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.

Herbicide warning

Before purchasing mulches or manures for your garden, ensure that they don’t come from pastures treated with a broad-leaf weed killer.
A relatively new herbicide (weed killer) ingredient, aminopyralid kills broad-leaf plants by disrupting plant cell growth. It does not affect grasses, but can remain active in them, and manures from animals that eat sprayed grasses, until it is completely broken down by composting or soil microbes.
Some readers may remember that, 16 months ago, I drew attention to the devastation this herbicide caused in UK gardens, rendering garden beds unusable for almost two years, after contaminated pasture was used as mulch, or uncomposted manures were dug into garden beds. UK residents were advised not to eat any produce from affected garden beds.
Despite extensive problems in the UK, and the fact that aminopyralid is highly mobile in soil, our Australian Pesticides and Veterinary Medicine Authority (APVMA) has approved herbicides containing aminopyralid under the names; ‘Hotshot’, ‘Starane’, and ‘Grazon’.
The APVMA Manager – Public Affairs told me that the APVMA requires products that contain aminopyralid to include on the label the following instructions: The herbicide is not be used on land to be cultivated for crops for up to 24 months. The herbicide is not to be applied to crops or pastures, which are intended to be cut for the production of compost, mulches or mushroom substrate to be used for susceptible crops or plants, as straw, hay or other plant material treated with this herbicide may damage the plants. Manure from animals grazing treated areas or feeding on treated hay is not to be used for growing broadleaf crops, ornamentals or orchards as injury to susceptible plants may occur.
However, these warnings do not help the many gardeners who are unlikely to ever see the herbicide label and, completely unaware of any potential problem, may inadvertently purchase contaminated products.
To test manures and mulch for herbicide residue, see: Manure and mulch warning update
If you are unfortunate enough to have beds affected by this herbicide, click here for treatment information.

Horse manure

Magic1One of our SA readers wants to know what to do about the weeds that sprout from their horse manure pile, as she is concerned about spreading the weeds through her garden. She also has a problem with millipedes. Interestingly, manure and millipedes have a relationship because millipedes feed on decaying organic matter and they can lay their eggs in faecal matter. Millipedes are related to slaters. See Slaters and earwigs for controlling them in the garden.
Horse manure is often the easiest manure to obtain close to metropolitan areas. We have found horse manure to be a good source of plant nutrients and our miniature Shetland, Magic, works 24/7 to keep up supplies. Small amounts can be fermented in a bucket of water, then diluted, to produce a fertiliser tea for plants that need a boost.
But, adding uncomposted horse manure to the garden can encourage millipedes, slaters and earwigs. This is more common if you use sheet composting for the manure, a process of spreading a layer of manure on an unused garden bed, dampening it and covering it with mulch.
The best way to use fresh horse manure is to put it into the compost heap as it is a good source of nitrogen and generates a lot of heat, but there is a big difference between an active compost heap and a pile of horse manure. The manure should be mixed with dry ingredients such as straw, mulch, shredded newspapers, or dry leaves to create a good nitrogen to carbon balance. The heap should also be turned regularly to aerate the heap because aerobic bacteria require nitrogen, moisture and oxygen to work efficiently. When the compost heap is turned, newly germinated weed seeds get turned into the mixture and provide more organic matter for the bacteria to feed on. In its early stages, the heap should generate enough heat to kill off pathogens and seeds. As the composting process continues, the heap reduces in volume. As it gets cooler, you will occasionally see earwigs, slaters or millipedes in the mix but they are helping to break it down so don’t spray them with anything. When the organic matter reaches a favourable stage, earthworms move into the heap if it has contact with soil, and they digest the decomposing organic matter and turn it into worm castings. The final product, ready to be used on garden beds, is about one quarter of the volume of the original heap, friable, very dark brown in colour, and has a earthy, rainforest smell.
If you just leave horse manure in a pile to break down, it will tend to pack down and anaerobic (without oxygen) composting will occur. This is much slower, and can generate unpleasant smells. A lot of the nitrogen can be lost to the air and other nutrients can leach away when it rains.
To get horse manure to work even more quickly in a compost heap, tip it onto a hard surface and mince it a bit with the edge of a spade, because bacteria only work on the surface area of the ingredients. Producing more surface areas to feed on by chopping ingredients will greatly speed up the process.

Bacterial wilt

The ABC’s Gardening Australia program on June 20th told gardeners how to identify plants affected by bacterial wilt but, unfortunately, did not tell them how to eliminate the disease.
Bacterial wilt (Ralstonia solanacearum) is a serious soil disease that can, like Fusarium and Verticillium wilts, spread throughout the garden on boots, gardening tools, and infected plant material and seeds. It grows best in temperatures of between 30 and 35° C. and the bacterium requires both heat and moisture to multiply. Consequently, it is more commonly found in areas with wet summers.
It can affect the entire tomato family, the banana family (including Heliconia), onions, papaya, ginger, mung beans, cashews and peanuts. Like the fungal wilts, it affects the water-conducting tissue of plants and causes rapid wilting. Diagnosis can be determined from a section of stem pruned from near the base of a suspect plant. Immediately after pruning the stem, suspend it in a glass of clean water for several minutes. Milky threads will begin to leak from the stem and the water will quickly become white if Bacterial wilt is present.
Remove all plants, tubers and weeds from infected beds and destroy them, or dispose of them in a sealed plastic bag. Any remaining plant material can infect future crops of susceptible varieties – do not compost this material.
Raising beds to 20 cm or more can help deter this disease. After working on affected beds, wash boots and garden tools and allow them to dry in direct sunlight.
Bacterial wilt often occurs in conjunction with root knot nematodes. These pests can be eliminated by growing a green manure bio-fumigant.
Allowing a fallow of at least 18 months will also help, especially if soil in the bed is kept dry. This can be achieved by covering the bed with clear plastic, anchored around the edges. This process is called solarisation and it works best in warmer months, as bacterial wilt pathogens cannot survive in temperatures over 41° C.
After solarisation and bio-fumigants, grow a green manure of corn or maize and dig it into the topsoil. This will restore organic matter to soil and encourage the growth of beneficial mycorrhiza fungi. Bacterial wilt is more likely to occur in soil that is low in nutrients and organic matter, and has a high pH. Before growing crops in the treated beds, add plenty of complete organic fertiliser and as much compost as you can spare. Also check that soil pH is in the 6.5-7.5 range. Avoid growing susceptible crops in the treated beds for at least 3 years after diagnosis of the disease. Maintaining organic cultivation methods and practicing an adequate crop rotation will help prevent recurrence of this disease.

For pumpkin enthusiasts

pumpkvine1As anyone who has had a vigorous, productive pumpkin vine emerge from an old compost heap will know, pumpkins l-o-v-e compost. If you are a pumpkin enthusiast, and live in an area with cold winters, you can prepare for pumpkin growing now.
Choose a sunny spot in the garden where you want to grow pumpkins next spring, and where the vine will have room to spread (but not the same spot where you grew them this year). Get a bag of horse manure and other compost ingredients, mix the ingredients together and pile it directly onto the soil where you will sow pumpkin seed.
Dampen the heap and cover it with black plastic but, and this is the important part, uncover the pile weekly and turn it to keep it aerated. Keep the heap just damp and the heat absorbed by the black plastic will help the materials break down over winter to about the quarter of the size of the original heap. Compost made from a mixture of ingredients will provide a full range of nutrients that pumpkin vines need for healthy growth. Soil under the heap will improve in structure, allowing better root penetration. Immediately after frosts in spring, you can sow seed directly into the compost to get your pumpkin vine off to a flying start, and you should get a very good crop.

Organic fertilisers

I sometimes hear garden experts say that organic fertilisers are not as high in nutrients as chemical fertilisers, so you have to use more of them. This is simply not true.
The American Association for the Advancement of Science (AAAS) reported in February, 2009 that hundreds of studies have shown that “incrementally higher levels of fertilizer negatively impact the density of certain nutrients in harvested foodstuffs.” They also reported that the complex way in which nitrogen is absorbed in organic cultivation results in more efficient assimilation of the nutrient, allowing organically grown plants more energy to produce antioxidants, and the formation of less nitrates. Nitrates in food can form carcinogenic nitrosamines in the digestive tract.
AAAS Conclusions

Excess use of nitrogen fertilisers (including uncomposted manures and manure teas) promotes bursts of soft, sappy growth that is much loved by chewing and sap-sucking garden pests. Overuse of a particular nutrient can block the absorption of other nutrients. Nitrogen and phosphorus compete for absorption. Overuse of chemical nitrogen fertilisers can also result in deficiency of the less mobile phosphorus.
Organic fertilisers don’t need to be as high in nutrients. Organic fertilisers made from a variety of recycled organic matter will contain a full range of major nutrients and trace elements. Organic fertilisers in the form of compost, castings from worm farms, animal manures, leaf mould, and broken down green manure crops and organic mulch add humus to soil, but chemical fertilisers do not.
Humus, the most stable form of organic matter, consists of electrically charged particles called ions. Nutrient elements also carry a weak electrical charge. Humus has a large surface area and many charged sites to hold nutrient elements through electrostatic force where they are easily accessible to plants, and regulate their absorption so that nutrients are not absorbed by plants in toxic quantities. Humus also provides a habitat for a group of beneficial fungi that assist nutrition in a wide range of perennial plant families. Some chemical fertilisers, such as superphosphate, suppress the activity of these fungi and other beneficial soil organisms.
Although clay particles in soil also carry an electrical charge and are capable of holding some nutrients, without humus in soil, phosphorus can become locked up with iron, manganese or aluminium, and unavailable to plants, and nitrogen and sulphur can leach from soil.
A suitable soil pH plays an important role in efficient absorption of a full range of nutrients. Adding extra fertiliser when soil is too acid or alkaline for particular species of plants will not help their growth. Humus in soil assists in maintaining a suitable pH. See:Changing soil pH

Although we tend to worry about plants getting enough fertiliser, fertiliser plays a relatively small, but essential, part in plant growth. The major contributors to plant energy are water and carbon dioxide. In the presence of sunlight, the green parts of plants can convert these into carbohydrates, which form the cell structure of plants. You could say, in fact, that plants are solar powered.

Herbicide problems

A cousin in the UK contacted me recently concerning a major problem that UK organic gardeners and farmers are experiencing. I would like to draw your attention to it because a similar problem could occur here.
The cause of the problem is a hormone-based herbicide (weed killer) Aminopyralid, which is an ingredient in several brands of herbicides produced by Dow Agrosciences. Aminopyralid has become popular because it is only effective against broad-leaf weeds and does not kill grasses. However, the herbicide binds to woody tissue in grasses and remains active in the grass, hay and silage fed to animals. The herbicide survives passage through mammalian digestive systems and remains active in manure produced by animals that consume contaminated feed.
Consequently, the herbicide affects a range of vegetable crops planted where contaminated manures have been added to beds, or where contaminated hay or straw is used as mulch. As soil bacteria begin to decompose the mulch or manure, the herbicide is released into the soil and absorbed by roots of broad-leafed plants.
Symptoms of affected crops are dying seedlings or curled leaves and gross deformity of plants and produce. Susceptible crops include peas, beans and other legumes, carrots and parsnips, potatoes and tomatoes, and lettuce and similar crops. The affected beds remain unusable until soil bacteria have completely broken down the herbicide. As you can imagine, there are a lot of angry gardeners in England and Wales where this problem is endemic, and there are calls to for this product to be banned.
The best Dow Agrosciences can offer is: the levels of amylopyralid in crops “are unlikely to cause a problem to human health”, although the Dow website says: “As a general rule, we suggest damaged produce (however this is caused) should not be consumed.”
Originally the Royal Horticultural Society (RHS) advised allotment gardeners that contaminated soil would not be usable for two years. However, more recent information indicates that it can be used as soon as soil bacteria break down the herbicide, so the faster you can get them to do this, the better. Aerobic bacteria that break down organic matter faster than anaerobic bacteria require moisture, air and nitrogen for fast consumption. They work faster in a soil pH that is close to neutral. If soil is acidic, raise soil pH to close to neutral (6.5-7) by watering in agricultural lime. Start with a generous handful per square metre, and mix it through the topsoil. This will also aerate the soil to provide air for the bacteria. Don’t use hydrated or slaked lime (builder’s lime) as this will lose nitrogen from the soil, and the bacteria will work more slowly when soil is low in nitrogen. Check soil pH every six weeks because manures produce hydrogen ions as they break down (making soil more acidic), and turn the topsoil regularly to maintain good aeration. Keep soil just damp. If uncontaminated mulch can be sourced, it can be applied to the bed surface to maintain consistent moisture content and keep soil warmer through the colder months, especially if mulch is applied in the middle of the day when the soil is warmest.

Other herbicide problems
Other herbicides, including glyphosate, increase the risk of soil diseases by damaging the good fungi in soils that keep soil diseases under control. Recent research published by the United States Department of Agriculture (USDA), found that herbicide-resistant GM crops are causing an increase in “Fusarium” diseases, and predicted that there would be an epidemic of soil diseases and a food crisis, before long. Food crops from plants badly affected by fusarium diseases can also contain the disease fungi, causing a range of health problems, or death, as happened in Mexico some years ago when tortillas made from affected corn were eaten.
Herbicides cause more problems than they solve but many people would rather believe the advertising hype put out by chemical companies because weedkillers look as though they will save the gardener some work. (See post ‘Wilting diseases’ in the Pest-free Gardening category.)

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 acidic 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.
Testing for pH level
The only way to find the exact pH of garden soil is to test it. The Manutec Test Kit is quite economical to use, is available from most large nurseries. I’ve found this test kit to be very reliable and have used it since it was first developed by the CSIRO.
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.
***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.


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.

The importance of humus

Soil without humus is lifeless – it’s dead soil. Humus, the indigestible part of decomposed organic matter, literally converts soil into a living thing because it provides a habitat for beneficial fungi that feed nutrients to many plant families, and microorganisms that keep soil-borne plant diseases under control. It also keeps soil more moisture retentive, yet better drained; improves soil structure; holds nutrients in a form that is easily absorbed by plants; insulates plant roots by keeping topsoil cooler in summer and warmer in winter, and acts as a buffer against extremes in soil pH through a complex exchange of electrically-charged particles in soil. Regular replacement of humus in soil is absolutely essential to healthy plant growth. Humus can be added to soil as compost, green manures, well-rotted, herbifierous animal manures, poultry manures, organic mulches, decaying roots and plants.
The vegetable garden will require the lion’s share of organic matter, but it is very important to fruit trees, and many foreign plants, too. Many Australian natives prefer humus supplied through leaf litter, or leaf mould. Plants from arid soil areas have evolved to require only small amounts of humus.