Changing soil pH

If the pH of garden beds needs adjusting, organic gardeners have a distinct advantage over “chemical” gardeners, because mature compost has a pH of about 6.5 where all the major nutrients are freely available to plants, essential trace elements are available, and aluminium is locked out. Adding mature compost to topsoil when preparing beds will help to lower pH of alkaline soils, and raise the pH of more acid soils, as well as buffering plant roots from an unsuitable pH in surrounding soil. Where the amount of mature compost is limited, green manures and well-rotted manures will break down to add nutrients, microorganisms and humus to topsoil. Worm castings and other solid organic fertilisers provide nutrients in easily absorbed form. Garden beds should be prepared a month before planting to allow soil chemistry to achieve a balance.

To raise soil pH
In all acid soils, pH can be raised by the combined use of organic matter and the addition of calcium ions in the form of dolomite or lime.
Agricultural lime – (Calcium carbonate) is finely ground limestone (chalk). Mined limestone, i.e. not chemically treated, is a safe choice to raise pH in garden beds. Although it takes several weeks to have an effect, it is longer acting than other sources of lime, and can be watered in around plants. Agricultural lime can be worked into the top 15 cm of soil when preparing garden beds. It takes less lime to raise the pH of sandy soils than it does to change clay soils. To avoid an excess amount of calcium in soil, apply as recommended in the test kit, and test soil a month later.
I must say here that I have not found the application rate recommended by Manutec for “organic soils” to be accurate, if soils contain compost. It may have been calculated for soils where only manures are added.
Dolomite – (Calcium magnesium carbonate) is limestone with a higher proportion of magnesium than agricultural lime, and is applied in the same way. It is a good way to raise soil pH on sandy soils with fairly low organic matter content because both calcium and magnesium leach easily from these soils. In soils with high magnesium content, such as in South East Queensland, agricultural lime is the preferred way to raise soil pH.
Quick lime – (Calcium oxide) is made by heating limestone in a furnace to remove carbon dioxide. It is very caustic and unsuitable for garden use.
Hydrated or slaked lime – (Calcium hydroxide) is also known as brickies or builders’ lime because it is used to harden mortar. Hydrated lime is made by soaking quick lime in water to form hydroxides. It is more soluble and faster acting than agricultural lime, but its effects do not last as long. This lime can burn roots and should not be used on beds that contain plants. It should also be applied a month before organic matter and fertilisers or nitrogen can be lost through conversion to ammonia. Gloves and a mask should be worn when applying hydrated lime because it is very drying to skin and throat. Apply hydrated lime to the soil surface, and water it in.

To lower soil pH
Adding organic matter as compost, green manures, and animal manures, without including lime or dolomite, can be enough to adjust the pH of slightly alkaline soils because organic matter produces hydrogen ions as it decomposes.
Manure from cows, horses and sheep that have grazed on herbicide-free pasture can be used more liberally on alkaline soils. It has been calculated that 2–3 kilos of manure per square metre of bed area will reduce soil pH from 8.0 to 7.0. Manures release hydrogen ions as they break down, replacing calcium ions on the charged sites.
Elemental sulphur, sometimes sold as flowers of sulphur, will assist organic matter in reducing soil pH in more alkaline soils. Elemental sulphur is available from produce stores, and some nurseries. For soils with a sandy structure, apply at 35 g per square metre, or 100g per square metre for clay soils. Test soil after one month, to see if further applications are necessary.
Please note – Lime sulphur is a fungicide, not a soil conditioner.
Acidic fertiliser can assist when alkaline topsoil contains some organic matter and herbicide-free manures are not available. Multicrop’s Ecofish liquid fertiliser is registered by NASAA as an input for organic cultivation. The concentrate is very acidic and diluting it in water should modify the acidity, somewhat. It can be watered into the soil or used as a foliar feed for plants in alkaline soils.

27 thoughts on “Changing soil pH

  1. What an excellent blog. And a very understandable desription of pH and its effects on soil chemistry. I have been a gardener all my adult life, and now I am teaching pupils at secondary school about fertilisers and the soil. Your article is so good I will copy it to my students if you don’t mind.
    No-one has ever told me that the “p” in pH stood for potential!
    Best wishes.
    Mike Bryan
    Kaitaia College
    New Zealand

  2. Happy to help people understand the important points of successful horticulture, Mike. I hope you will teach your students the benefits of organic fertilisers over synthetic ones, too. Lyn

  3. A soil supplier recently provided me with a cubic meter of composted cow poo. I tested it’s pH with a csiro kit and it has tested highly alkaline >10. The supplier has advised me that cow poo is alkaline – is this so? I am concerned about using it in my garden (fruit, veges and natives).

    Hi Kathryn, cow poo is not alkaline – poultry manure is more alkaline than cow and horse manure. In fact, well-rotted cow manure can be used as a mulch and fertiliser for acid-loving plants. Well-made compost has a pH of around 6.5, which is perfect for growing most vegetables and exotics. It sounds as though they added too much lime to the mix. If my supplier told me porkies like that, I’d be looking for a more trustworthy supplier.
    I also would not add this mix to the garden, unless the soil was extremely acidic. You can recompost it with some fresh horse manure (which has a good nitrogen to carbon balance). As the horse manure breaks down, it will replace some of the calcium ions with hydrogen and bring the mix back to a more suitable pH as long as you don’t add lime or dolomite. – Lyn

  4. Hi, I brought some little blueberry plants today and want to plant them into a new garden bed. Unfortunately I didnt research first and have already dug through a good amount of chicken manure. I have also added alot of mushroom compost, but worried that the chicken manure will turn the soil too alkaline. Is there any way to lower the ph now that it has chicken manure dug through? Thanks, Joseph
    Joseph, blueberries grow best in a soil pH of around 5.5, but the mushroom compost is more likely than the poultry manure to cause any problems because mushroom compost can have considerable amounts of lime added to it before it is bagged for sale. It is impossible to guess how much the pH would be raised by the combination of these two soil amendments. The way to reduce soil alkalinity is with the application of elemental sulphur (flowers of sulphur) but you will need to test the soil pH to know how much to apply. – Lyn

  5. oh! I didn’t know that. well I’ve decided to change the plan. I’m keen to incorpate some permacultre priniples to my landscape and had the idea of almond trees and mass plantings of blueberries. I enthusiastically dug the beds with the compost and manure went and brought the plants and now researching and realise its not possible also the trees will provide too much shade. so now I will plant the blueberries in wine barrels with azalea mix (no mushroom compost ) and in sunny position. my next question is now I’m hoping the almonds will still be happy in the bed and is there some other edible plants I can plant arround the almonds to fill in the spaces. I hope to use the space and not waste it on ornamentles. its a totally new garden and I’m a new gardener with big dreams but many mistakes. thank you – joseph
    Joseph, you are right in thinking that the almonds would have shaded the blueberries too much. As far as planting food crops under the almond trees, I would suggest a perennial understory food plant rather than crops that need regular cultivation because almond trees are prone to producing suckers if their roots are damaged by mowing or cultivation.
    As the selection of plants will depend on where you live, I recommend Bill Mollison’s book “Introduction to Permaculture”. This book has suggestions for suitable understory plants in orchards, for different climate zones.

  6. Thank you for this very useful information! I finally think I understand my soil ph. Ive prepared a couple of vegie patches and did a soil test which shows it is too alkaline. I would really like to plant in as soon as possible (maybe 2 weeks until my seedlings are ready). I purchased some manutec sulphur. Do you know whether this is safe to use on an organic veggie garden? I figure its just safer to check on such commercial methods before use.
    If you are plannning to plant in a couple of weeks Leah, I’d mix a good trowel full of compost in each seedling planting hole. This will buffer the seedlings until the soil in the bed adjusts as it can take a month for pH changes to take effect.
    I didn’t know Manutec produced sulphur, but it should be ok if it is elemental sulphur without any additives, as it is a basic plant nutrient present in varying levels in all plants. We use elemental sulphur as a feed supplement for our miniature Shetland – (it helps him resist ticks) and it is available, quite cheaply, from most produce stores. So gardeners don’t need to worry if they can’t find the Manutec product. – Lyn

  7. I stupidly watered my roses with lime sulphur, instead of spraying the soil around them. As soon as realised what I had done, I satured the soil. Have I killed my lovely old roses, and worse still ruined my garden soil?
    Lindy, you may not have killed your roses but you would not have done a lot of good to the beneficial fungi in your soil because lime sulphur is a fungicide. I don’t agree with Yates advice to spray soil with fungicide. It increases the risk of your roses being affected by soil-borne diseases which means you will have to buy another Yates product to solve that problem. The fact your roses need lime sulphur means that they are not getting a fully balanced diet and they have weak immune systems – or your soil is too acidic or alkaline for the roots to absorb all the fertiliser you have given them. Give them a complete organic fertiliser, and water in some seaweed extract diluted to weak black tea strength. Seaweed contains the trace elements that plants need to strengthen their immunity to pest and disease attack – but don’t overdo the seaweed as it contains a lot of potassium and too much potassium can upset the balance of the nutrients they can absorb. – Lyn

  8. I am trying to grow a lemon tree down here in Tas, and I have it in a hothouse more for wind protection than anything else. The ph is 7.5+ and I’ve been using goat manure (which I have in abundance) and worm wee. The leaves are going yellow and the flower buds fall off and don’t fruit. I also have a lime tree which doesn’t have the same leaf problem but the fruit falls off when they are only a couple of mm big. Can you suggest a solution?

    There are a variety of causes for yellow leaves Fairybreadgirl, and it is the pattern of the yellowing on the leaves, and whether the yellowing is isolated to young or old leaves that helps identify the cause.

    The fact that you have asked your question in a post on soil pH indicates that you suspect soil pH could be the problem, and you may be on the right track. At a soil pH of 7.5, inadequate iron is available to plants and while we associate nitrogen with green leaves, iron is essential to the formation of chlorophyll – the green colouring in plants. At 7.5, manganese and zinc are also becoming less available to your trees, and these trace elements are important in citrus fruit production.
    Manures are generally rich in phosphorus, and too much manure can result in a deficiency of other nutrients. This may also cause yellowing and fruit drop. Fresh manures can cause fruit drop if they are supplying too much nitrogen to the trees. Manures are best applied after they have been composted. Uncomposted manures will acidify soil as the break-down releases hydrogen ions into the soil, but this is a slow process best used in areas that do not yet contain plants, and should never be used on potted plants. Worm pee or worm castings are the best fertilisers for these. However, without seeing the pattern of the discolouration, it is impossible to be more precise.
    Fertilisers are applied to citrus after harvesting time, so do not add any more fertiliser to your trees now. Water in some elemental sulphur at the rate indicated for your soil type on the pH test kit, and water trees thoroughly whenever the top centimetre of soil is dry. Normally, Tasmania is too cold for lime trees – be especially careful of your tree during the winter. – Lyn

  9. I would really like your advice. I purchased some “premium garden soil” and added a lot of straw, lucern, manure and bagged mushroom compost thinking I was making fantastic vege garden soil. I since tested it and found it to be very high ph of 9-10. I think I added too much bagged mushroom compost. I added agricultural sulphur about 2mths ago but it doesn’t seem to be working or is very slow. After reading some posts above it seems it might be too difficult to reduce the ph to a normal level as the ph is way too high and all the organic materials may be acting as a buffer against the sulphur too. Do you think the soil will gradually come down or should I get rid of it and start again? Should I add more manure and just plant in it using a potting mix and hope that it comes down? Please help. Thanks for your advice.

  10. Hi Terry, you are correct in thinking too much mushroom compost is the problem because mushroom compost often has lime added to it before sale. For future reference, it is always a good idea to test the pH before deciding how much to add to garden beds, as too much can be a problem, as a high pH means that phosphorus is not available to plants and this makes it difficult to get your veges growing . You could try adding some cow manure (quite acidic) to the garden bed. As organic matter breaks down it releases hydrogen ions into the soil. These displace the calcium ions from clay and compost particles and the pH gradually drops. If you don’t need all the bed in the next couple of months, you could grow a green manure grain in part of the bed area, slash it and dig it into the topsoil. As the green manure breaks down it will also help to lower the pH. – Lyn

  11. Hi Lyn, thanks for your advice. I originally put a lot of cow manure in it with the straw and mushroom compost etc. I made no dig style gardens and bought some soil to mix with the organic stuff. I have just tested the cow manure and it is off the scale alkaline. The brand is called Moo Poo. How can this be so alkaline too? Do they add lime to this as well? So frustrating. The only other thing I can think of is to find some acidic soil, either a screened topsoil or organic soil that is already acidic and mix it with my alkaline soil. Would this be ok? Thank Lyn.

  12. Terry, for cow manure to be alkaline it must have had a lot of lime added too it. Cow manure tends to pong a bit and the lime has been added to reduce the smell. It sounds as though some organic topsoil is the way to go for your problem. It will help to bring the pH reading down a bit. After you mix the topsoil through, if you want to get a few things growing soon, you can add a rockmelon-sized amount of certified-organic compost to each seedling planting hole before planting the seedlings, or add some to a furrow where you will be sowing seed. One of the marvelous things that mature compost does is buffer roots from an unsuitable pH in surrounding soil. Hopefully, by the time the seedling roots grow into the other soil the topsoil will have a more suitable pH. Amgrow Organix is a good brand of compost to use; I use it myself. It has a pH of around 6.5 – perfect for seedlings. – Lyn

  13. Hi Lyn, These comments explain the problems I am having after successful vege patches for many years. Thankyou to all who contributed. This winter I made compost differently; layering a variety of wet and dry materials with a sprinkle of different fertilizers (commercially prepared chicken or cow manure, dynamic lifter, blood and bone, and lime). I was trying to follow the steps used in the garden books and TV shows. The compost broke down much quicker than my usual method (chuck everything willynilly in the compost bin).

    In late winter I prepared the garden beds with the compost and bagged chicken and cow manures. A few weeks later I planted. It is now 2-3 months later and the plants have died or not thrived. I uprooted some tomatoes a month ago and the roots were minimal- no fine hair roots and the larger roots wound in a corkscrew pattern. I tried replanting, and used diluted seawood concentrate without success. Last week I tested the pH and then took a few samples to be retested independently. The soil pH in this bed was 8. I have only had the test kit for a year, but a year ago I tested some areas in the vege gardens and the pH was 6.5 Today I tested sections of the flower gardens. In some areas that I have never ‘touched’ the pH is 10, but most areas are between 7- 8 . A few areas are good with a pH of 6.5

    It seems I may have done everything wrong to increase alkalinity. Soils was trucked in 6 years ago after rebuilding the house. To improve the soil we added a small mountain of mushroom compost, and my regular applications of homemade compost, bagged manures, blood and bone and a sprinkle of lime. Every local nursery will tell me that my suburb has acid clay soils, but I have ignored the building works on my property and the imported soils and added a light lime dusting to the vege garden twice a year. On reflection, I can see that my vege harvest have been poor in at least one part of my garden patch each year, but ok where I had previously had the compost bin. Yesterday I added and watered in Manutec agricultural Sulphur (it is 99.9% sulfur) to the worst of the vege patch. What more can I do? Fixing the vege patch is a priority and then I can gradually work on the flowerbeds. I am wary of buying more commercially bagged cow manure and would prefer to leave my compost in case it is still. Someone has suggested a place where I may be able to get ‘used’ horse stable manure/straw. I have also read that adding sulphate of ammonia, gypsum or Yates Acitone may help. Would you recommend any of these to reduce the acidity? Thanks

    Don’t go down the suphate of ammonia, Acitone path Hobbit, as they will affect the beneficial microorganisms in your soil, and gypsum won’t change the pH. It is virtually impossible to adjust a high pH quickly with just sulphur alone. It is more effective when pH gets closer to 8.
    First of all, lime should only be added to soil where testing of soil pH shows a need for it as 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. 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.

    My advice is to not add any more lime and use the soil where the pH is 6.5 for current vegetable growing. Where the soil is between 7 and 8, I would grow some suitable legumes as green manures. (You haven’t said where you live so I can’t be more specific.) Legumes can usually handle a soil pH in that range, whereas some other green manure crops may struggle. Slash them as they start to flower, and dig them into the top 10 cm of soil. 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. 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.

    Where the pH is higher than 8, 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). By the way, relying on seaweed as the source of fertiliser for your vege garden is not adequate as it is a supplementary fertiliser only. See: http://aussieorganicgardening.com/?p=94

  14. Hi Lyn I wonder if you can help. I purchased soil to add to raised garden beds and was told by the supplier that I did not need to add anything to the soil as it was already added, so I planted my seedlings and seeds, but after six weeks there was little growth, so I tested the soil and found it was too alkaline, is there any way I can remedy this, or will I have to pull everything out and start again??? Hope you can help, Annie.

    Hi Annie, several other gardeners have had a similar problem recently, so I’ve answered with a new post. See: Soil pH is so important – Lyn

  15. I have just planted 18 blueberry plants into a garden bet that has a PH of 6. (I added a handful of rusted nails under each plant to help with iron absorption). I dug my compost (PH 7) into the beds before planting. I want to mulch with compost but used up all my compost digging it into the soil. I have some lucerne hay that I could use to mulch with but don’t know what PH it is likely to be. I could leave them un-mulched for the time being and wait until I have made some more compost but that will take months. I would use the lucerne hay and dried leaves and kitchen fruit and vegetable scraps to make the compost. Whilst I have the lucerne hay and dried leaves now the kitchen scraps will take weeks to become available. Would you put on the lucerne Hay now and then add the compost later or leave them un-mulched and add the compost in several months. I am in Australia and it is spring now so not too hot. Thanks, Anne
    Hi Anne,
    Compost should always be covered with mulch and not used as mulch because it dries out on the soil surface and you lose the benefits of the microorganisms in compost. I’d put the lucerne hay on as mulch to help keep the soil surface damp and prevent weed growth. If you apply it early in the day, it will also help to keep the blueberry roots cooler as mulch also works in a similar way to root insulation.
    Keep an eye on the growth of the plants and if they seem slow or show some leaf discolouration, rake back the mulch and add some old cow manure (that has not had lime added) to the soil surface, then re-cover it with mulch. As the cow manure breaks down, it will acidify the soil and as blueberries perform best when soil pH is about 5.5. However, the amount of compost you have added should help modify any pH variations as it helps the plants absorb the nutrients they need. – Lyn

  16. Hi Lyn

    Thanks for the excellent information. I too added mountains of mushroom compost to my garden only to find that my vegetables (everything except broccoli) failing to thrive with many plants showing signs of chlorosis. This growing season is a write off but in autumn I plan on collecting lots of leaves (oak, dawn redwood, chestnut etc ). A these beneficial in lowering the ph and should I compost them first or can I just shred them and use them as mulch? Thanks again

  17. stay away from the chestnut leaves, they can be toxic to other plants. Oak is good. Don’t know About the redwood. If you have pine needles or pine bark, that is considered best.

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  19. Thanks for all this excellent advice. I’ve only just started learning about pH and am trying to prepare my soil for the next lot of planting after a very average summer harvest – at least I know why now! Hopefully I’ll be better prepared in the coming year.

  20. Fantastic advice. I’ve been rotating five raised beds for vege gardening over the last four years with great success in the first and second & minimal after that. I started off as a veritable black thumb and it is progressively getting greener but initally my beds were created with a mix of our natural soil, organic compost and blood and bone and I added organic compost as needed. I knew very little to start with and this is the first year I am incorporating pH testing and the use of green manures. Two years ago ( the year my problems started) I added mushroom compost to all my beds as I said knowing nothing of pH or soils at that time I went on the recommendations of a friend who’d used it herself. I’ve had unending problems with my plants since, my tomatoes and capsicums ( I know they’re acid lovers so it makes sense) were pathetic and my tomatoes had what I thought were fungal problems two years in a row despite rotating the beds, my cauliflower and broccoli bolted and went to seed without ever growing heads, zucchinis never really fruited etc etc. I tested the beds two weeks ago and they were 8-8.5, added organic compost and they are still sitting at 8 today. I live in the coastal perth region of western australia and our natural soil is sandy, incredibly hydrophobic & full of limestone (eg we have an old limestone quarry not far from us) and having tested it today I can tell you the pH is 8.5-9 at least. I’m unsure of whether I should ditch the soil from the beds completely and start afresh or add the sulfur and green manure and organic matter. Any suggestions?
    No wonder you are having problems. Plants can’t absorb some of the nutrients they need for growth at that pH level. Lime is often added to packaged mushroom compost and manures because some people find the natural smell offensive. However, the liming effect of the mushroom compost should have dissipated by now and, as organic matter breaks down, it has an acidifying effect on soil. From your e-mail response, I think it is very likely that the packaged compost you used had a high pH. Please see my post Test compost pH before use.
    I would apply elemental sulphur to your beds at about 30 grams per square metre, and water it in, to help reduce the alkalinity. Try liquid feeding your existing plants with something like Multicrop’s Ecofish (stocked by Bunnings) at one teaspoon per litre of water fortnightly until plants are looking healthy again – it has a pH of 3.6-3.8 and should help to reduce the alkalinity. (It is also advertised to improve water-holding capacity of soil so that might help your other problem.)
    Or, if you know someone who has a horse (or cow/sheep) that grazes on herbicide-free pasture, you can make a manure tea by fermenting a shovelful of the manure in a bin of water with a loose cover. Dilute it to weak black tea strength as this can also help to reduce alkalinity. Don’t use packaged manures as these are very likely to have lime added.
    Try making your own aerobic compost over winter without adding lime or dolomite unless it develops a sour smell, and then use it very sparingly to keep the compost heap working. A well-made compost has a pH of around 6.5. After applying sulphur, you can also grow some peas as a green manure over winter. Or, you could try some wheat – it did well in the alkaline soils where we used to live. Chop them up when about knee high and dig them in to the top soil.
    Regarding your sandy, hydrophobic soil – adding 10 % bentonite to the topsoil is helpful in improving the water-holding capacity of sandy soils. After adding compost to your soil, always cover soil with mulch. This helps keep the soil damp, saves you water and allows all the microorganisms to work at full capacity.
    If your green manures don’t do well, you may have to replace the topsoil, but that is a last resort – and make sure you test the pH before you accept delivery. – Lyn

  21. Hi all even tho I have been gardening for many years I just can’t grow any vegetables since I have lived in my new home which we had built almost 8 years ago. My garden area is just a metre wide down each side of the house and any soil we have hear we had to have bought in as we have very hard solid limestone. My poor hubby spent a whole day jack hammering a hole to plant a apricot tree and I honestly don’t know how it manages to grow so well . Back to the. Vegetable problem after another disappointing year trying to grow a few Roma tomatoes I bought myself a test kit and despite us adding heaps of purchased smelly compost i my soil has come up on the test as high alkaline in the range of 8 to 9 I am really confused as I have had different answers from different garden centres on what to do about the problem

  22. Hi again forgot to mention I live in an country area 500 metres from a beach so we receive a fair bit of wind but despite not being able to grow any vegetables successfully the 4 year old peach tree I have produced 75 kg of fruit last year which we dried heaps and just as well as we had 6 peaches this summer
    I also despite not having a lot of room have a lemon (lots of lemons) apricot,plum apple and kiwi fruit vine

  23. A great big thank you for all the helpful comments ,questions and answers. We purchased some ‘compost’ only to find it had a pH of 10 and like all the other situations here are trying to reduce it. Very disappointing that the reputable “brand” company that we purchased the 3 tonnes of material from haven’t found time to respond to our request of finding a solution. Excellent site, keep up the good work. Happy gardening to all.

    Heather, according to their website, Jeffries compost is “Certified Organic to NASAA Standards 5125M.” The 5125M is probably their certification number.
    It is impossible to grow anything in pH 10 because the calcium has become insoluble.
    It is unforgivable that Jeffries, who advertise themselves as ‘compost, soil and mulch experts’, have not responded to you and sought to remedy the problem, as they would not retain their certification if they are producing products of such poor standard.
    As a certified organic farmer myself, I know that members have to conform to very strict guidelines and it is annoying when some companies fail to adhere to the organic standard as it undermines the public’s confidence in other organic certified or registered products.
    I would make a complaint directly to NASAA. I suggest you e-mail the General Manager, Ben Copeman: ben.copeman@nasaa.com.au and send a copy to their Certification Officer, Paul Fowler: paul.fowler@nasaa.com.au
    I hope you have this matter resolved very quickly. – Lyn

    http://www.jeffries.com.au/store/view/jeffries-organic-compost

  24. Hi Lyn, can you please advise how I should approach helping my magnolia Vulcan which each year starts off with beautiful green leaves but by November has yellow leaves with dark green veins on all leaves but much worse on new leaves. The leaves quickly turn yellow, before dying and falling off in summer. It is five years old and under planted with iris. I fertilised with a camellia fertiliser, added home made compost and then applied a mulch of lucerne a couple of months ago hoping this would help – it has a ph of 7, which I understand is too high for magnolia. My local nursery sold me a liquid acid fertiliser advising this would lower the ph and that I should see an improvement in leaf colour in a few days but if anything it is now worse a week later. They said this would be better than adding Epsom salts or iron chelate as it is a balanced fertiliser. My next plan was to add iron chelate to the soil but not sure if this would work if the alkaline levels are the problem. I saw a packet of sulphur in the nursery which is used to help acidity soil. Is this something I should look at using next?

    Hi Ro, the water supply where you live is probably not contributing to the alkalinity problem. These trees are not usually bothered by a lot of problems. If the leaves dry before dying, it could be that the irises are absorbing more than their share of water. If the leaves drop while yellow, it can indicate poor drainage. Forget the Epsom Salts. Magnesium deficiency shows up first in older leaves as this element is transferred by the plant to newer growth.

    It may be difficult to make the soil suitable for both bearded iris (pH neutral to alkaline) and your Magnolia (pH 5.5-6.5). Japanese irises would be more compatible with this tree as they like a more acidic soil. I understand that you didn’t want to disturb the Magnolia roots, but the flag iris rhizomes should sit half out of the ground and they are not difficult to ease out, so that you could grow something that enjoys a similar soil pH to your Magnolia. However, it is unwise to grow understory plants too close to tree trunks as the trunk tends to stay damp due to poor air flow and this encourages collar rot. That is why mulches should be kept at least a hand span from tree trunks.

    Although the pH is not extremely high, the leaf symptoms you described suggest an iron deficiency as iron availability is somewhat restricted at 7. Soil can contain adequate iron but it becomes locked out at an unsuitable soil pH and some plants can’t then absorb it from soil, I would apply iron chelates as a foliar spray. This should provide a better response if iron is the problem. However, without a photo of the leaf patterning, I can’t be positive that it is the deficient trace element.

    If you remove the irises, you can apply elemental sulphur to the soil. For loam soils, apply at the rate of 50 grams per square metre. Repeat application at 20 grams per square metre one month later, if you want to reduce it further. – Lyn

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