Grow Your Own

by Angus Stewart

  Microbial contaminants – there are some dangerous microbes living in soil, manure and water that we need to be aware of when growing and harvesting food on urban farms. In most cases these microbes do not build up to a point where they are dangerous, but sometimes they do – so it is ALWAYS important to wash or process your produce thoroughly before it gets to your table. This is particularly vital for root crops and plants that may be affected by rain splash of soil, such as lettuce.

  Remove most lateral shoots on tomato plants to encourage larger fruits and better air circulation.

  RIPENING TRICK

  Ethylene is a naturally occurring gas that regulates various growth responses across a wide range of plant species. Among these fascinating processes, a particularly vital one is that ethylene is generally the key trigger for fruit ripening. Placing unripe fruits – such as tomatoes and avocados – in an airtight container with ripe fruits (particularly bananas), hastens the ripening process. You will soon have perfectly edible fruits, and avoid the devastation of discovering pest-damaged fruits in the garden.

  FERTILISERS

  THE IMPORTANCE OF ENRICHING YOUR SOIL

  Why do we need to ‘improve’ or ‘fertilise’ soils to use them for horticulture? In most parts of the world, soils in their natural state do not have sufficient nutrients to grow the types of plants we want to cultivate in urban farms. It is usually high-yielding vegetable and fruit crops we want to grow, and all of these – with very few exceptions – need exceptionally fertile soils to produce the quality and quantity of produce we require.

  Even where natural soils are fertile enough to support essentially zero-input farming, they start to decline in fertility as nutrients are exported in crop produce. With the halt in the normal system of organic-matter cycling, these soils soon exhaust their organic matter, and consequently their physical soil fertility is reduced. No soils can run on zero-input farming forever. In this sense, farming is a bit like mining for plant nutrients.

  The classic example of zero-input farming is the traditional slash-and-burn farming practised by most traditional tribal groups in rainforest areas all over the world. They cut down and burn the vegetation from a patch of mature rainforest, and the ash returns the minerals from the vegetation to the soil. In the first couple of years, yields are worth the effort of farming the plot, but after three to five years the plot is abandoned and the process starts again. The abandoned plot takes a minimum of 20 years to recover, as nutrients slowly dissolve from the weathering of rock and bioaccumulate in the soil again.

  This is a reasonably stable system when there is low population pressure. However, as the population increases, the period before returning to previously slashed and burned plots diminishes, so the soil has not had time to fully recover. Rainforest areas all over the world bear the ruins of early civilisations that collapsed thanks to the loss of soil fertility.

  It follows that if we want agriculture in urban areas to be truly sustainable, we have to not only improve soil fertility to support the crops we want to grow, but also replace those nutrients that are lost in the export of produce. The weathering of rock and the subsequent process of bioaccumulation is simply too slow to maintain soil nutrients at the levels we need to support the productivity of an urban farm.

  A clay soil has too few nutrients to support food plants, so it needs to be fortified with quality fertiliser.

  Thick organic mulch both protects and eventually improves the soil in this street-side raised bed.

  Using a side dressing that contains soluble forms of nutrients is a great way to encourage spring onions to maintain their high yield.

  A HISTORY OF FERTILISER

  While there is clear evidence that the ancient Romans widely used the ash and slag from steelmaking (as a source of lime) to boost their soils, it was only in the first half of the twentieth century that industrial-scale manufacture of fertilisers became a reality. So, what did we do before that?

  For most of the long history of adding nutrients to soil, organic fertilisers and improvers were the only options. Agriculture and horticulture traditionally relied on the recycling of nutrients from everyday materials, such as manure and urine. Indeed, human faeces and urine were mainstays of plant nutrition in some places. While there are definitely health and hygiene considerations in using such materials, it makes sense that the nutrients being removed from the soil via the crops are returned in pretty much the same balance via the manure and urine that animals, including humans, excrete. As late as the 1940s, my grandfather in Yorkshire took the ‘night soil’ can to his allotment down the lane, where he grew beautiful potatoes (and brussels sprouts) that caused no apparent health problems for his wife and four daughters when they ate them. This makes me think that we sometimes exaggerate health risks.

  Exploration and knowledge

  By the beginning of the nineteenth century, the importance of bone in agriculture was well established. The blood and bone-rendering industry forged ahead, at first using the bones of soldiers killed during Napoleon’s wars of conquest, but later relying on the efficient re-use of all abattoir wastes.

  Exploration of the oceans during the nineteenth century revealed a raft of new fertilisers. There was worldwide industrial-scale mining of ‘guano’, the fossilised droppings of seabirds, and extensive quarrying of the rock phosphate reserves of Morocco and many islands of the Pacific. So important was rock phosphate to the feeding of the burgeoning populations of Europe, that most of the world’s high-grade reserves are now depleted, including those on Christmas Island and Nauru. Long used to make gunpowder, Chile saltpetre (sodium nitrate) was present in such large quantities in the Chilean desert that it was mined and bagged as fertiliser.

  In the late nineteenth century, with the advancement in knowledge about chemistry and the elements that make up living things, agricultural science made great strides. We started to understand that it was deficiencies of certain chemical elements that held back agriculture and plant growth. The new science of chemistry explained that the success of blood and bone as a fertiliser was due to the phosphorus and calcium in the bones, and that guano was rich in nitrogen and phosphorus, essential for plant growth.

  ORGANIC OR CHEMICAL?

  In a scientific sense, everything we can see and touch is made from chemicals. The word ‘chemical’ usually has negative connotations, but for horticultural purposes it’s the same as saying ‘mineral’. Scientifically, the word ‘organic’ means anything that is or was once living and made of carbon. Unfortunately, ‘organic’ has been so heavily entrenched as meaning ‘natural’ – with its implications of wholesomeness and purity – that it is almost impossible to knock it off this lofty pedestal. To be clear, this is how we will use various terms when discussing fertilisers in this chapter:

  organic – derived from once-living things (for example, compost, urines, manures, guano and vermicast)

  mineral – not organic, but essentially ‘natural’ materials dug up and used as fertiliser (for example, rock phosphate, lime, kieserite and gypsum)

  synthetic – made during an industrial process, either deliberately or as a by-product (for example, highly refined urea, potassium nitrate and other purified materials used in hydroponics).

  In terms of plant nutrition, you just need to remember one thing: plants only take up nutrients in mineral form. All organic nutrients have to decompose or break down into their mineral constituents before plants can take them up, usually via the roots. This breakdown is mediated by the living organisms either in soil or in the compost heap. One of the most important reasons for composting is to promote the rapid breakdown of the organic molecules and to solubilise the minerals so they are readily available to plants.

  Organic fertilisers and composts

  There is a very broad range of products in this category, and, unfortunately, they differ greatly in their nutrient content and balance, and hence the purpose for which they are used. Some are so low in nutrients that they actually consum
e nitrogen, while others are so high that they work as fertilisers but are not really a good source of organic matter. The Types of Organic Fertilisers table on the opposite page outlines the various organic fertilisers and their usage.

  To grow really well, densely planted vegetables and lush culinary herbs need a soil that has a very high fertility level.

  TERRA PRETA SOILS

  These are palaeological urban soils that show greatly increased levels of black carbon ash (charcoal) in them. Ancient peoples made them by burning forests and adding the ash to their garden soils. Many benefits are extolled by true believers in its modern reincarnation, known as biochar, and it is said to have permanently solved the problem of soil fertility. Keep in mind that every society which produced terra preta soils is now extinct. Not only was this process not a permanent solution to the problem of growing high-yielding crops on poor soils in essentially urban environments, it was so environmentally destructive that the people practising it were exterminated by the environmental havoc they wrought.

  TYPES OF ORGANIC FERTILISERS

  ORGANIC PRODUCT (PERCENTAGE OF ORGANIC MATTER) COMMENTS USUAL APPLICATION PER SQUARE METRE

  Sawdust, pine bark, peat, straw (very high, 80–95 per cent) A good way to introduce bulk organic matter to improve soil, but will generally require the addition of other fertilisers to compensate for extremely low nutrient levels; it can cause nitrogen drawdown; source of organic matter only, not nutrients 10–20 litres

  Commercial composted green waste (moderately high, 45–70 per cent) A good way to introduce bulk organic matter to improve soil, but may require the addition of other fertilisers to compensate for low nutrient levels; about 1 per cent nitrogen; some fertiliser value 5–20 litres

  Composts containing food waste (moderately high, 45–70 per cent) A good way to introduce bulk organic matter to improve soil, and will generally provide enough nutrients for crops with moderate nutritional requirements; moderately high nutrient levels; moderate fertiliser value 5–10 litres

  Domestic compost and worm castings (moderately high, 45–70 per cent) A good way to introduce bulk organic matter to improve soil, and will generally provide enough nutrients for crops with moderate nutritional requirements; moderately high to high nutrient levels; moderate fertiliser value 5–10 litres

  Composted manures and mushroom compost (moderately high, 35–50 per cent) A good way to introduce bulk organic matter to improve soil, and will generally provide enough nutrients for crops with moderate nutritional requirements; moderately high to high nutrient levels; moderate fertiliser value 2–10 litres

  Raw manures (low to moderately high, 25–70 per cent) A good way to introduce bulk organic matter to improve soil, and will generally provide enough nutrients for crops with moderate nutritional requirements; moderately high to high nutrient levels; moderate to high fertiliser value 2–5 litres

  Pelletised fortified manures (low to moderate, 25–50 per cent) A concentrated source of nutrition that will also introduce small amounts of organic matter for soil improvement; high to very high fertiliser value 100–300 grams

  Blood and bone, rendered animal by-products, fishmeal (low, < 20 per cent) A concentrated source of nutrition that will also introduce small amounts of organic matter for soil improvement; high to very high fertiliser value 100–300 grams

  Note: A standard household bucket holds 10 litres. A standard 250-millilitre cupful of organic fertiliser weighs about 100 grams.

  Fortified organic-based fertilisers add a good balance of nurients to the soil, so they have a high fertiliser value.

  This table gives some important clues as to what to use in each situation. For example, if you want to improve organic matter without adding much in the way of nutrients, you would not use pelletised poultry manure – as this is really only applied to the soil for its nutrient value. You would use green- or garden-waste compost instead. You could even use sawdust, as long as you compensate for the nitrogen drawdown by adding some blood and bone or mineral nitrogen.

  Manures as fertiliser

  We do not recommend utilising raw animal manures for any soil used to grow salad greens or vegetables that are eaten raw, because of the possibility of salmonella poisoning. However, they may be suitable for other urban farm plots.

  One of the most fascinating things about manures is that the nutrient balance can vary dramatically between the different types. This is related to the particular diet of the animal whose manure you are using. Here are a few generalised observations on such materials.

  WHAT IS NITROGEN DRAWDOWN?

  Woodchips make excellent mulch, but they cause nitrogen drawdown. Soils topped with this mulch need extra nitrogen added to them.

  Nitrogen drawdown occurs when microbes consume the available nitrogen in the soil as they break down high-carbon materials (for example, woody items such as bark or woodchips, or fibrous components such as straw). This often leads to plants becoming deficient in nitrogen, and their foliage begins to turn yellow. It is commonly seen when straw, woodchips or bark are used as mulches, and insufficient supplementary nitrogen is supplied.

  Feedlot cattle manure Because it is very easy and economical to collect manure from feedlots, this is what tends to be available for purchase at garden centres. Feedlot animals are often fed diets that are high in mineral salt, protein and sometimes zinc. Manure products from feedlots are consequently usually high in nitrogen and salt, and have zinc levels that can be toxic to plants. Obviously, it is better if you can source manure from free-range animals if at all possible.

  Poultry manures Chickens and other commercial poultry are fed a diet that is very high in protein (for meat production) as well as phosphorus and calcium (for bone and egg production). Consequently, their manure tends to have excessive amounts of phosphorus and calcium, and to be deficient in potassium. For this reason, extra potassium is often used to supplement pelletised poultry manures.

  Manure from naturally fed animals The manure from farm or zoo animals fed on a diet resembling that of animals in the wild will be generally better balanced than feedlot manure. We find that manure from pasture-fed animals has the best balance of all, although it can sometimes cause weed problems – especially if the animals in question have been kept in a weedy pasture and consumed its seeds.

  Large amounts of green waste are often commercially composted to make an excellent organic fertiliser that can improve the physical fertility of soil.

  MANUFACTURED FERTILISERS

  As seen here, the common soil ions can be mixed and matched to make just about any mineral fertiliser you like. Actually, many of them are simply dug up as minerals, ground down and put in a bag. Unprocessed mineral fertilisers include things such as lime (calcium carbonate), dolomite (calcium magnesium carbonate), kieserite (magnesium sulphate), gypsum (calcium sulphate) and glaserite (potassium, sodium sulphate).

  Some minerals are dug up and then refined by dissolving them in liquid and precipitating them by drying. Sulphate of potash and Epsom salts (magnesium sulphate) are made this way.

  One mineral fertiliser that seems to receive unwarranted bad press is superphosphate, which is rock phosphate that has been dissolved in sulphuric acid (a waste product from the metal-smelting industry). Rock phosphate is insoluble, so it is generally unavailable to plants; dissolving it in acid changes it to a soluble form that plants can take up. For many years superphosphate has been the mainstay for broadacre farming, but for intense urban farming it is inefficient and unnecessary. It is worth mentioning that there are soil microbes which help make phosphates more available to plant roots, so applying compost to your soil will help this process.

  The most synthetic of all manufactured fertilisers are the nitrogen fertilisers: urea, ammonium salts (for example, ammonium sulphate) and nitrates (for example, potassium nitrate and ammonium nitrate). These are made in a similar manner to the way bacteria ‘fix’ nitrogen from the atmosphere. Called the Haber process, it was invented by German chemist Fritz Ha
ber before the First World War (where it was used to make nitrogen-based explosives). Basically, it’s a way of extracting the abundant nitrogen in the atmosphere to make ammonia gas, which is then combined with carbon dioxide to produce urea, the simplest naturally occurring organic molecule. Urea is also a waste product found in the urine of all animals.

  Pros and cons

  We support the use of manufactured fertilisers if there is a sound reason to introduce them, and in some situations, where only a single element is missing (for example, potassium), there is really little alternative. Some organic certifying bodies will now accept minerals that have been purified from natural deposits by simple processes such as precipitation, but they will not certify manufactured products such as superphosphate and urea.

  This doesn’t make much sense scientifically, as the plant doesn’t care where the nutrient ion comes from. However, we can see a couple of good scientific reasons why you might object to the use of manufactured fertilisers:

  They don’t represent a balanced diet – it’s a bit like feeding kids sugar and vitamin tablets, instead of fruit and vegies. Hence, using a fertiliser that just supplies nitrogen, such as urea, will tend to encourage soft, leafy growth but will not give as good a yield for fruiting plants or nutritionally dense food.

  They consume large quantities of fossil fuels during their production and distribution, which is detrimental to the environment as a whole. If the same nutrients can be supplied from locally recycled wastes, then we avoid creating pollution. This same argument, however, could be levelled at compost made commercially from green waste that is picked up by trucks, transported to centralised facilities, chipped, turned and watered for 12 weeks, screened, loaded back into trucks and transported to retail outlets – all using pollution-causing diesel engines and electricity. Nevertheless, the essential message remains the same: we should maximise our own recycling and the utilisation of suitable local wastes.