Grow Your Own Read online

  Sulphur (S) is important for protein synthesis, and it is usually supplied in combination with other elements as sulphates – for example, gypsum (calcium sulphate). In its elemental form, sulphur is used to acidify soil. Some plants, such as brassicas, have high sulphur needs, hence the ‘sulphurous’ smell of cabbages.

  Yellowing between the leaf veins of new leaf growth is often caused by iron deficiency in the plant.

  ELEMENT LEVELS IN PLANTS

  ELEMENT DEFICIENCY SYMPTOMS APPROXIMATE CONCENTRATION*

  Nitrogen (N) General paleness, overall yellowing of leaves, browning starting with oldest leaves, stunted growth 2–4 %

  Potassium (K) Lacklustre growth and flowering, yellowing then browning of leaf margins of oldest leaves, low wear tolerance in turf 1.8–4 %

  Calcium (Ca) Blackened and distorted stem and root tips, abnormal development of flowers and fruits, ‘end rot’ of flowers and fruits, poor root growth 0.3–0.5 %

  Phosphorus (P) Stunting, poor branching, overall lacklustre growth, red or purple tinges on leaves and stems 0.25–0.6 %

  Magnesium (Mg) Mottled yellowing on leaves 0.2–0.4 %

  Sulphur (S) Overall yellowing of leaves, slow growth 0.2–0.4 %

  Iron (Fe) Yellowing between veins of youngest leaves 100–300 mg/kg

  Manganese (Mn) Yellowing between veins of youngest leaves, deformity of growing tips 50–500 mg/kg

  Zinc (Zn) Yellowing between veins of leaves, deformity of growing tips 15–50 mg/kg

  Copper (Cu) Yellowing between veins of leaves, deformity of growing tips 5–20 mg/kg

  Boron (B) Deformity of growing tips, ‘hollow stem’ in brassicas, stem and tuber browning 10–50 mg/kg

  Molybdenum (Mo) Yellowing of leaves, ‘whiptail’ in brassicas 0.03–0.1 mg/kg

  *Note: We usually measure macronutrients as a percentage by dry weight. Micronutrients are present in much lower amounts, however, so we express these as milligrams per kilogram, which is a more sensitive unit of measurement.

  Micronutrients

  Iron (Fe) is essential during chlorophyll production. It is insoluble, and its availability to plants is dependent on soil pH, as it becomes unavailable when the soil is alkaline.

  Manganese (Mn) is very much like iron – it is necessary for manufacturing chlorophyll, and its availability is dependent on pH levels. To some extent these two elements can substitute for one another (for example, iron deficiency can be alleviated somewhat by adding manganese, but not completely). An excess can be toxic to many species, such as clover; manganese toxicity is common on acid soils in manganese-sensitive plants.

  Zinc (Zn) and Copper (Cu) are essential to many enzyme pathways, synthesising myriad organic compounds that make up plants. Generally, they are less available when the soil has a high pH level. They are often deficient in rural soils, but rarely deficient in urban soils due to pollution with metals. Sometimes this pollution rises to levels that are toxic to plants.

  Boron (B) is vital for growing cells. Deficiency is more common in certain groups of plants, such as brassicas (for example, cabbage, broccoli and cauliflower). However, deficiencies are uncommon in urban soils.

  Molybdenum (Mo) is required by plants for nitrogen metabolism, such as turning nitrates into proteins. Fortunately, due to human activity, obvious molybdenum deficiency is very rare in urban soils; it is really only seen in legumes, such as clovers, that are growing in pasture soil. Organic fertilisers and composts contain all the molybdenum you will need for your urban farm.

  NUTRIENT DEFICIENCIES

  Plants need about 15 elements from the soil to live and grow. An inadequate supply of one or more of these elements leads to an array of problems, from discolouration of foliage to distortion of growth. The Element Levels in Plants table on the opposite page summarises the symptoms of nutrient deficiencies in plants and gives an estimate of the concentration of nutrients within living plant tissue. The last column provides other vital information, because it tells us the best ratio of nutrients we need to put back into the soil after we have removed them by harvesting a crop.

  An important principle in feeding your plants is that of the most limiting nutrient. A plant will only grow as well as the most limiting nutrient allows. For example, if a soil is high in all nutrients except nitrogen, any plants growing in that soil are likely to be pale and sickly – despite having most of the nutrients they need, they still will not thrive. If we add nitrogen to the soil, the plants will respond rapidly without the need to apply any other nutrient. The same issue can happen with trace elements such as iron, where an application of iron will often cure yellowing of the new vegetative growth.

  DOES YOUR PLANT HAVE A NUTRIENT DISORDER?

  Most people are quick to blame poor nutrition when things go wrong with their plants, but it’s important to eliminate other possible causes first. Here’s a list of things to check.

  □ Water content Dig a hole to about 150 millimetres deep near a struggling plant. Does the soil look right for the desired moisture content? Or is it too wet or too dry?

  □ Root problems Have a look at the plant roots that are protruding into the hole. Are there signs of galls or swellings that could be due to nematodes? Are the roots black and mushy, which is an indication of root disease? If the plant is healthy, then the roots should be fleshy and white.

  □ Foliage issues Examine the leaves for grubs, aphids and scale insects.

  □ Stem trouble If you find holes in the stem, and frass (which looks like fine sawdust) around the holes, then you may have borers.

  If you have checked the above list and you have not discovered any obvious problems, then you can start to think about possible nutrient disorders. Some of the most frequently seen disorders are given in the table here If issues persist or symptoms are not clear, you should have your soil or plant foliage tested by experts.

  COMMON PLANT NUTRIENT AND OTHER DISORDERS

  APPEARANCE SYMPTOMS LIKELY CAUSES CURE

  Wilting and dieback, perhaps with marginal burn on leaves Root-rot diseases

  Waterlogging

  Excessive fertiliser or salts Try a fungicide drench

  Stop watering, and dry out soil

  Leach heavily to remove soluble ions

  Pale spindly plants, poor growth Nitrogen deficiency

  Multiple nitrogen/

  phosphorus/

  potassium deficiency Apply a high-nitrogen side dressing

  Apply composted manure tea, liquid fertiliser from a worm farm or mineral NPK fertiliser

  Marginal yellowing or burn on leaves, poor growth, poor fruit set Potassium deficiency Water with compost tea, worm juice or sulphate of potash (2 g/L)

  Stunting, yellowing and reddish colours on leaves Phosphorus deficiency Use a high-phosphorus organic fertiliser or water with monoammonium phosphate (1 g/L)

  Yellowing between veins of leaves, usually worst on youngest leaves Iron and/or manganese deficiency due to high pH level, excessive phosphorus or both Spray foliage with chelated* iron and manganese

  *Note: ‘Chelated’ means the element is bound to an organic molecule that acts as a carrier and keeps the element in solution (otherwise it would precipitate because it is not very soluble). Chelated elements can be purchased from many nurseries and garden centres; sometimes they are sold as part of balanced mixtures containing a number of micronutrients (trace elements).

  A ‘cold’ compost bin like this one is the perfect way to recycle household wastes and return vital nutrients to the soil.

  NUTRIENT CYCLES

  In the wild, essential plant nutrients are accumulated and efficiently recycled in natural soil/vegetation systems. This is achieved via the all-important litter layer on top of the soil in forests, and the ‘thatch’ layer at the base of plants in grasslands. In many forests around the world (and especially within Australia), nutrients are released back into the soil by fire followed by rain; the water dissolves the nutrients and washes them back into the soil, thereby completing t
he nutrient cycle and fertilising the plants.

  In human farming systems, these natural nutrient cycles need to be replaced or mimicked in order to maintain the correct balance of soil nutrients for optimum plant growth. Large farms use inputs of fertiliser and the rotation of crops to help maintain fertility, but even these farms will eventually need to address the inevitable run-down of organic matter and physical fertility if they are to maintain soil productivity. In intensively used urban-farming land, these nutrient cycles are even further interrupted – there is no forest litter or thatch layer, and seldom do we have the space to rotate crops.

  We can grow crops purely on synthetic fertiliser inputs – as hydroponic culture proves – but, fortunately, urban environments also produce a lot of waste organic materials that we can use for composting and worm farming. This returns precious nutrients and organic matter to soil, thus creating, preserving and even restoring fertility. Many of these inputs are so cheap that we often find urban soils actually increase in fertility over time. Read Composting and Mulching for information about the composting process that substitutes for natural cycling and the many ways it can be adapted to create superbly sustainable urban farms.

  SOILS AND SOIL FERTILITY

  WORKING FROM THE GROUND UP

  Soil can be thought of – especially in a farming context – as a delivery system for the minerals and gases that plants need to grow. Plants can also grow in the complete absence of soil, as long as the nutrient minerals are carefully balanced in the proportions that the plant needs, by either creating fully balanced organic fertilisers or by using purchased mineral fertilisers dissolved in water (see the section on Hydroponics. The principles discussed in this chapter relate to the fertility of all growing media, from natural soils to lightweight potting mixes for rooftop gardens.

  Depending on your urban farm’s particular circumstances, you will most likely use either natural or artificially constructed soils for your garden beds. The first preference is to utilise your on-site soil if at all possible, as this is the cheapest and easiest option. Well-managed natural soil is very forgiving of our mistakes and the imbalances we can create in soil fertility, and it helps to ‘even out’ nutrient supply. We can do many things to boost this ‘buffering’ capacity of soil, so that most of us can grow healthy urban produce without having to fully understand all the technicalities of soil science.

  NATURAL SOILS

  Most people have heard of topsoil and subsoil, but few know that there are three basic soil horizons, or layers: A, B and C. Technically, there are actually five horizons – the O horizon comprises the organic matter that lies on top of any healthy natural soil, and the R horizon is the bottom layer of rock (this horizon is more accurately called ‘parent material’, as it’s not always rock). The Soil Profile diagram on the opposite page shows the different soil horizons.

  In natural systems, the weathering of the rock (parent material) releases dissolved nutrients; the vegetation takes up the nutrients it requires and discards any elements it does not need. As plants die and decay, the nutrients that they have bioaccumulated are returned to the soil in roughly the same proportion as the plants took them up. Nutrients released by the decomposing vegetation in the O horizon make their way into the topsoil, where plants take them up; this nutrient cycle repeats again and again. Over time, the important plant nutrients accumulate in the topsoil, improving its fertility relative to that of the original parent material. This is the essential basis of nutrient cycling or bioaccumulation in natural soil systems.

  In urban soils, we don’t have the decomposing vegetation to allow nutrient cycling – or do we? Mulches are a human-created O horizon, and some – such as pea straw or lucerne hay – will break down and enrich the topsoil in an urban farm. In the absence of mulch, compost becomes an urban farmer’s best friend. Composting is an artificial way of cycling nutrients, and it is vital to maintaining the high level of soil fertility required to grow extremely productive and nutrient-dense horticultural crops. It circumvents the need for natural bioaccumulation to maintain the fertility of soil, and it does not take thousands of years. In a very real sense, composting allows us to create our own topsoil.

  Preservation and enhancement of the precious existing bioaccumulated topsoil should be a priority for every farmer and gardener. However, thanks to our current knowledge of soil chemistry and plant needs, we now have the means to quickly improve a soil’s fertility and productivity if necessary. In order to understand how to do this, we need to look at the concept of soil fertility and how it relates to our urban farm.

  Urban soils can have very different profiles to the normal A, B and C horizons of natural soils.

  NOT ALL ROCKS ARE EQUAL

  I once heard a NASA astronaut talking about the soil samples he obtained from the moon. He was quite wrong to use the word ‘soil’, because there is no soil on the moon – only powdered rock. You need living things – including microbes, plants and animals – to work on powdered rock before you get soil to form. Because of bioaccumulation, we find that soils all over the world show remarkably similar properties, although it is certainly true that parent materials with better levels of the essential nutrients (such as igneous rocks created by volcanic activity) form better and more productive soils more quickly than those with poorer geology.

  If your topsoil has a high level of fertility, it will be dark in colour and loamy in texture.

  SOIL FERTILITY

  In The Needs of Plants, we saw that plants absorb carbon, hydrogen and oxygen from the atmosphere, but they obtain the great majority of their nutrients from the soil – the nutrients are dissolved in water and taken up by the roots. Because plant roots need elements both in gaseous and liquid form, it is useful to divide soil fertility into three parts:

  Physical fertility – the soil has optimum amounts of air and water.

  Chemical fertility – the nutrients are in the right amount and balance.

  Biological fertility – this is important for particular plants that form symbiotic relationships with microbes (for example, legumes and Rhizobium bacteria).

  If a soil has excellent physical fertility, then it is able to provide the physical support, water and aeration that plant roots need. Note this focus on the role of the roots – they must remain healthy so they can support the plant in an (often) upright position, allowing the leaves to get maximum sunlight, and take up water that is needed for both nutrient uptake and photosynthesis. Also, the uptake of water is an active process that requires energy from respiration, so therefore the roots need oxygen. No oxygen, no root function – and the plant wilts, as it is unable to transport water and nutrients to the leaves. Thus, somewhat ironically, the first sign of waterlogging in plants is wilting!

  The chemical fertility of a soil is its ability to supply the elemental nutrients that plants require, in the ratios and proportions they need them (The Needs of Plants chapter discusses in detail the essential plant nutrients and the amounts needed for healthy growth). This chemical fertility can be supplied from either organic or inorganic sources, and nutrients such as nitrogen are often derived from both. Thus, we can build chemical fertility with different fertilising strategies.

  Physical and chemical fertility are intimately related. Plant roots take up their chemical elements dissolved in water. The oxygen needed by roots to do this is in itself a chemical, and it could be thought of as an essential plant nutrient. Cut off the oxygen to the roots, and there is no water or nutrient uptake. Therefore, physical fertility is the most important consideration when it comes to soils, because plants will die much faster from a lack of water or oxygen in the soil than from any nutrient deficiency. A soil with good physical and chemical fertility will usually also show good biological fertility.

  If a soil has excellent physical fertility, then it is able to provide the physical support, water and aeration that plant roots need.

  PHYSICAL FERTILITY

  This is possibly the easiest thing f
or urban farmers to judge for themselves – you just have to answer the question, ‘What does a root need?’. In a nutshell, a root requires:

  physical support

  water

  oxygen (gaseous exchange)

  ease of penetration (the more energy it takes to penetrate the soil, the less energy is available for production of edible structures)

  a nutrient supply (again, the harder a root has to work for this, the less energy is available for producing a useful yield).

  To assess whether your soil has physical fertility, you just need a nose, an eye and a hand, and maybe a couple of tools. Use all your senses to examine your soil, and you will soon learn about its density, structure and texture.

  Soil density

  The first issue to consider is soil density. Compression of a soil by either foot or machinery traffic in urban environments causes compaction and an increase in soil density, and this is the great enemy of root expansion. The denser the soil, the harder the roots have to work to penetrate it, and to access water and nutrients.

  Use a hand trowel or spade to obtain a soil sample – the very act of digging will tell you plenty about the soil’s density. Now look at the topsoil. Is it dry, yellowish powder, or dark and crumbly aggregate? Dark soil indicates that organic-matter levels are high (which is usually good), while pale yellow or red soil with no peds signifies a poor structure and low organic-matter levels (which is bad, unless you have a sandy soil that naturally drains well). The Soil Density table below describes various levels of soil density, and how to use each soil type.

  Soil structure

  Grab a handful of soil, and loosely crumble it in your hand. Can you see little grains or big chunks of soil? Structure is concerned with the size and shape of soil particles, and how well they bind together into crumbs (which are known as peds or aggregates). These peds have different shapes – such as ‘granular’, ‘blocky’, ‘platy’ and ‘prismatic’ – and inside them are small pores in which water is held. In the spaces between the peds are large pores through which water drains by the force of gravity, allowing air to enter. A well-structured soil has a balance between the two types of pores, making it both free draining yet able to retain a good amount of moisture for plant growth. The soil is similar to a good sponge cake: light, fluffy and full of air.