Soil Quality

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<img src="https://counter.theconversation.edu.au/content/68499/count.gif?distributor=republish-lightbox-basic" alt="The Conversation" width="1" height="1">
GUEST OBSERVER
In our recent study we found that 40 percent of 203 Sydney homes we sampled contain lead in garden soil above the Australian health guideline of 300 milligrams per kilogram (mg/kg). 
This presents a hazard because soil lead can adhere to or get absorbed into edible plants. An additional pathway of exposure occurs when contaminated soil dust enters homes and is accidentally ingested. Lead is a potent neurotoxin that affects childhood development. 
<h2>Urban agriculture and VegeSafe</h2>
Urban agriculture is becoming more popular across Australia. Almost half (48%) of all households in metropolitan areas are now growing some form of edible produce.
Most lead contamination is a result of the historical use of lead petrol and lead-based paint (now phased out) and previous industrial emissions. Scientists and regulators are well aware of these legacy issues, but the general public remains underinformed about the potential risks. 
To help urban gardeners assess contamination risks associated with their garden soils, we started the community science initiative VegeSafe in 2013. This program offers free soil metal screening to participants.
Each participant receives a formal report on their soil metal results and advice about what to do next if soils contain elevated concentrations of metals. We have provided 5,500 free soil metal tests to over 1,300 homes and community gardens (Australia-wide), the largest program and study of its kind in Australia. 
<h2>What did we find?</h2>
As well as the 40% of Sydney gardens containing soil above the 300 mg/kg Australian health guideline, approximately one in seven homes had soils lead levels greater than 1,000 mg/kg. Soil metal concentrations were typically greatest around drip lines. 
Soil lead concentrations were greatest in the City of Sydney and former local government areas of Leichhardt Municipal Council and Marrickville Council, which had mean soil lead concentrations of 883 mg/kg, 960 mg/kg and 689 mg/kg, respectively. 
<a href="https://www.propertyobserver.com.au/images/2017/01/17/image-20161122-14964-1w7krx6.jpg" target="_blank">Click to enlarge</a>
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Soil lead concentrations of vegetable garden soils from 141 Sydney homes. Map represents one of four areas around homes (front yard, drip line, back yard and vegetable garden) in this study. Source: Rouillon et al. 2016
Homes with painted exteriors built before 1970 were more likely to have soils contaminated with lead. The highest levels are at homes 80 years or older. This is likely to have been caused by lead-rich paint, which contained up to 50% lead prior to 1970. Lead in paint was reduced to less than 1,000 mg/kg (0.1%) by 1997. 
We observed the environmental benefit of the withdrawal of lead from paints and leaded petrol (removed in 2002) in our study. Garden soils at newer homes contain the least lead. Soil lead concentrations decrease with distance from Sydney’s city centre, where there are more old homes and greater density of traffic and industry. 
<a href="https://www.propertyobserver.com.au/images/2017/01/17/image-20161122-10973-1aft5jx.png" target="_blank">Click to enlarge</a>
<img src="https://www.urban.com.au/images/2017/01/17/image-20161122-10973-1aft5jx.png" alt="">
Cross-section schematic of a typical inner-Sydney residential home with median soil Pb (lead) concentrations for painted pre-1970 homes, non-painted pre-1970 homes, post-1970 homes and reference homes. The vegetable garden is displayed at the rear of the back yard, as this was the case for the majority of homes. Source: Rouillon et al. 2016
<h2>Public health</h2>
Lead exposure is especially detrimental for children because their neurological and skeletal systems are developing. Adults are also adversely affected, with studies showing increased blood pressure and hypertension associated with sub-clinical exposures.
Toxicological evidence also shows that exposure reduces semen quality and extends the time to pregnancy. In short, lead is detrimental to all human systems and exposures should be avoided or minimised at all times.
Our study demonstrates lead contamination in garden soils is greater at painted homes than non-painted homes. Many pre-1970 Australian homes still contain paint with up to 50% lead on exterior walls, fences, eves, doors and window frames. 
The main risk of exposure arises when lead-based paint deteriorates or is removed improperly. Indeed, many home renovators unwittingly expose themselves and others due to a lack of knowledge of lead hazards.
Paints containing lead are no longer used in Australia, but remain on countless homes.
<h2>Reducing exposure</h2>
The United States Environmental Protection Agency introduced legislation specifically targeting houses with lead paint to prevent contamination and to minimise avoidable lead exposures. 
Unfortunately this regulatory gap has not been filled in Australia. Despite the widespread historic use of lead-based paints and the high proportion of exposure related to it, our data reveals a concerning legacy of soil lead contamination in older suburbs. 
We recommend that people residing at or planning to purchase or renovate homes built before 1970 should get their soils and paint tested for lead. Using a qualified lead-abatement decorator in older homes would also help prevent exposure. Where parents and homeowners think they may have caused exposure, their GPs can provide a blood lead test. 
Reducing even low-level exposures is critical, as demonstrated by Bruce Lanphear’s “prevention paradox” (see the image below). The graphic illustrates that the most IQ points across a population are lost from low-level lead exposures. 
<a href="https://www.propertyobserver.com.au/images/2017/01/17/image-20161122-14944-ruxd0e.jpg" target="_blank">Click to enlarge</a>
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Bruce Lanphear’s prevention paradox. The majority of IQ points lost to lead exposure occurs in children who have low-to-moderate exposure to lead. Adapted from Reference (Lanphear 2015)
<h2>What can gardeners do?</h2>
Where non-food-growing soils exceed the Australian soil metal guidelines, we recommend maintaining year-round cover of lawn or mulch to minimise dust generation. 
Where metal guidelines are exceeded in food-growing soils, we recommend either replacing existing soil with new, uncontaminated soil, or relocating the food garden to an above-ground vegetable plot (again with new soil). 
In this way, gardeners can exercise our motto, which is to carry on gardening knowing their soils are clean.
<a href="https://theconversation.com/profiles/marek-rouillon-139693" target="_blank">Marek Rouillon</a> is PhD Candidate, Macquarie University.
<a href="https://theconversation.com/profiles/louise-kristensen-124418" target="_blank">Louise Kristensen</a> is postdoctoral research fellow, Macquarie University.
<a href="https://theconversation.com/profiles/mark-patrick-taylor-11394" rel="author">Mark Patrick Taylor</a> is professor of Environmental Science, Macquarie University.
<a href="https://theconversation.com/profiles/paul-harvey-106637" target="_blank">Paul Harvey</a> is researcher of Environmental Science, Macquarie University.
<a href="https://theconversation.com/profiles/steven-g-george-317844" target="_blank">Steven G George</a> is researcher of Environmental Science, Macquarie University.
All are authors for The Conversation.
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Elevated lead levels in Sydney backyards: here’s what you can do

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<img src="https://counter.theconversation.edu.au/content/65332/count.gif" alt="The Conversation" width="1" height="1">
GUEST OBSERVER
Most people think of soil only in terms of the dirt that sticks stubbornly to their hands and shoes. But soil is much more than that.
A handful of soil is a small and very complex ecosystem which includes soil particles, pores, aggregates, organic matter and a staggering number of microorganisms, all of which interact to keep the soil healthy and productive.
Each soil is different. The primary factor determining the physical properties of a soil is particle size. Sandy soils have a coarse structure, large pores and little ability to hold on to water and nutrients, which makes them prone to leaching.
Clayey soils hold nutrients and water, but because the pores are small they are prone to waterlogging.
The pores between soil particles are important for water flow but also for the movement of gases. Particularly important is the exchange of oxygen with the atmosphere, because plant roots and most soil organisms need oxygen to breathe.
Sand, silt and clay particles are bound together to form aggregates. These aggregates form the skeleton, or matrix, for chemical and biological processes which are critical for soil functions.
<h2>Nitty-gritty</h2>
Organic matter and soil microorganisms play key roles in the formation and stability of aggregates.
Think of organic matter as being the glue or mortar between soil particles. But the organic matter needs be decomposed to be most effective. And this is where soil microorganisms come in.
Each gram of soil contains a billion bacteria, which is almost as much as the human population of China or India, and about 50 times the number of people in Australia.
And this number does not even include soil fungi. All these bacteria and fungi can coexist because they have different needs or don’t interact.
For example, a bacterium on one side of the aggregate is quite isolated from a bacterium on the other side of the same aggregate. So even if these two bacteria have similar needs, they don’t compete.
They may also not compete because they are not active at the same time. In a given soil condition, only a small proportion of the bacterial population is active. The majority are inactive (dormant) and come to life only when conditions are optimal for them.
Another reason for the staggering number of microbes in a gram of soil is the huge diversity. A gram of soil contains around a million different species. Among them are generalists but there are also specialists, such as those capable of fixing nitrogen from the atmosphere or degrading wood.
The food of the vast majority of soil microbes is organic matter, which is mainly plant matter. When plant matter gets into the soil, it is attacked by soil microbes for energy and nutrients.
This attack is aided by soil animals such as earthworms or mites. They mix the plant matter in the soil and its microbes, fragmenting it and thereby improving contact between plant matter and soil organisms.
During decomposition of plant matter, microbes multiply and produce carbon dioxide and nutrients for plant uptake. The dead microbes and the components of the plant matter that are very difficult to decompose help to bind the soil particles together.
This binding is also aided by slimes produced by microbes and the long filaments of <a href="https://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/Thallus">fungal hyphae</a>. The aggregates formed in this way are stable, which is important for adequate water and air supply for plant roots and soil organisms.
If aggregates are not stable, such as in sodic soils, water and air supply are restricted and plants and soil organisms will suffer.
<h2>Feeding your soil</h2>
Maintaining a certain level of organic matter in soil is critical for sustained soil health and function. Organic matter not only ensures that aggregates are stable, but also acts like a sponge, holding water, which is particularly important in sandy soils.
Another important function of organic matter is supply of nutrients required by plants. As mentioned above, decomposition of plant matter releases nutrients and it also can bind nutrients which can become available later.
But organic matter also decomposes, so needs to be replaced regularly. You can do this by adding plant matter growing in your garden or from outside to the soil, either directly or after composting.
Compost is more stable in soil than litter or straw because it is already decomposed during the composting process. It is a good glue for soil particles and binds nutrients, but it will not supply much nutrients for plants.
Fresh young plant matter is a good source of nutrients, but also decomposes very quickly and may initially release more nutrients than the plants can take up.
Mature cereal straw is nutrient-poor and decomposes slowly. So for good nutrient supply and long-term soil productivity, you may have to supply a mix of different types of organic amendments.
Of course, you can bypass this organic loop by adding inorganic fertilisers. Plants will grow well, but you have to match supply and plant demand carefully, because most fertilisers are dissolved quickly.
In your garden, you may get the best results by combining organic and inorganic fertilisation: various organic amendments for long-term soil health and stability, and inorganic fertiliser to overcome short-term nutrient deficiencies.
Petra Marschner is professor of Agriculture, University of Adelaide and author for The Conversation. She can be contacted <a href="https://theconversation.com/profiles/petra-marschner-299098" target="_blank">here</a>.
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