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Information about plants & gardens for Brisbane & Qld |
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This page is intended to provide information primarily about pH, CEC, wettability and salinity of soils, or their physical composition and structure. The topics of soil fertility, soil structure and soil chemistry can't be separated completely because many fertilisers and mulches contribute both plant nutrients and add organic matter. On the other hand, some materials added to correct pH problems can also contribute plant nurients. Furthermore, issues like pH and aeration can have an effect on the availablility and uptake of plant nutrients present in the soil.
An Australian study has shown how plastic in the garden - whether through careless disposal of waste or deliberate use - can impact soil health and the wider environment. Soil samples were collected in the vicinity of two plastic items in an ordinary South Australian garden. One was a piece of discarded polyethylene bubble wrap. The other was polypropylene weed control fabric. Both had been in contact with the soil for seven years. Analysis showed that degradation over that time had released plastic particles into the soil. These ranged in size from visible fragments to microplastic particles a fraction of a millimetre in diameter. Although the techniques in this study were not suitable for detection of ultra-small nanoplastics, the results suggest that these would also be present. Source: Reducing plastic in gardens (October 2021)
Removing beach-cast seaweed (sea wrack) to manufacture garden products or make the beaches appealing to tourists could impact sea bird populations. In "the first study of its kind," ecologists at the University of South Australia found that fresh and aged wrack of various depths had different temperature profiles, allowing shore birds to be able to take advantage of the warmest spots available at various times of the day to conserve energy. Source: Conserving coastal seaweed: a must have for migrating sea birds (June 2021)
Older news at bottom of page.
Cation exchange capacity (CEC) is a property of soils and growing mixes that is rarely discussed in mainstream gardening literature. However, it plays a very important role in nutrient-retention by soils which subsequently affects fertiliser efficiency and plant growth.
Many important plant nutrients, when added as fertilisers or released from breakdown of insoluble minerals and decomposition of organic matter, exist dissolved in in soil moisture (the soil solution) as positively-charged ions (cations). Examples are potassium, magnesium, calcium and ammonium nitrogen.
In dissolved form, there's a limit to the amount of nutrients that can be stored in the soil and they are vulnerable to leaching.
Fortunately, certain types of negatively-charged particles in soil have the ability to cling onto cations, while keeping them relatively available for uptake by plant roots. Cations can re-enter the soil solution by swapping places with other cations in solution.
The "cation exchange capacity" of a given soil will depend on the amount and type of such substances it contains. Clays generally have a high CEC (although it it varies with the type of clay). Sand has none. Humus also has a high CEC, which is one of the ways in which organic matter improves soil.
Note that sodium is also a cation. Overwhelming clay particles with positive charge affects soil structure and is one of the problems associated with salinisation of soils.
Most gardeners have probably heard of humus, but what is it really?
"Soil organic matter" is a broad term encompassing plant roots, soil organisms, root exudates, charcoal and organic matter in various states of decay. "Humus" is the dark-coloured end product. It's composed of complex organic molecules that are relatively resistant to microbial decomposition and under suitable conditions can persist in the soil for a long time.
Benefits of humus include cation exchange capacity (see above), pH buffering, water retention and improved soil structure. For these reasons, gardeners and farmers strive to build the humus content of their soils.
Besides soils and composts, "humic substances" can be found in coal, peat, streams, lakes, organic sediments and some seaweeds. Extracts of coal and seaweed are often used to produce commercial soil improvers. Some vendors claim a variety of plant growth-enhancing properties for certain products.
"Humic acid" and "fulvic acid" and "humin" are components distinguished by their chemical behavior. Humic acid can be precipitated with acid from a sodium hydroxide solution. The resultant solid is called "humate", although this term is sometimes applied to humic subtances more generally. This is a complicated subject and loose usage of these terms in horticultural literature makes it even more confusing.
A few weeks ago, a research report was published concerning the amount of lead in Australian yards, followed by some disturbing headlines in mainstream media stories.
It's quite detailed, but if this topic is of special interest to you, do download the report and pick apart the findings for yourself: A citizen science approach to identifying trace metal contamination risks in urban gardens.
It presents analysis of data collected by the Vegesafe program based at Sydney's Macquarie University. They used soil samples from across the nation, sent in by people wanting to find out if their soils had dangerous trace metal contamination.
While a number of metals are potentially hazardous, most of the discussion concerns lead. There is some breakdown by capital city, including information about Brisbane.
Some properties are more risk than others (especially inner city locations and those with older, painted houses) and some parts (front yards, under the house dripline) are more likely to have elevated lead levels than others.
Fortunately, there are many actions that food gardeners can take to reduce their lead risk. Some practices - like using raised beds with new soil, using composts and mulches, maintaining a pH close to neutral, wearing gloves and thoroughly washing hands and harvested produce - may already be a part of their routine.
A downside to the focus on vegetables by Vegesafe and others could be complacency among city residents who don't grow edible plants. Most of the lead risk involves soil or dust contact, inhalation or ingestion. This can happen through all types of gardening and other soil disturbance.
There's plenty of information from health authorities online if you'd like to know more. A good place to start could be Vegesafe: research.science.mq.edu.au/vegesafe/. You could even send in some samples of your own soil for testing. They do ask for a donation of $20 or more to help keep the program running. You'll find instructions at the website.
Uptake of Nanoplastic particles (which are less than 100 nanometres in size) has been observed in a plant. Now there's a report of larger microplastic particles being absorbed by plants. In this case, the particles squeezed between root cells or entered through natural cracks formed where new roots emerge. They were then transported upwards with normal water movement. Food crops particularly at risk of plastic contamination would be those growing with wastewater or sewage sludges. Source: Crop Plants Are Taking Up Microplastics (July 2020)
Pollution of aquatic environments is getting a lot of attention, but now there's evidence of a terrestrial plant absorbing nanoplastics under 200nm in size. Under laboratory conditions, Arabidopsis thaliana was shown to accumulate two types of particle with similar properties to weathered and degraded plastic found in the environment. Both types inhibited growth, which has big implications for both natural and agricultural systems. Of the two plastics tested, the one with a positively charged surface had the biggest effect on the plants, although it was mainly confined to the root tips. The negatively-charged plastic was, on the other hand, seen to move more within the plants. Source: Research in Land Plants Shows Nanoplastics Accumulating in Tissues (June 2020)
When soil organic matter is too high, there can be an excess of nutrients and microbial activity that damages plants. Samples from urban gardens near Oregon State University, averaged 13% organic matter, while 3-5% is recommended. One garden visited had 30% organic matter and dying plants. High figures were associated with raised beds, which are typically filled with materials other than the sites' natural soils. The researcher recommends balancing high-carbon materials with nutrient-rich manures and composts, and blending with good soil. Source: Study shows some urban gardens contain too much organic matter (January, 2020)
Antibiotics are used extensively in animal production and some wind up in the manure. Research from America comparing fields fertilised with manure from treated and untreated dairy cattle has indeed shown multiple effects. Changes in soil microbiology and the use of carbon and nitrogen by plants was demonstrated. Less carbon was stored in the soil. For some time, the potential for development of resistance has been a major concern with respect to the overuse of antibiotics, but these results point to a range of possible impacts on agriculture and environment as well. Source: New research finds multiple effects on soil from exposure to manure from cows administered antibiotics (October 2019)
Surprisingly, the effects of discarded cigarette butts on terrestrial plant growth has not been studied until now. Recently released experimental results from the UK, however, have shown that they do indeed impact plants - and not in a good way. Germination success and subsequent growth of clover and ryegrass was found to be reduced by the presence of cigarette butts in the soil. It would appear that most of the damage was caused by the filters, regardless of whether the cigarette was smoked or not. It's thought that the cellulose acetate component and/or plasticising chemicals used in their manufacture caused the observed seedling inhibition. Source: Cigarette butts hamper plant growth - study (July 2019)
One of the potential drawbacks of edible gardening in urban areas is the possibility the soil may have be contaminated from prior industrial uses or fallout from heavy traffic. A study conducted in Baltimore (USA), in which community gardeners were surveyed about their knowledge of these issues, has shown insufficient knowledge and expertise in assessing the risks of a site and how to minimise exposure to contamination that may be present. Source: Urban gardeners may be unaware of how best to manage contaminants in soil (March 2014)
While stimulating marked increases in plant growth associated with brassinosteroids and auxin gene activation, a biochar study also showed that plant defence genes were suppressed by the soil supplement. This could have serious implications for the suitability of biochar for crop enhancement and carbon sequestration. Source: New study finds biochar stimulates more plant growth but less plant defence (March 2014)
Mexican researchers are developing an organic hydroseeding techniques to stabilise sloping roadsides that is actually better than alternatives employing artificial polymers, adhesives and fertilisers. Components of the new technique include mycorrhizae to encourage soil aggregation and an adhesive made from the nopal cactus. Gardens used to reduce landslides (September 2013)
One of the risks of vegetable gardening in urban areas is soil contaminated by toxins. Soil tests may indicate problems, but researchers in Detroit evaluating sampling strategies in a garden plot found that some of the methods failed to detect a lead "hotspot". They suggest that standard sampling procedures be reassessed, and that if possible garden planners check historical records to ascertain what activities the land was previously used for. Source: Detecting Lead Hotspots in Urban Gardens Requires Different Sampling Strategies according to Wayne State research (June 2013)
Research suggests that a significant proportion of soil organic matter may not derive from plant material directly, but from the remains of the soil microflora that feed on it. Bacterial cell wall fragments, resistant to further decay, thus contribute to the long-term soil carbon store. Bacterial products also coat mineral soil particles with an organic film, on which such particles can be accumulated. Source: Fertile soil doesn't fall from the sky. The contribution of bacterial remnants to soil fertility has been underestimated until now (December 2012)
Scientists have discovered a bacterium that can incorporate carbon dioxide into limestone, with the the help of a tropical tree and a fungus. Under certain conditions, the tree combines soil calcium with atmospheric CO2 and the bacterium creates the conditions under which it can be converted into calcium carbonate, depositing limestone around the tree's roots. This is a way of both improving the soil for agriculture or reforestation as well as locking away carbon in the soil and is already being trialled in several tropical countries. Source: Bugs in key role of CO2 storage method (June 2012)
Soil microbe erosion
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