Things about soil acidity and alkalinity
|Monty Don once said every garden that can grow this plant
should have one (acid)
I asked soil scientist Peter Williams what his three headline bullet points would be about soil acidity/alkalinity for amateur gardeners. His first was that usually it did not really matter! That’s quite something from a man who restored the North’s slag heaps from a sulphur induced pH of 1.5 by adding massive doses of lime! Peter did qualify himself by noting as his second point that there are several acid loving plants such as most rhododendrons that won’t grow on alkaline soils. As his great love is rhododendrons it is fortunate that his own silty soil is about pH 5.5.
More technically, Peter’s third bullet was that the most important thing about the pH scale for a gardener to understand was that it is logarithmic and that a fall of one unit represented an increased acidity by a factor of ten. For example PH 5 is ten times more acid than pH 6. Similarly if you raise an alkaline soil say from pH 7.2 to 7.3 it’s a greater rise in alkalinity than you might imagine.
|Most plants are versatile regarding pH - my captions today state’ ‘acid’ or ‘wide range’ (wide range)
My own four bullet points are
- You can usually learn much about the likely pH of your garden by walking around the neighbourhood and seeing what grows well.
- That pH testing kits are quite useful (Unlike most amateur soil testing paraphernalia) but be careful what you measure - do you want an average for the generality of a fairly uniform garden or do you want a precise pH for an unrepresentative spot?
- I have lived the last twenty years quite happily without carrying out a pH test
- Although it is easy to make your soil more alkaline it is barely worth trying to make it more acid because it is both difficult, short term and imprecise. Just grow what you can.
|When I moved in to Boundary Cottage I planted ‘an acid border’. I later found I could plant acid lovers anywhere in my garden (acid)
What is pH?
Although lemon juice might be pH 2 and vinegar pH 3 a UK soil is unlikely to be below pH 4. Nor is it ever going to be higher than 8.5 even when the mineral particles might be composed of chalk or limestone.(Which when crushed are lime). In contrast caustic soda is pH 11.
The traditional story is that if a gardener could chose his optimum pH it would be the magic pH 6.5 .At this level there would not be much he could not grow - but it’s far too high for blueberries!
pH stands for ‘concentration of hydrogen ions’ Pure water is pH neutral at pH 7.
In technical terms pure water has an equal number of hydrogen and hydroxyl ions. As pH rises above pH 7 hydroxyl ions will be in the ascendancy.
Considering water a little further it might be noted that ‘hard water’ is alkaline because it contains calcium (and other) ions. By way of illustration when calcium compounds dissolve, calcium ions displace hydrogen ions, thereby increasing the proportion of hydroxyls - ergo pH rises.
If I go further it will hurt my head. We gardeners don’t really need to know very much more.
What we do need to know is the significance of varying pH to our plants.
(Note for nerds. Our school chemistry books say that the pH scale runs from 0 to 14. In truth with exceptional chemicals it can go higher or lower)
|In the pink (acid)
Availability of nutrients
Although the effects of pH on plant growth might be very subtle perhaps the greatest significance to we gardeners is on the solubility of nutrients and their availability to our plants.
For example positively charged iron and manganese ions are freely available when soil water is acid. As pH rises these nutrients become ‘locked up’ in the soil and exist in insoluble forms. For plants that have a high iron or manganese requirement when the soil becomes less acid these nutrients become less available and plants become deficient.
The other side of the coin is that aluminium (not normally a nutrient) when it becomes too freely available at very low pH becomes toxic.
pH also effects how soil nutrient components chemically combine. For example phosphate tends to form insoluble compounds with calcium when a soil is alkaline and similarly with iron when a soil is too acid. Phosphate’s maximum solubility - and therefore availability - is in the range pH 6 to 7.
|Flag iris do not do very well in my own pH 6 soil … but I don't know whether it is because of the pH! (wide range)
Changing your soil pH
I am reluctant to write much about this as there is so much information on the net and I have written about it before. (Although I can’t find where). Suffice to say that pH is easily raised by adding various forms of lime. Professionals tend to use ground up chalk or limestone and amateurs hydrated lime - perhaps because the latter is slightly quicker acting and can be applied to the soil surface - often in Winter - and will bring about fairly rapid change. Crushed chalk and limestone is particularly suitable when making up composts when it is intimately mixed with the bulky ingredients.
|Hydrated lime has a higher liming value than chalk or limestone and is quicker acting but might be best left at the builder’s merchant!
|My lime outlives its paper bag!
Making a soil more acid is rather hit and miss. You might use sulphur or ‘acidifying’ fertilisers such as ammonium compounds or rarely, exceptionally, perhaps foolishly and precariously actual acids! You might conclude I am not very keen on this last one.
|Variegated pieris in its ‘lily of the valley’ phase (acid)
|In the dry cold east camellias are pretty dodgy. I got lucky this year (acid)
If you have an alkaline soil it is perhaps best if you want to grow acid loving plants such as pieris and camellia to grow them in tubs and other large containers using acid materials such as peat and bark based compost. For smaller plants raised beds have possibilities.
The most successful - but not perfect - method to grow acid lovers in alkaline soil is not to try to change the pH but to use those fertilisers described as sequestrenes or chelates and (usually) apply them as foliar or liquid feeds.
Four final bullet points
- Although most plants grow without difficulty at higher pHs it is difficult to find examples of plants that must have levels above pH 7
- My pictures today are mainly plants that must have acid conditions. Those not so indicated are examples of the majority of plants that will grow in any normal garden.
- If your soil is on chalk or limestone it is a waste of time trying to reduce its alkalinity
- If your soil is pH 7.5 or higher some everyday plants might start to have problems
|pH 7 is getting high for citrus - even so (wide range)
|I guess this very fine York garden is about pH 7 (wide range)
I can perhaps tell you more about pH by telling you some stories
- When I was a lad in industrial Yorkshire, allotment vegetable growers would find after a few years that the effects of sulphur in soot from the polluted atmosphere and the use of the then popular acidifying fertilisers such as sulphate of ammonia would make the soil acid and it was common practice to apply lime every four or five years. Usually before the brassicas in any rotation. My advice these days would be only to lime allotments if a pH test shows the pH to be falling. Perhaps below six? Many gardeners sensibly lime clubroot infested soil to over pH 7 which enables them to grow excellent brassicas. It s not that the brassicas need to be alkaline, its just that the disease hates it!
- When forty years ago some commercial soft fruit growers started to use minimum cultivations made possible by the then new herbicides some were caught out by the soils becoming more acid and requiring lime. I did not actually know how general this was as an effect of minimum cultivation but it did encourage me to suggest it was potentially a benefit of minimum cultivation for acid loving plants. My further research finds no-till farmers often do find the top two to three inches of their soil becomes acid. It is due to the bacterial nitrification of their ammonium fertilisers near the soil surface and is easily corrected by normal levels of lime.
- When a former colleague moved into his new house on an alkaline York terminal glacial moraine soil he could not understand why his plants were so sick. That was until he found that the delusional former occupant had applied lime every year! I wonder if his soil attained pH 8 - perhaps even more?
- A correspondent on my post about iron sulphate to control moss in lawns said that in the US they worried that this would make the soil acid (which is true). That is great for me as I am besotted with my fine leaved fescues that love soil acidity. The thought does occur that not all lawn grasses around the world like acidity nor does our own ever popular ryegrass need it. There can be situations to use lime on a lawn!
- Last week I received an advert from the Hardy Plant Society for two new peat based Levington composts. I recommend Levington as an excellent compost (especially when peat based). Their stated pH was pH 4.8 and pH 5.5. This seems very low compared to a mineral soil for growing the target plants. I was reminded that when peat based composts superseded John Innes compost they were fairly acid too. Clearly the optimum pH for best nutrient uptake will sometimes vary with the growing medium.
- I think a fact that it is often overlooked is that lime apart from raising pH is a source of the nutrient calcium. Arch rhododendron grower, friend Peter sows his rhododendrons in pure peat to which he has added chalk at a tiny 1 gram per litre. (But don’t do this if you have hard water).
- When we moved into Boundary Cottage we were delighted to be able to grow acid lovers on the pH 6 soil. The only evidence of any planting by the previous owner was a very sick pieris! He had unwittingly found the only alkaline place to plant it - at the base of a mortar rich wall!
|Daphne ‘Everlasting’ is very well named regarding its long flowering and thrives on most soils (wide range)
|No good trying Hamamelis if your soil is not distinctly acid - and it gets rather big in a tub (acid)
|Wonderful cherries thrive on chalky soils - but they ain’t half good on Cathi’s wet acid sand (wide range)
Read about Peter's reclaimed slag heap 35 years on