compostbins2 Composting recycles organic waste into a product that makes garden soil healthy. Mature compost is a dark brown, sweet-smelling material that can be added to topsoil.
There are two ways to make compost – aerobic, which requires aeration during the process, and anaerobic, which is a slow, rather smelly process. Mature aerobic compost can be produced in about 6–8 weeks in most areas of Australia.

How does compost make garden soil healthy?

  • Compost keeps soil more moisture-retentive, yet better-drained.
  • Compost provides food for earthworms that increase the depth of fertile topsoil by leaving digested food along their deep tunnels.
  • Compost provides food and a home for the many helpful bacteria and fungi that help protect soil from soil-borne diseases.
  • Well made compost has a pH of 6.5 – where all plant nutrients are fully available, and the perfect pH for the majority of plants.
  • Compost buffers plant roots from an unsuitable pH in surrounding soil.
  • It also insulates plant roots from temperature extremes so that soil stays cooler in summer and warmer in winter.
  • Compost contains all the minerals that plants, animals and humans need for good health, most of the soil’s nitrogen, plus lots of humus that forms the most stable part of recycled organic waste.
  • Humus and the minerals that plants need both carry a weak electrical charge. The electric charge holds the plant food minerals close to plant roots and prevents them from washing away in heavy rain.
  • Humus is able to control the release of trace elements needed in tiny amounts, and block absorption of poisonous metals in soil so that they do not end up in our food.
  • Humus stores carbon in soil for very long periods of time.
  • Humus, in compost, provides a habitat for a soil community of billions of beneficial bacteria and fungi that perform important functions.
  • Some bacteria species in humus make a ‘glue’ that is able to hold soil particles in a way that improves the flow of water and air through soil. This improves the structure of soil so that plant roots grow more easily. Strong roots help plants to resist the effects of drought and storms.
  • Mycorrhiza fungi in humus stick like hairs to the roots of plants, helping them absorb water and nutrients in exchange for sugars produced by plants during photosynthesis. Some 95% of perennial plants rely on mycorrhiza for healthy growth.

Test compost pH before use


It is wise to test the pH of compost (including homemade compost) before adding it to your topsoil because the pH of topsoil determines which nutrients are available to plants. Well-made aerobic compost has a pH of around 6.0 to 6.5. Unfortunately, I didn’t follow my own advice recently when, temporarily short of homemade compost, we purchased from our local Bunnings store some bags of Richgro ‘All Purpose Organic Compost’ registered as an allowed input by BFA.

I noticed that the transplanted seedlings where I had applied the compost were not making any growth although the rest of the vege patch was doing well. And, in the area where I had mixed extra compost through each planting hole, the seedlings had turned yellow – a clear sign that the soil pH was too high for the plants to absorb phosphorus for root growth, or iron. Iron (along with nitrogen and magnesium) is essential for chlorophyll (green colouring) formation in leaves that allows plants to produce carbohydrates for growth. Iron deficiency (iron chlorosis) starts in the tips of plants and the leaves of the whole plant turn yellow between the veins or, in extreme cases, white. In contrast, nitrogen chlorosis starts in the older leaves because, when nitrogen is in short supply, plants transfer the available nitrogen to the growing tips, and magnesium deficiency starts in the margins of older leaves.
I tested the topsoil of the affected areas and, finding it above 7.5, I then tested the Richgro compost and the reading was between 8.0 and 8.5. As the pH scale is expressed as negative logarithms, a reading of 8.0 means that the soil is ten times more alkaline than a neutral reading of 7.0. This week I received a question from a blog reader who is having difficulty reducing his soil pH. He had also used the same brand of compost, bought from Bunnings in WA.
Richgro’s response that their “internal records” of the batches in question show a pH of 7.1 and 7.2 and that the test kit I used “do not work well with high organic fraction soils or composts” blah, blah, is not consistent with the physical symptoms of the seedlings. I suspect the problem may have been caused by the type of timber used to produce biochar added to the product. Wood ash contains a very reactive form of calcium and, as the ‘Gardening with Biochar’ website states, Raising soil pH is biochar’s most important contribution to influencing soil quality.
* I must add that Bunnings were quite happy to accept return of the unopened bags of compost after I explained the problem.

To test pH
Add equal quantities of topsoil from the bed you are preparing and the compost you intend to use to a clean bucket. Mix thoroughly, then mix again. Test a sample of the mixture according to the instructions on your soil test kit. Ideally, for healthy growth of most plants, the pH reading should be between 6 and 7.2.
If soil pH is higher than this, please see: ‘Changing soil pH’
Also see: ‘What’s soil pH?’

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.

Soil for Magnolias

Recently, Anthea wrote to me about the problem of growing her Chinese Magnolia where soil is alkaline.
I was just reading your article on changing soil ph and was hoping you could give me an idea on how to fix a problem I have with my magnolia x soulangeana.
I bought the tree approx 4 years ago and, although it has grown well and has good leaf coverage, it has never flowered. My thoughts at first was that it was still too immature to do so, but I have since discovered that the ph level of our soil is very alkaline (we live on the side of an extinct volcano towards the coast). Over the last year, I have been trying to reduce the alkalinity of the soil with a general garden sulphur, but this does not seem to have worked. Once again, no flowers/sepals this year. I have checked the soil ph again, and it is still as alkaline as it was a year ago. I do not want to be as drastic as to uproot the tree and replant in an acidic base soil as I am aware that magnolias don’t take kindly to transplants but I am at a loss as to what else I could do. Do you have any suggestions as to how to rectify this problem? I live in Mount Gambier in the south east of SA. Any help/advice would be greatly appreciated.

It is a long job to reduce very alkaline soil with sulphur alone Anthea, and when soil is very alkaline plants can’t absorb the nutrients they need to produce flowers (or fruit, in suitable species).

I would try putting some aged cow or horse manure on the soil surface around the tree (keeping it well clear of the trunk) after a thorough watering, and covering the manure with about 5 cm of organic mulch to keep it damp. As the manure (and mulch) break down they will release hydrogen into the soil. The electrically charged hydrogen ions will replace the calcium ions in the soil and the pH will drop. It won’t happen overnight, but it will happen. One of the problems on dairy farms is that the soil becomes quite acidic due to the constant manure deposits.

Then, when the tree is in leaf, spray the foliage with organic seaweed extract diluted to weak black tea colour. Seaweed is high in potassium that plants require for good flower formation, and it also contains a range of trace elements that plants need but can’t absorb from alkaline soils. This may be enough to assist flowering next season. I would spray as soon as leaves form and again in early summer.

By the way, volcanic soils are usually rich in nutrients and Plants usually grow well in them when pH is adjusted. As the pH problem is likely to affect your entire garden, it might be worth your while to invest in a pH test kit and and make annual adjustments to your soil where necessary.

What’s soil pH?

Soil pH is the measure of the acidity or alkalinity of soil from an extremely acid pH of 0 to an extremely alkaline pH of 14. A soil pH of 7 is neutral, neither acid not alkaline. Knowing the pH of your garden soil is important because soil pH controls the availability of nutrients and the number of microorganisms that improve soil structure. Plants can only absorb nutrients as electrically charged “ions” that attach themselves to clay and organic matter ions with the opposite electrical charge. Depending on the level of acidity or alkalinity of soil, varying amounts of different nutrients can be taken up by plant roots. At some pH levels, nutrients can become bound to other elements, or to soil, and become “locked out” and unavailable 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 pH Table below) Most vegetables and exotics will remain healthy if grown in a pH range of 6.0–7.0, but potatoes and strawberries do best when pH is around 5.5, and Brassicas and beetroot require a pH close to neutral. However, few plants will survive when the soil pH is below 4.5 where major nutrients are strictly limited and trace elements become available in toxic quantities, or above 9.0 where calcium becomes insoluble.
On the pH scale, the “p” stands for potential, and “H” is the chemical symbol for hydrogen. The more acid your soil is, the more hydrogen ions in your soil. As hydrogen ions are replaced by calcium ions on the charged sites, soil pH rises. Just to make it confusing, the pH scale is shown as a negative logarithm so that the more hydrogen ions in topsoil, the lower the pH number. Because soil pH is expressed as a logarithm, a pH of 6.0 is ten times more acid than a pH of 7.0, and a pH of 5.0 is a hundred times more acid than 7.0. Adjusting pH without the buffering effect of decomposed organic matter is difficult.
The only way to find the exact pH of garden soil is to test it. Test kits for this purpose are available at most large nursery and hardware stores for around twenty dollars, or so. The Manutec Test Kit is quite economical to use, and Bunnings are selling them, at the moment, for $15.50. Testing involves taking samples of topsoil from across the growing area, and mixing them thoroughly in a bucket. A small sample of the mixture is placed on a supplied sheet and moistened with a liquid dye. The damp mixture is then dusted with barium sulphate, and the resulting colour matched to a pH range on the kit’s colour chart (–see image below).
***All garden soils should be tested at least annually, because exudates from plant roots and the decomposition of organic matter release hydrogen ions into the soil, replacing calcium ions and increasing acidity.

Black sections show level of availability to plants.

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.