines how readily plants can access the soil’s nutrients, the precious humus in the soil (spongy, dark, moist, colloidal substance composed of broken-down organic matter) forms a kind of buffer against extremes of pH by ‘unlocking’ and dissolving nutrients, converting them to a form that’s ready for plants to suck up. So ‘organic soils’ (i.e. those with a high content of compost, manure etc) can overcome unfavourable pH to a certain extent, as long as they are treated nicely (e.g. kept moist and protected with mulch so that all the micro-organisms have a comfortable home). For this reason and so many others (water retention, improved soil structure, recycling and general plant health), compost well and often, regardless of what else you do to manage pH.
The most direct way to neutralise soil alkalinity is to add acidic substances. Agricultural sulphur is the most commonly used one, and it is acceptable in organic systems, but needs to be handled with care and kept away from children and pets. It’s available in powder or granular form and the amount required depends on the soil texture (sand, loam or clay) so follow the directions carefully. Although not water-soluble, it needs to be watered into the soil well. It can take several months for the effect to be evident, so re-test before treating again. pH can also be very stubborn to change (e.g. when underlying limestone maintains soil alkalinity) so you might need to repeat the treatment each year or so – but test first.
Another acidic option to help neutralise soil is pine mulch – e.g. composted pine needles, pine bark or chips. However, this can inhibit the growth of some plants so I’d suggest testing on a small patch of the garden first. Acid-loving plants such as blueberries are said to respond well to mulching with old pine needles. Be aware that using fresh timber mulch products (as distinct from composted pine) on the garden leads to ‘nitrogen draw-down’, where the nitrogen in the soil is tied up in the process of breaking down the wood for a while and is therefore less available to plants. As nitrogen is so essential to plant growth (especially leafy green vegetables), you would need to supplement nitrogen, e.g. by adding blood and bone and/or extra manure underneath the mulch. The breakdown of wood products engages beneficial fungi and can contribute to fantastic humus development over time, but in vegetable gardens the temporary lack of nitrogen is probably not worth it – better to opt for composted products there and save any fresh wood mulch for paths and native garden areas where nitrogen is not so crucial!
Until your pH is optimised, what can you do to help your plants along? Well, although plants’ roots are the main means of taking up nutrients, their leaves can also absorb a certain amount. So spraying with foliar supplements (e.g. seaweed extract, fish emulsion, worm juice) can help to compensate by supplying some nutrients and trace elements that they might be struggling to find in the soil. Liquid fertilisers are quick but short-acting compared with slow-release fertilisers in the soil, so apply little and often (e.g. diluted to colour of weak tea or as per pack instructions, once a fortnight). They also offer the bonus of helping to strengthen plants against pest attacks.
How about just importing garden soil? You could certainly try this, especially if you’re contemplating a contained garden area such as a raised or wicking bed – BUT don’t assume that commercial garden soils are neutral and better than what you have already. Check a sample first – feel it, smell it and test its pH. Sometimes they turn out to be more alkaline or more sandy than the soil they are supposed to improve. Giveaway soil from excavation sites can be a nightmare too – often heavy clay or lifeless sand with weed seeds – so check this carefully too.
Steer clear of… mushroom compost, large quantities of chicken manure, dolomite and agricultural lime! All these products are often promoted in gardening programs, and they have their place – in acidic soils, especially prior to planting crops like cabbages and broad beans that do well with a higher pH than the crops grown before them. But not in a soil that’s already alkaline! This bit often gets omitted from general product advice.
And finally – as nature always fills a niche, there are plenty of plants that have adapted to love an alkaline soil. Obviously these will be the best to start out with while you’re working on the rest of your garden. In SA, we have a terrific range of native plants from areas like Yorke Peninsula which have grown forever on limestone and therefore are good at coping with alkalinity. The State Flora catalogue is a great place to check for examples. In the vegetable garden, you might have more success with legumes (peas and beans) and brassicas (cabbages and their relatives) while the soil is on the alkaline side.