Organic Soil Guide - Introduction and Chapter 1


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The introduction and first chapter for the Organic Soil Guide, a book written to help you understand your garden soil.

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Organic Soil Guide - Introduction and Chapter 1

  1. 1. Organic Soil Guide Healthy Soil, Healthy Plants, Healthy You Alisa Bryce
  2. 2. It took me nearly a decade of farming to accept that healthy soil means healthy plants - Jackie French
  3. 3. Organic Soil Guide | Author Alisa Bryce @ 2012 Alisa Bryce ISBN: 978-0-9873291-0-3
  4. 4. Organic Soil Guide Contents Introduction 1 What is soil? 2 Where does soil come from? 3 Why Organic? 6 Chapter 1 Physical Soil 7 Texture 8 Structure 14 Depth 18 Colour 19 Chapter 2 Soil and Water 22 Infiltration 23 Drainage 27 How much water can my soil hold? 27 Chapter 3 Soil (and Plant) Nutrition 30 Why plants need nutrients 31 Where nutrients come from 31 Nutrient deficiency 33 Soil pH 37 Chapter 4 Organic Matter and Soil Life Forms 41 What does organic matter do? 42 Soil carbon 43 Soil dwelling creatures 44 Worms and worm farms 45 Bacteria 47 Soil Fungi 47
  5. 5. Chapter 5 Composting 49 Manure 51 Compost for nutrition 52 Compost recipes 53 General composting rules 56 Common problems & solutions 56 Mulch 58 Chapter 6 Common Soil Problems 61 Waterlogging 62 Water repellence 63 Shading and tree root competition 64 Salinity 65 Surface crusting, sodicity and dispersion 66 Perched water 69 Frozen soil 69 Chapter 7 Preparing Your Soil for Planting 70 Preferred soil type for common vegetables 73 Chapter 8 Water Wise Gardening 77 Glossary 81 References 83 Appendix A How do I fix it? 84 Organic Soil Guide
  6. 6. Introduction Soil. The life source of your garden. The soil is where plants get the water and nutrients they need to grow and thrive. A malnourished soil will breed malnourished plants. You wouldn’t build a house on a poor foundation, and neither should you plant a garden on poor soil. Jackie French herself said “it took me nearly a decade of farming to accept that healthy soil means healthy plants” and Franklin D Roosevelt quoted “a nation that destroys its soil destroys itself.” The importance of soil has been known since Egyptian farming, yet too often we don’t give our soil the care it deserves. If you want to grow your own healthy, vibrant, organic vegetables and plants, this is the book for you. Organic gardening is rising quickly in popularity, as health concerns over chemical pesticides and fertilisers encourage people to utilise more natural solutions. Luckily, organic gardening is something you can easily do yourself. And that is done by managing your soil properly. How you treat your soil is the key difference between conventional and organic gardening. How much do you really know about your soil? Do you know if it’s a sand, clay or something in-between? Do you know how deep it is? Is it constantly dry, or always waterlogged? Even after adding piles of organic fertiliser or compost, do your plants still struggle? A problem in your soil will inevitably translate into a problem in your plants. This book will teach you how to assess your garden, pinpoint problems and provide options to fix them. The different chapters cover soil pH, mulching, making a water wise garden, assessing the size of your ‘soil water tank’, worm farms and composting. Compost and organic gardening should be inseparable. Compost provides the nutrients to rebalance your soil and improve your plants health. If you’re already an inspired organic gardener, recycling and waste management should be part of your lifestyle as well. The recipes in the composting chapter will help you turn scraps and organic waste into potent tonics and composts for your garden. This book could be the answer to your problems. Read the chapters and follow the self assessment instruc- tions. You may find the source of your problems and that your gardening problems are easily solved. Organic Soil Guide 1
  7. 7. What is soil? The soil is the life source of your garden – the foundation. You wouldn’t build a house on a bad foundation, and neither should you plant a garden on poor soil. Soil provides not only the water, air and nutrients for plant growth, but physical support as well. Without a suitable soil, plants will continually struggle. When you get down to it, soil is complex. It is more than just ‘dirt’. The soil itself is made up of organic matter, minerals, living organisms (fungi, microbes), as well as containing air and water as life support for plants and soil dwelling creatures such as worms. The type of soil you have can dictate what type of plants you can grow, and how well they will grow. Depend- ing on your soil, plant roots might only grow 100mm deep, or they might grow to their full extent. Did you know the amount of sand or clay in your soil will determine how fertile your soil can be? The soil affects many different aspects of your garden, including: • The types of plants that will grow • Plant root depth and vigour • How much water you need to apply, and how frequently to apply it • Soil fertility - how nutrient rich your soil can become • Drainage - will your soil be waterlogged, dry, or something in-between? • Frequency of disease and weeds - believe it or not soil quality plays a big role in the presence of weeds and disease. For example, crab grass absolutely loves waterlogged soil, while most other plants don’t. It’s almost guaranteed that in an open field, if there are areas of soil that are compacted and waterlogged, crab- grass will set up house! Organic Soil Guide 2
  8. 8. Where does soil come from? Just as milk does not come from the supermarket, soil does not come from your local nursery. Milk comes from cows and soil comes from rocks. Rocks are weathered to make the mineral part of the soil. A mineral is a solid substance that occurs naturally. Minerals range from pure elements such as nitrogen, phosphorus, etc. right up to complex structures such as quartz, gold and opal. The most common minerals in soil are the nutrients – nitrogen, potassium, phosphorus, calcium, magnesium, sulphur. Common rocks (or parent materials) include granite, basalt, quartz, sandstone and limestone. Differ- ent soils form based on the type of parent material, the climate, and how the parent material was weathered. Weathering is the process where rocks are broken down into smaller rocks, particles and eventually soil. Types of weathering include: • freezing and thawing • heating and cooling • wetting and drying • grinding – rocks grind or rub against each other • organisms – can slowly breakdown rocks and minerals Organic Soil Guide 3
  9. 9. The soil profile If you dig deep enough, or drive by an exposed soil surface, you’ll probably see a ‘soil profile’. Profiles have dif- ferent layers or ‘horizons’, each with different physical and chemical properties. Horizons form because soil is constantly changing. Weathering causes rocks and soil to become smaller and smaller, eventually leaching (being washed) deeper into the soil profile. The rate of weathering, type of parent rock, climatic conditions and what plants grow in a particular area all interplay to create different soil horizons of varying colours, textures and depths. Human intervention also plays a big role – see “But I have clay!” on page 13. Broadly speaking, we can divide a soil profile into 4 separate horizons: These 4 layers are, of course, a very basic outline of a soil profile. If you want to delve into the various sub lay- ers, consult the Australian Soil Classification, or the soil classification relevant to your country. For the moment, we are only interested in the first 2 layers, the O and A. This is where it’s at! Where you will be working, and your plant roots growing. Microbial activity, root growth, nitrogen fixation - almost all biological activity takes place in the O and A horizons. Throughout the rest of this book, these layers will be referred to as the ‘topsoil’. • O horizon – the ‘organic’ layer, where most of the organic material is • A horizon – where the most biological and micro biological activity takes place. Soil organisms and plant roots are concentrated in this layer. • B horizon – the subsoil. In residential properties, this usually means clay. • C horizon – parent material and the ‘R’ (rock) Organic Soil Guide 4
  10. 10. Soil layers Volcanoes, glaciers wind and water all move soil around, forming different soil profiles and layers. There are a few different terms to describe how soil is moved around: Eluviation – soil particles are washed away Illuviation – soil particles are deposited/accumulated Leaching – soil particles and minerals are washed down through the soil profile. Podsolisation – when strongly acidic soil water breaks down clay and washes it away Gleying – in waterlogged soil (no oxygen), iron in the soil is either removed or forms clumps. A wetland will often have a gleyed soil. In your garden, leaching is the most common type of soil movement. As you water, or it rains, soil particles and minerals at the top of the soil are washed deeper into the profile. Sometimes they can be washed com- pletely out of the soil, and into the groundwater or local streams. Erosion or human activity also removes the topsoil, leaving you a nutrient poor subsoil to work with. So try to keep your soil planted at all times! Bare soil is bad soil. Organic Soil Guide 5
  11. 11. Why Organic? Organic gardening has numerous benefits for your soil, your plants, and if you are eating what you grow - you. Chemicals, whether pesticide, herbicide or fertiliser, can migrate through your soil, damaging or killing benefi- cial soil life such as fungi and earthworms. Once in your garden, chemicals can affect insects and fauna that come to visit. Colony collapse disorder, the inexplicable loss of bees, is wreaking havoc on beekeepers and farmers alike. Bees and other pollinators are essential for plant pollination, and their decline may be because of the chemicals applied in our gardens and farms. Birds and mammals that visit your garden may also be harmed by plants sprayed with chemicals. Some common garden chemicals are known to be carcinogenic, can cause liver and kidney failure, and in some cases cause birth defects. Rachel Carson’s 1962 book Silent Spring chillingly describes the effects DDT use was having on both humans and the environment, from the deaths of thousands of birds and fish, to rapid increases in the incidences of cancer. DDT has since been banned in many wealthier countries, but some poorer nations with limited other options still use DDT. Glyphosate, one of the most commonly used herbi- cides, has been heavily used as it was thought to not persist in the soil. But more recent research is showing Glyphosate can make plants more susceptible to disease, and restrict the plants ability to take-up nutrients (Johal and Huber, 2009). This means produce grown in soil where Glyphosate has been applied will be nutri- ent deficient, leading to inferior quality food. And unfortunately to combat this, chemical fertilisers are the most frequent solution... Chemical fertilisers were first created and used in the 1830’s, by German chemist Justus von Leiberg. The ‘father of fertiliser’ was at first very encouraged by increases in yield, but after a few years yields declined, and farmers needed to add more and more synthetic fertilisers to achieve the same results. In some cases, pro- longed use of fertilisers reduced the fertility of the soil. Yet instead of returning to organic methods to replenish the soil, fertiliser use increased exponentially, particularly during the food shortages of World War I and World War II. By the early 1940s, experiments in the U.S. illustrated plants developing dependence on synthetic ferti- lisers. In an organic system, soil nutrients peak naturally during the growing season. But with chemical use, the relationship is more akin to a drug addict and their substance. Plants need a ‘fix’ from fertilisers to grow. By choosing to garden organically, you can prevent your plants becoming fertiliser addicts. In your home, organic gardening can be cheaper, as you don’t need to pay for chemical inputs. Using your household waste (e.g. lawn clippings, food scraps) to make your own fertilisers is a great way to both nourish your garden and reduce your waste footprint. Finally, organic gardening makes your garden a part of nature - not separate from it. Birds, insects, and fauna that visit can do so without the threat of encountering nasty chemicals. Organic gardens are harmonised with nature, ecosystems that support diverse species of flora and fauna, and can be replenished naturally. Organic Soil Guide 6
  12. 12. Chapter 1 Physical Soil
  13. 13. Texture Soil texture is the ‘feel’ of your soil. You would have noticed when gardening, if you have a very sandy soil, or a clay soil. Sandy soils are rough and gritty, and don’t stick together very well. Clay soils on the other hand can be too sticky, compact easily, and make it difficult for plants to grow. The proportion of sand, silt and clay determine a soils texture. A ‘sandy’ soil will be mostly sand, with only a small portion of clay and silt. A ‘silty’ soil will be mostly silt, with some clay and sand. When describing a soil, the more clay and silt a soil has, the ‘heavier’ it is. So a clay soil is ‘heavier’ than a sandy soil. Size is how we differentiate between sand, silt and clay. Clay is smallest in size, and sand the biggest. From smallest to largest they go: clay, silt, fine sand, coarse sand. Most soils are a combination of sand, silt and clay. This is because erosion, rainfall and human activity move and mix different soils together. The soil texture triangle (below) can be used to work out what texture your soil is. For example, a soil with 20% clay, 10% silt and 70% sand will be a sandy loam. The texture triangle is best used when you know exact per- centages of sand, silt and clay. If you don’t know exact percentages (which is more commonly the case) you can use the ‘ribbon test’ (see Page 9) to work out your soil texture. Organic Soil Guide 8
  14. 14. 9 Why texture is important Soil texture is important because it influences so many other soil properties. For example, a loam will hold more water than a sand, but a sand will compact less readily than a clay. Read on for a more comprehensive list of different textures and their properties. • Water holding capacity – the more clay/silt in a soil, the greater amount of water it can hold • Nutrient holding capacity – a straight sand holds almost no water or nutrients. A loam on the other hand will hold plenty of nutrients. • Drainage – the sandier the soil, the greater the capacity for drainage. This is why a straight sand holds minimal water – it’s lost to drainage. • Compaction – clay soil compacts more easily than sand. It is still possible for a sandy soil to become compacted, but it will take longer than a clay • Organic matter content – a heavily textured soil (i.e. more clay) can hold more organic matter than a sandy soil. This is because a) clay particles are more ‘charged’ and can hold more organic matter, and b) organic matter decomposition occurs faster in a sandy soil. • pH buffering capacity – the ability of a soil to resist pH change. Fine textured clay soil usually has a better buffering capacity than a coarse sandy soil. It’s easier to change the pH of a clayey soil than a sandy soil. Exercise 1 Your soil texture – the ribbon test 1. Take a handful of soil. Don’t take the soil directly from the surface because you’ll get mostly plant roots. Dig a small hole and take the soil from about 5 - 10cm depth. 2. Add water slowly until you can make a wet ball with the soil – a ‘bolus’. Be careful not to add too much water, as it’s very difficult to texture a soil that is saturated. 3. If you can’t make a ball, and the soil feels like sand, you’ve probably got a sand. 4. Keep working the soil around in your hand. Using your thumb and forefinger, try and make a ‘ribbon’ out of the soil 5. Use the flow chart and table to work out your soil texture. Slowly add water to soil work soil in hand to make a ‘bolus’
  15. 15. Makes a ball Ball falls apart easily Rolls into a ribbon Silty/soapy feel Fells sandy/gritty Ribbon cracks when bent Sandy feel predominates Hand heavily stained No No No No No No No No Yes Yes Yes Yes Yes Yes Yes Sand Loamy sand Sandy clay loam Clay loam loam Clay Silt or silty loam Start Here Organic Soil Guide 10
  16. 16. Texture Description Ribbon Length Sand Sand granules up to 2 mm in size. Sand can be described as either ‘fine’ or ‘coarse’. A sandy soil is less chemically active than a clay soil. Nil Loamy sand Approximately 85 % sand and 15 % clay & silt (fines). A loamy sand resembles sand in that it is loose and single-grained, and most individual grains can be seen and felt. Loamy sands are slightly cohesive when moist. 5 mm Sandy loam Approximately 60 % sand and 40 % fines. Many of the individual sand grains can still be seen and felt. When wet, the soil is easily rolled into a ball. 15 – 20mm Loam Approximately 40 % sand, 40 % silt and 20 % clay. When worked into a bolus, a loam is smooth and spongy in the hand, and very coherent. 25 mm Sandy clay loam Easily forms a bolus – sand grains can still be seen and felt 25 – 40 mm Clay loam Bolus strongly coherent and plastic - smooth to manipulate. Bolus can feel spongy 40 – 50 mm Silt Feels like talcum powder and has a sliding feeling between your fingers 50 - 75mm Clay Clays have a high surface area and can have a high amount of chemical and physi- cal activity. A clay will become more plastic when worked in the hand. When textured, clay soil often stains your hand > 75mm Heavy clay Feels like plasticine when worked in the hand >75 mm Organic Soil Guide 11
  17. 17. Exercise 2 Particle Size Analysis There’s a quick and easy test you can do to see the proportions of sand, silt and clay in your soil. All you need is a glass jar (an old jam jar works well), a scoop of soil and some water. 1. Half fill the jar with soil from your garden 2. Add water to the jar until it is almost full. Leave about 5 cm air space at the top 3. Screw on the lid and shake the jar vigorously for a few minutes. You want to try and break up all the soil clumps (peds) 4. Stand the jar somewhere it won’t be disturbed. Let the soil and water settle for at least 8 hours. After at least 8 hours, look at the soil in the jar. You will see layers. Sand will be at the bottom, silt in the middle and clay at the top. You may also have pieces of organic matter floating on the top. The percentage of each layer tells you what kind of soil you have. You can try and use the soil texture triangle on page 8 to work out what type (texture) of soil you have. Note how easily you can see the three different layers in the photos below. This soil is about 50% coarse sand (bottom layer), 35% silt (middle layer) and 15% clay (top layer). The water at the top still hasn’t cleared, indicating there is some very fine clay that is still yet to settle. Organic Soil Guide 12
  18. 18. But I have clay! Do you know whether soil was removed from your property before your house was built? Common practice is for most of the original topsoil to be carted away to create a stable building site on the clay subsoil. Later, new (and inferior) topsoil is often trucked in. This is known as ‘cut and fill’, and presents a problem for you as the topsoil (that was carted away) is where most of the nutrients, microbes, fungi and other living parts of the soil are. If you have a clay ‘topsoil’, you will need to improve it. This is best achieved by: • Digging in coarse compost to the top 50mm • Digging in sand (about 1 parts sand to 10 parts soil) Keep digging in organic matter, compost and small amounts of sand until you’ve achieved the soil texture you want (should be similar to a loam). Soil Layering You may find you have layers of different textures in your soil, like the picture below. Clay Loam Sandy Loam Clay Loam Loamy Sand Clay Loam Sandy Loam Clay Loam Sand This is most commonly caused by topdressing and re-turfing. Turf usually comes with a clay loam underlay which helps kept the turf intact when it’s being moved and layed. When you lay turf, the clay loam layer stays, and often compacts to form a seal near the surface. On a soil profile like this, water infiltration is impeded - each time the water hits the heavier textured clay loam layer it will start to pond. Layering is good for cake, but bad for soil infiltration and plants. You need to mix the different layers of the soil together to try and make a uniform profile. Organic Soil Guide 13
  19. 19. 14 Structure The structure of your soil is so important, but commonly ignored. The structure of a soil determines how easily plant roots will penetrate. If your soil is as hard as concrete, the plants are really going to struggle to establish their roots. Similarly, if your soil has no structure, just like a pile of sand at Bondi Beach, plant roots may find it difficult to establish. You need a good combination of soil chunks (peds – see page 17) and pores to allow plant roots to establish and grow. We can divide the soil up into 3 major fractions – air, water and solids. An ‘ideal’ soil will be about 50% sol- ids (minerals), 25% air and 25% water. When soil is compacted, the air and water portions decrease, to look something like the ‘compacted’ picture above. A compacted soil is more dense, and has less pore space (less room for water and air). Compaction can lead to a waterlogged (saturated) soil, and the formation of depres- sions in the soil surface. These depressions then fill with water during rainfall – this is known as ‘ponding’. As a minimum, 10% of the soil should be air space. This is in addition to the water. Oxygen is vital to let roots breathe. It is also required for the absorption of some essential nutrients by plant roots. Roots and micro-organisms not only absorb oxygen but release carbon dioxide during respiration. Accumulation of carbon dioxide could result in toxicity to the organisms growing in the soil. If a soil has poor drainage, it may look something like the ‘waterlogged’ picture above. Poor drainage is as bad as a compacted soil. Once all the soil pores are filled with water, an anaerobic (no oxygen) environment is cre- ated. To fix a waterlogged soil, make sure your soil is well aerated. If the soil is not compacted but water is still ponding, you may be adding water faster than the soil can take it. Also check the slope of your land. Is all the water draining towards one corner of your yard? If this is the case, you probably need to adjust the drainage patterns – install a swale or build up the sunken corner by adding more soil. But remember! The water will still have to drain somewhere, so make sure it doesn’t drain towards your house.
  20. 20. The most important structural issue you will face, particularly on your lawn, is compaction. The more compact- ed your soil, the harder it is for plant roots to grow. A compacted soil also has less room to hold water, which as well as stressing your plants, can lead to problems such as water repellency (See page 63). If your soil is compacted, you need to aerate. The simplest and cheapest method is to take a garden fork and work it into the soil to create holes or ‘cores’. If you have a large area of lawn, you might aerate using a push corer. But I heard that ‘no till’ was the way to go… “No till’ or the practice of not cultivating is a great idea, particularly considering the poor quality of most Aus- tralian soil, but unfortunately not always practical. On a lawn (turf) surface, you need to aerate the soil. Other- wise, as you walk or drive on your lawn, the soil will continually compact, reducing the air space in the soil and suffocating your plants. In your garden beds, depending on the soil type, you may be able to get away with no till. No till, or no dig gardening lets roots grow deep into, and create pores within the soil. No dig gardening is also a great way to maintain a healthy soil structure. No-dig Gardening is becoming increasingly popular. Layers of organic material are built up on top of the soil, rather than dug into it. No-dig gardens are great because they can be set up anywhere, they are easy to make and are very useful if you struggle to grow veggies or plants in your current soil. What to do... A no-dig garden can be whatever height you like. Begin by laying down a layer (about 6 sheets thick) of wet newspaper. Use more newspaper if there are weeds. Alternate with thin layers of brown organic material (e.g. pea straw, lucerne, dry grass clippings), com- post and manure, wetting each layer thoroughly. Keep adding until the garden is the height you want. Make sure that the top layer is the ‘brown organic material’, which acts as a great mulch to suppress weeds, hold water and insulate the soil. Water once more and leave for at least 2 weeks before planting. Make sure your no-dig garden is in a fairly sunny position. What to plant Vegetable seedlings, flowering annuals, herbs, bulbs and strawberries all thrive in a no-dig garden. To plant seedlings, pull aside the mulch and add one or two handfuls of compost to the hole that you’ve cre- ated. Make a hole in the compost and plant the seedling into this compost. Your no-dig garden will approximately halve in height in the first six months as it composts and breaks down. So if you eventually want a 30cm high bed, initially make your no-dig garden 60 cm high. To maintain the health of the no-dig garden area, add home-made compost at least once , but preferably twice a year. Spring and Autumn are the best times to add more compost. Organic Soil Guide 15
  21. 21. Exercise 3 Structure To check your soil structure, you will need a shovel. Try to cut a ‘cube’ of soil out of the ground. Once you have the square out, look at the profile. • Does it appear completely solid, like a brick? • Does the soil crumble into peds (chunks)? • Is it single grained like sand? Using a shovel, cut a ‘cube’ of soil from the ground This soil looks ‘crumbly’ and has peds, a sign of good structure If your soil is like a brick, it is compacted, and will need aeration. If you have a ‘single grained’ structure, you most likely have a sand. Adding compost and organic matter helps aggregate the sand, making a better soil structure. If you can see individual peds and are feeling eager, have a go at classifying them. Organic Soil Guide 16
  22. 22. Ped Shapes Soil structure is the shape that the soil takes based on it’s physical and chemical properties. Each individual unit of soil structure is called a ped. Take a sample of undisturbed soil in your hand. Look closely at the soil in your hand and examine its shape.               Columnar Vertical columns of soil Blocky Irregular blocks that are usually 1.5 - 5.0 cm in diameter Platy Thin, flat plates of soil that lie horizontally. Usually found in compacted soil Single grained Soil is broken into individual par- ticles that do not stick together. Sand is a common example Prismatic Vertical columns of soil that might be a number of cm long. Usually found deeper in the soil Massive Soil has no visible sructure, is hard to break apart and appears in very large clods Granular Resembles cookie crumbs and is usually less than 0.5cm in diam- eter. Commonly found near the surface where roots have been growing Organic Soil Guide 17
  23. 23. Depth Plant roots need somewhere to go. You may have the best soil around, but if there’s not enough of it, your plants will still struggle, especially over the summer. It’s as simple as, ‘the greater the topsoil depth, the greater the medium for water and nutrients to be held’. Most plants will like a soil depth of at least 200 mm. If your soil is less than 200mm deep, you need to add more soil. 300mm of topsoil 60mm of topsoil Remember! Even if you have a deep topsoil, the ability of the soil to hold water and nutrients strongly depends on the texture of the soil. The more fines (clay/silt) in a soil, the greater the capacity to hold water and nutrients. Exercise 4 Your soil depth Go to your garden or lawn, and dig a hole. You could just poke a screwdriver into the soil to see how far it goes in, but it’s better to dig a hole so you can see what the soil looks like. Dig down until you hit clay, or until the soil becomes too hard to dig anymore. Take a tape measure and measure away. This is your soil depth. If you already have a clay soil at the surface, it’s time to start improving (see ‘But I have Clay!’). Organic Soil Guide 18
  24. 24. Soil Colour The colour of your soil can tell you a lot about what’s happening down there. That is of course, if you have remnant soil left in your garden. If you’ve imported a whole bunch of potting mix, or bought soil you might not have much colour variation. There are 3 main components that affect the colour of your soil: • Minerals derived from the parent material. Iron is a good example. In well drained conditions, iron produces a red colour, so the more red your soil, the more iron it has. • Organic matter - Although it is difficult to assess the organic matter content of your soil, the colour of the soil will help you. There are two basic ‘rules of thumb’ to follow: • The darker the soil is, the more organic matter it contains • Dry soil that leaves your hands ‘dirty and dusty’ has a high organic matter content • Moisture content. Pour water on a soil and it becomes darker. This we all know. But a soil that is frequently waterlogged produces long term colour changes. Waterlogged soil is often mottled, and a dull bluish-grey colour. Well drained soils are usually brighter in colour. One sign of a soil that has been waterlogged for a long time is the presence of manganese nodules or concretions. These nodules are black in colour. This mottled soil has a variety of grey and black nodules. This indicates that the soil has been subjected to waterlogging for a long period of time. A bit about iron.... The iron type and amount in your soil can lead to a variety of different colours. When a soil is dry, iron oxides become yellow. Under normal (i.e. good drainage and moisture content) conditions, it is red. But when a soil is waterlogged (a lack of oxygen), the soil often becomes a green, grey or bluish-grey colour. Organic Soil Guide 19
  25. 25. 20 The colour chart above illustrates how iron content and organic matter (in this case carbon) can affect the colour of soil. It’s not as simple as just red, brown and black - the proportion of iron and organic matter affects the degree of colour. For example, a soil with a high carbon content, but low iron content could be pink, pale yellow or a grey colour. Black and brown soil These soils often have higher levels of organic matter, although some minerals can also give a dark colouring. Sodium can also make a soil look darker. Sodium causes the organic matter particles to disperse, making the soil appear blacker (See Chapter 6 for more information on dispersion). Red soil A red soil indicates good drainage. Iron found within the soil is more easily oxidised because there is more oxygen available. This causes the soil to develop a ‘rusty’ colour. Organic matter in the soil might make the soil appear as a darker red. Yellow and yellowish-brown soil Yellow and yellowish-brown soil often has poorer drainage than red soils. The iron compounds in these soils are in a hydrated form and therefore do not produce the red colour.
  26. 26. Grey and blue grey soil Grey and blue grey soil is usually waterlogged or has poor drainage. Light grey and white soil A light grey or white soil indicates a high amount of rainfall and good drainage. Excessive rain can ‘wash out’ the iron and manganese particles, which give the soil some of its colour. This washing out is known as leach- ing. White soil can also indicate the presence of quartz or the presence of salts. In very dry regions, the mineral calcite can also make a soil appear white. The following table from DERM, QLD will help to explain the different properties of coloured soil. Remember these descriptors are indicative only, and not steadfast rules. Soil Property Black Light Grey Red Brown Yellow Grey/Blue Drainage Often slow Well drained Well drained Well - moderately Less well drained Poorly drained Water-logging potential Medium Low Low Low Low to me- dium High Organic Matter Accumulation High Low Medium Medium - High Medium - low Low Leaching of nutrients Low High Medium Medium Medium Low Nitrogen loss Medium Low Low Low Low - medium High Key Points to Remember • The three main physical properties of soil – structure, texture and depth are all as important as each other. If you make sure these three aspects of your soil are well looked after first, your nutri- ent, water and plant management will be much easier. • For the most part, your soil texture and structure will influence many other soil properties i.e. the water holding capacity, infiltration rate, susceptibility to compaction and drainage. • If your soil is compacted, you need to aerate it • Try to have a minimum soil depth of 200 mm. Organic Soil Guide 21