Soil pH is so important

I’ve had several e-mails recently from gardeners who have used purchased soil or organic fertilisers and found that their plants were sickly or not growing because they were growing in soil that is too alkaline.
Soil pH (acidity or alkalinity of soil) is extremely important because determines which nutrients are available to plants. All the major nutrients are only freely available to plants within a narrow soil pH range of 6.5 to 7.5, where essential trace elements are also available, and aluminium is locked out.
See What’s soil pH?

Lime should only be added to soil where testing of soil pH shows a need for it. Mushroom compost and poultry manure (including dynamic lifter) can be quite alkaline, and some suppliers are now adding lime to bagged cow manure and horse manure because some customers objected to the smell, (usually caused by nurseries leaving bags sitting in hot sun). And, only agricultural lime (calcium carbonate) should be used when liming is necessary as hydrated lime can burn plant roots and reduces nitrogen levels through conversion to ammonia.
When purchasing soil, always check the pH before adding fertilisers or lime to it, and always test the pH of your compost before adding it to the garden (well-made compost has a pH of around 6.0-6.5). If soil pH is not higher than 8, you can reduce the pH using elemental sulphur (according to the instructions on the pH kit) and dig into topsoil a 5cm layer of well-made compost, which will buffer plant roots from the unsuitable pH. Or, grow a legume as a green manure while waiting for the pH to reduce. Slash the legumes as they start to flower and dig them into the top 10 cm of soil. See Soil pH too high? , also Changing soil pH.

If the pH is higher than 8, it is not easy to reduce it with sulphur alone. As organic matter breaks down in soil it releases hydrogen ions that will replace the calcium ions in the soil, and gradually reduce the pH. Cow, horse or sheep manure (but not poultry manure) under mulch will also reduce the soil pH gradually as it breaks down. However, be very cautious where you source manures and mulch as more farmers are using herbicides that remain active in manures and mulch materials (except lucerne and pea straw) until they are broken down by soil bacteria. Test soil every 6 weeks after digging in the green manure, and you can use it for general vege growing when it gets below 7.5.

7 Replies to “Soil pH is so important”

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  3. HI I have your book and I love it! most useful gardening book I have. Got so excited to see someone actually addressing alkaline soil and presenting some solutions instead of saying “its hard to do but you can just chuck on some sulphur.”
    I went outside today to get my organic bagged sheep manure ready to go, but tested it in case and lo and behold its well above 8 closer to 10 actually. should I just try green manures like millet, mustard and marigolds I have problems with root knot nematodes in the same bed. Thanks

    Hi Mel, I am pleased you are finding my book helpful. It is a shame that suppliers feel obliged to add large amounts of lime to their products because some people find the smell of manure from herbivorous animals offensive. IMHO, such people should be restricted to growing pot plants because this recent practice causes problems for genuine organic gardeners.

    Regarding your root knot nematodes (eelworms), I’d skip the marigolds because they are effective against northern hemisphere eel worms and ours are different. However, the other green manures you mention are good and you can find more information in this article: Root knot nematodes.
    The best thing to do with your high pH sheep poo, is get some straw (high carbon content), dampen them down and compost them together (without adding any lime). As the manure is broken down it produces hydrogen ions that replace the calcium ions and thereby lowers the pH. If kept damp, the resulting pH should be around 6.5 – perfect for most crops. The microorganisms in the mature compost will also help keep nematodes under control in your garden. – Lyn

  4. Hi Lyn, The soil we have is from an excavated site. We have added an organic compost to it and tested the ph. which comes in at 8.5. The drainage is poor, under 20 mmph.
    Are you able to tell me if I add a mature cow manure will help reduce the Ph and at what ratio? Also, our local nursery does a veggie mix and it has mushroom compost in it. Is this bad? I have been reading your comments and I don’t know if i should buy it?

    You haven’t said what area you live in Freda. Is it a clay soil? If your soil is poorly drained gradually increasing the organic content will improve drainage and structure among other benefits.
    As I have said in the article, mushroom compost is usually quite alkaline and lime is now often added to bagged manures because some people objected to the smell. I’d avoid these at the moment because adding them to your soil will not do much to lower the pH. I would rely on green manures to provide organic matter and help reduce the pH but I need to know what climate and soil you have to recommend a suitable green manure at this time of year.
    If you are anxious to get started on gardening, you could buy some of the veggie mix and grow some veges in large pots or even foam boxes if they have plenty of holes in the base and you line the box with a couple of sheets of newspaper. – Lyn

  5. Despite the passage of time your information above seems to be a secret, at least judging by the perplexed or nonsensical responses I’ve received virtually everywhere else when trying to find out why my lemon tree wasn’t growing, & in fact showing signs of iron deficiency (& subsequently responded quickly to sprays of iron chelate) after I surrounded it with commercial cow manure which (when I, baffled at my soil now reading ph 9 when I thought to test it again, after previously reading ph 7.5) turned bright purple when tested with 2 commercial testing kits (the manure that is). As far as I can see, bright purple must indicate a ph of at least 12 or even the maximum of 14. This max doesn’t allow citrus or anything much else to grow. This problem has caused me huge waste of time, money & effort including physical effort – & simply bafflement & frustration although I must admit I was starting to find references to the addition of lime to manure for the smell & was wondering about this as a cause. Also whether elevated levels of sodium – in ??sodium chloride?? could also be a factor (I’d read about high sodium producing a high ph)
    Many thanks & apologies for my rambling. I will now take this matter further with appropriate bodies.

    As explained in Yellow or pale leaves in citrus it is more likely the lime added to the cow manure is the cause of your problem. You have not said where you live but, if in an area where irrigation water has a high saline content, it is more likely that your tree would show other symptoms. Plants absorb water through osmosis, where a strong solution draws a weaker solution to it. Where water has a high salt content, moisture is drawn away from the roots. A pH of 12 cannot support plant growth. I would remove the cow manure and compost it with other ingredients before using it around plants.
    This website explains the impact on saline excess: Impacts of salinity

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