Angus Stewart: How to make urban soils safe for food plants

Are contaminants in city soils of real concern if you’re growing food? In their book Grow Your Own – How to be an urban farmer ANGUS STEWART and SIMON LEAKE describe which pollutants or nutrient overloads need to be addressed, and how best to do it

Cities are particularly conducive to plant life because of the incidental nutrients that are provided by simply being in an urban environment. The atmosphere is enriched with nitrogen, carbon and sulfur; soils can be enhanced with phosphorus and calcium; the pH levels are usually higher (the soils are less acid); and the important micronutrients zinc, copper and boron are available from a variety of sources. However, poor management choices and overzealous fertiliser use on urban farms can lead to soil chemistry imbalances. The ‘Symptoms of

Soil Chemistry Imbalances’ table (see page 63) details some common imbalances and their cures.

While nutrient deficiencies are the most frequently encountered problems with overworked urban soils, past practices can lead to nutrient excesses. The three elements most likely to accumulate to undesirable levels are phosphorus, zinc and calcium.

PHOSPHORUS As some organic fertilisers, such as poultry manure-based products, contain high levels of phosphorus, and as it adheres strongly to certain soil minerals (such as iron minerals) and does not leach (in other words, it cannot be washed out of the soil with either rainfall or irrigation), phosphorus often accumulates to exceptionally high levels in overused and urban soils. This is a problem, as phosphorus ‘fixes’ or reduces the solubility of – and therefore availability to plants of – important micronutrients such as iron and manganese.

ZINC This essential micronutrient is very commonly used in urban environments. It is present on galvanised objects (roofs and gutters),

in car tyres (up to 2 per cent by weight) and in many metal items. Feedlot cow manure is often contaminated with zinc, as feedlotters have the mistaken view that very high levels of zinc are good for cattle. Thus, run-off from roads, roofs and many organic fertilisers has elevated zinc levels. After many years, this zinc accumulates in soil, largely because it does not leach quickly. High levels of zinc can be toxic to some of your plants.

CALCIUM Because there is so much concrete, lime and cement in urban environments, run-off from these surfaces contains large amounts of calcium; once it reaches the soil, this calcium does not leach easily and can sometimes accumulate to excessive levels.

The other source of surplus calcium is urban farmers following well-meaning advice from gardening books and putting lime on legumes at planting. So, every year you apply lime to your garden, and you end up with inordinate amounts of calcium in the soil, which makes it alkaline.

pollution in urban soils

Although the dangers of pollution in urban soils are perhaps not as great as some would have us believe, there are a few potentially harmful contaminants of which we need to be aware. First, it is important to understand the three principles that determine the toxicity of an element in an urban farming context:

It is harmful to mammals (that’s you and me). Some metals (such as lead and mercury) are quite toxic, while others (such as zinc and copper) are of no real concern. It is harmful when taken up by plants (not all toxic elements are taken up by plants; we can manipulate the situation if this occurs).

It is deposited on the outside of the edible portion (leaf or root) as dirt or dust and is dangerous when directly ingested.

Let’s consider the most likely contaminants to cause problems for either people or plants.

ZINC As discussed earlier, zinc is the ‘heavy’ metal element most likely to be elevated in urban soil. Since it is not toxic to mammals in any conceivable amount, it is of no concern to human health. It does impact on plants, however, because many species will absorb significant amounts, and it can poison them. While it is an essential element – or perhaps because it is an essential element – an excess of it will antagonise the uptake of other elements, such as copper, causing deficiencies of these elements. In some very old urban soils (for example, around historic houses), the zinc level is now so high that any seedlings planted there are severely injured – with symptoms including yellowing, stunting and deformity – or even killed.

COPPER Many of the objects we use in irrigation and urban water supply are made of copper or brass (an alloy of copper and zinc), so soils can easily become contaminated with copper. Old orchards where ‘bluestone’ (copper sulfate) was used as a fungicide can also show elevated levels. Copper, like zinc, is not toxic to humans in almost any conceivable amount, but it can cause toxicity in plants. The main symptom of copper toxicity is the bleaching of the old leaf tissue between the leaf veins.

LEAD This is the second most likely element to be elevated in urban soil, thanks to lead in petrol, old paint, roof flashings and batteries. It is taken up by some plants, is present in dust and is poisonous to mammals. Lead is not very poisonous to plants, which explains why very high levels – in excess of 1000mg per kg (normal soils have around 10–30mg per kg) – are needed to cause any problems for plants. However, it is the most common toxic element to cause problems for humans, especially children. Poisoning does not usually result from eating the produce of urban farming, but from ingesting contaminated dust. If you are on a main road, lead levels can be elevated in soils due to years of lead-contaminated vehicle exhausts. Lead has been removed from modern fuels, but should be considered in a historical context.

ARSENIC This can accumulate in urban soils affected by industrial pollution. Arsenic was used as a termite and insect killer in the old days, so it may still be encountered around old houses and stock dipping pens. We have seen seaweed products being sold for their soil health benefits, but they contain a very high proportion of arsenic at levels likely to cause toxicity in humans. Not all seaweed is high in arsenic, but some is, so it is highly desirable to see an analysis of any seaweed products you might use on your soil.

CADMIUM Toxicity is unknown in plants, and very rare in animals. When cadmium toxicity does occur, it is quite dangerous to humans as it accumulates in the bones over time. Cadmium appears in rock phosphate deposits, and the older forms of phosphorus fertiliser had restrictions on how much cadmium they could contain. Today most forms of phosphorus fertiliser are safe.

NICKEL The most common source of nickel is the metal industry, where it is used in alloys and for plating metals. While certain enzymes need nickel, an excessive amount of it interferes with the metabolism of other metals, such as manganese and zinc, reducing plant growth. In animals, nickel inhibits certain enzyme and hormone pathways, but fortunately it is not readily absorbed and is rapidly excreted, so toxicity is very rare.

MERCURY Toxicity in plants is almost unknown. When it occurs in mammals, the mercury does not usually come from plants grown in soil – problems are far more likely from contaminated seafood. Plants will take up mercury from soils, but less readily from well-aerated, iron-rich and lime-rich soils. The main cause of elevated mercury levels in soils and plants – the use of mercuric fungicides – has now been completely banned.

golden rules to follow

Keep in mind that soils in urban areas are not always natural, and they may have been radically altered from their natural state. Research the history of the area if you can. If your site was once part of an industrial development, or is an

old inner-city housing area, it pays to have your soil analysed in depth by professionals.

If your soil is significantly elevated in any of the elements toxic to humans (lead, cadmium, mercury, arsenic and nickel), take the following precautions:

Always wash your hands after gardening; if the conditions are dry and dusty, wear a protective dust mask while working in the garden. Dust and dirt are a more likely source of contamination than eating the produce from your garden. Don’t let children play in the dirt if it is high in lead.

Always wash produce to remove any potentially contaminated dust from it before you consume it.

Lime your soil to increase the pH to above 6.5. Contaminant metals are more soluble in acid soils, so liming reduces their uptake by plants.

Ensure that the soil has plenty of organic matter, as this ‘chelates’ or locks up contaminant metals.

Make sure there is plenty of phosphorus in the soil. The phosphates of these contaminant metals are insoluble, and this will also lock them up.

If your soil is moderately elevated in metallic contaminants, it may be best to avoid root crops such as potatoes. These are in direct contact with the soil, and the metals can accumulate in the edible part, particularly in the epidermis or ‘skin’ of the tuber. If you really want to grow them, then always peel your root crops. This will get rid of most of the contamination.

If your soil is really elevated, apart from perhaps not growing root vegetables, also don’t consume too many leaf vegetables, particularly silverbeet or spinach, which are known accumulators. Grow fruit-like vegetables such as tomatoes and eggplants, or fruit trees. Developing fruits are each protected by a structure similar to a placenta, which largely prevents toxic elements from getting into the fruits and future seeds. Fruits are perfectly healthy to eat even when grown in quite high contaminant levels.

If your soil is very highly contaminated with mercury, cadmium, arsenic and lead, consider soil replacement. These metals cannot be removed from soil, and replacement is the only option. This applies to zinc and copper, too, as they are toxic to plants.

Using raised garden beds that have a growing medium you have created yourself is a sure-fire way to avoid any issues with contaminants. There are also some ingenious proprietary growing systems available to urban farmers that completely circumvent problems with pollution.

Contamination should not be a problem where soils and growing media are purchased from reputable suppliers, but some manufacturers recycle soils from skip bins and construction sites, so ask for an analysis. Alternatively, make your own from ingredients you know are free of contaminants.

See page 35 for our story on growing edibles