While nutrient deficiencies are usually the most frequently encountered problems with overworked urban soils, past practices can sometimes lead to nutrient excesses. The three elements most likely to accumulate to undesirable levels in urban soils are phosphorus, calcium and zinc.
Phosphorus Because some organic fertilisers, such as poultry manure-based products, contain high levels of phosphorus, and because 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, because phosphorus ‘fixes’ or reduces the solubility of – and therefore the availability to plants of – important micronutrients such as iron and manganese.
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 eventually you end up with inordinate amounts of calcium in the soil, which makes it alkaline.
Zinc This essential micronutrient is, unfortunately, 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 plants.
A SIMPLE WAY TO MEASURE SALTS IN YOUR SOIL
You can get an indication of soil salinity by using a hobby multimeter that is set to measure electrical resistance. Opposite to conductivity, resistance is the measure of the ability of a solution to oppose the flow of an electrical current. Set the meter to resistance, and tape the red and black probes together so there is a 0.5-centimetre gap between them. Make a 1 gram per litre solution of common salt by dissolving a flat teaspoon of salt in a litre of water, and immerse the probes into it. Set the range of the meter to give a reading on scale. After taking the reading, dip the probes into clean tap water to wash off the salt.
Now make a solution of equal parts soil and fresh water, shake it very well, and leave it for an hour. Immerse the probes in the water, and take a reading. If the resistance is significantly less than that for the salt solution, chances are that salinity is a problem in your soil. If the resistance is significantly greater than that of the salt solution, it may be that there is very little nutrition in your soil and your garden would respond well to an appropriate application of fertiliser.
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 sulphate) 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.
SYMPTOMS OF SOIL CHEMISTRY IMBALANCES
SYMPTOMS LIKELY CAUSES CURE
Plant is spindly, weak and yellowish; older leaves drop early Usually nitrogen deficiency; less commonly sulphur deficiency Apply chicken manure, high-nitrogen compost or nitrogen fertiliser
Youngest leaves are yellowish, especially during growth spurts Usually iron and manganese deficiency; less commonly zinc and copper deficiency; sometimes toxic levels of zinc Identify the deficiency, then correct with soil applications or foliar sprays of appropriate chelated elements
Margins of the leaves are burned Salt burn; ammonia burn; fertiliser burn Deeply water the plant to clean leaves and leach salts from soil
Yellowish margin to leaves, with leaf tips burned; worse in older leaves; poor fruit set Potassium deficiency Apply potassium-rich compost or sulphate of potash
Growing tips and leaf margins distort, yellow and then blacken Calcium deficiency Apply lime (if soil is acid), or gypsum if it is not
Even today, soils located next to major roads may be contaminated with lead, thanks to decades of exposure to the exhausts of vehicles run on leaded petrol.
Lead This is the second most likely element to be elevated in urban soil, thanks to lead having been used 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 1000 milligrams per kilogram (normal soils have around 10–30 milligrams per kilogram) – are needed to cause any problems for plants. However, it is the most common toxic element to cause problems for humans, especially children. Fortunately, poisoning does not usually result from eating the produce of urban farming, but from ingesting contaminated dust. Little children, for example, often put odd things in their mouth (such as soil), and this can be an issue if they play in inner-city backyards where the lead from old plumbing and paint has made its way into the soil. If you are on a main road, lead levels can be elevated in soils because of years of lead-contaminated vehicle exhausts. Lead has been removed from modern fuels, but it should be considered from 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.
Be very careful of toxic dust settling on the edible leaves of plants, such as parsley. Always wash your farm produce well.
BONUS NUTRIENTS IN URBAN SOILS
Contrary to popular opinion, various discharges and incidental substances in urban environments can be quite beneficial to plants, as they offer extra nutrition. Oxides of nitrogen in the atmosphere are rapidly converted to valuab
le nitrate within both the plant and the soil, while sulphur dioxide from smelters and diesel emissions quickly becomes soil sulphates that are a source of sulphur for plant growth.
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 placed on them regarding 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. Mercury problems are far more likely to arise from consuming contaminated seafood. Plants will take up mercury from soils, but less readily from well-aerated, iron-rich and lime-rich soils. Fortunately, the main cause of elevated mercury levels in soils and plants – the use of mercuric fungicides – has now been completely banned.
In heavily built-up urban areas, it is strongly advised to have soil-contaminant testing done. For larger communal urban farms, testing for nutrients is also useful.
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 your 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 the 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 chemically.
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, not only consider not growing root vegetables, but 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. The 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 the dangerous metals 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 with a growing medium that 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 commercial suppliers, but don’t take this for granted – some manufacturers recycle soils from skip bins and construction sites, so always ask for an analysis. Alternatively, make your own growing medium from ingredients you know to be free of contaminants (see Constructing Beds and Plots).
Fruits are usually quite strongly protected from moderate levels of soil contaminants.
PROFESSIONAL SOIL TESTING
There are soil-testing laboratories in every state, and an internet search for soil-testing services should locate one that can give you the analysis you need. Be warned: it can be quite expensive, and should be considered only when all the simple and obvious problems such as pH, drainage and disease have been discounted, or if you suspect that your soil has substantial levels of elements that are poisonous to humans.
CONSTRUCTING BEDS AND
PLOTS
PERFECT PLACES TO GROW YOUR PLANTS
Urban-farming plots will use anything from natural soil profiles to hydroponic culture in pipes and gutters without any soil whatsoever. Where natural soil is available, this is usually the lowest-cost alternative, but in rooftop or balcony areas this is obviously not an option. Saving space and protecting young plants are also important considerations, so this is where green walls and greenhouses come into play. Regardless of the size of your urban farm, there is always a way to create efficient and productive garden beds or plots that receive the perfect amount of sunlight and nutrients for plant growth.
USING NATURAL SOIL
Examine your soil profile to a depth of at least 500 millimetres. While the usual rooting depth of crops is seldom more than 300 millimetres, you need to know if there are clay layers or hard pans (for example, a rock seam) that might interfere with drainage. Make a note of the colours, textures and structures you encounter in each horizon as you dig down. A dark, loamy topsoil overlying a well-structured clay subsoil (bright red or orange in colour) with no signs of impeded drainage is ideal.
If you find very hard layers, you will have to think about an initial cultivation to loosen the soil to at least 200 millimetres. If your testing shows that the soil also needs some conditioners (lime, dolomite or iron sulphate to adjust the pH level, or gypsum to ‘break’ the clay) and initial fertilisers (to increase the level of phosphorus or potassium), add these to the surface first, and then work them in to depth. For big urban farms, it certainly pays to hire or borrow a cultivator (a tractor for large areas, or a walk-behind mechanical hoe for medium-sized areas). For smaller areas, forking is the best way to add conditioners and fertilisers to the soil (and it’s good exercise!).
If you need to increase organic-matter levels, add some compost. Apply a 20–50-millimetre layer of either homemade or commercial compost to the top of the soil – along with the other conditioners that may be required – and work the material into the soil to a depth of 200 millimetres. If you don’t want to do any digging, or your soil doesn’t need loosening up, then just leave the compost for the worms and other soil organisms to do their job. This will be much slower, but will eventually achieve the same aim.
Plants are traditionally grown in the ground, but today there are many choices when it comes to planting places, from raised beds to soil-less hydroponic set-ups.
Sand and clay
With extremely sandy soils, organic matter is certainly helpful – but adding some clay will also improve water- and nutrient-holding capacity. Using purchased bentonite clay or another clayey soil, apply it to a depth of about 10 millimetres and work it in to 200 millimetres. You have to work it in, as leaving it on the surface will cause crusting and water run-off.
If you have a light topsoil that is dominated by sand, and a heavy clay subsoil, then it i
s a good idea to turn over the soil to mix it up a bit so there is not such a contrast in texture between the two soil horizons. If there are indications of poor drainage (for example, white or poorly structured clay, or obvious soil wetness), then you will need to think about improving the drainage. See the Water and Drainage for more information.
Vertical gardens are often called green walls. They are becoming increasingly popular for urban farms with little horizontal space.
To ensure you have healthy crops, it is vitally important to use a suitable growing medium as the foundation for your garden bed.
COMMERCIAL SOILS
There are soil yards in most urban centres that will supply soil for making gardens where needed. Often the manufacturers of soil also run composting systems using locally available garden or yard waste and other organic and solid wastes from industry.
Commercial soils vary greatly in quality. There are some good and reliable products, and others where the manufacturer has not done the proper product-development work to get the soil right. Some are downright toxic to plants. In our consulting and laboratory-analysis work, we see a few major problems that you need to be aware of when purchasing commercial soils.
Poor structure Commercial soils have very little to no structure. This makes them potentially poorly drained and subject to low aeration. They will develop structure with time, but make sure subsoil drainage is excellent or the raised bed is at least 500 millimetres high, or the soils can tend to become waterlogged.
Too much compost Organic-matter levels in commercial soils are often very high – in fact, higher than they need to be or should be. This is because good mineral soil is hard to get as an ingredient, so manufacturers tend to incorporate plenty of compost in their soils. We have seen ‘soil’ that is 95 per cent compost and so high in nutrients that it is actually saline and acutely toxic to plants. About 40 per cent compost by volume is as high as you would want to go, and 30 per cent is a more sensible limit if the compost is rich. A high level of organic matter combined with poor drainage capacity causes anaerobic conditions if the soil gets wet at depth. Ideally, do not use organic-rich soil any deeper than 250 millimetres. If your garden or raised beds are deeper than this, use a sandy, well-drained, low-organic mix beneath the organic-rich soil. Ask the producer how much organic matter they have put into their mix. If it’s any more than around 40 per cent, it is probably best avoided.
Grow Your Own Page 11