Soil organic matter

Organic matter is the lifeblood of fertile, productive soil. Without it, agricultural production is not sustainable.

Organic matter is any living or dead animal and plant material. It includes living plant roots and animals, plant and animal remains at various stages of decomposition, and microorganisms and their excretions.

On farms the main sources of organic matter are plant litter (plant roots, stubble, leaves, mulch) and animal manures. Earthworms and microorganisms decompose these materials. The process of decomposition releases nutrients which can be taken up by plant roots. The end product of decomposition is humus, a black crumbly material resistant to further decomposition. A complex chemical substance, humus stores plant nutrients, holds moisture and improves soil structure.


The rate of decomposition of organic matter depends on the soil’s temperature, moisture, aeration, pH and nutrient levels.

The warmer and wetter the climate, the faster the rate of organic matter breakdown. Cooler areas have higher levels of soil organic matter because it does not break down as quickly in low temperatures.

Waterlogged organic matter breaks down very slowly because microorganisms necessary for decomposition cannot exist where there is no oxygen. Soils formed from waterlogged organic matter are known as peats, and contain a high percentage of organic matter.

Acid soils with low pH usually contain greater quantities of organic matter because microorganisms become less active as soil acidity increases.

Benefits of organic matter

  • Improve soil structure
    As organic matter decays to humus, the humus molecules ‘cement’ particles of sand, silt, clay and organic matter into aggregates which will not break down in water. This cementing effect, together with the weaving and binding effect of roots and fungal strands in the decomposing organic matter, makes the soil aggregates stable in water.
  • Improves drainage
    These larger, stable aggregates have larger spaces between them, allowing air and water to pass through the soil more easily.
  • Holds moisture
    The aggregates are also very effective in holding moisture for use by plants. Humus molecules can absorb and hold large quantities of water for use by plant roots.
  • Provides nutrients
    Organic matter is an important source of nitrogen, phosphorus and sulfur. These nutrients become available as the organic matter is decomposed by microorganisms. Because it takes time for this breakdown to occur, organic matter provides a slow release form of nutrients. If crops are continually removed from the soil, there is no organic matter for microbes to feed on and break down into nutrients, so fewer nutrients are available to plants.
  • Improves cation exchange capacity
    Humus molecules are colloids, which are negatively charged structures with an enormous surface area. This means they can attract and hold huge quantities of positively charged nutrients such as calcium, magnesium and potassium until the plant needs them. Clays also have this capacity, but humus colloids have a much greater CEC than clays.

(For more explanation, see Cation exchange capacity.)

How to increase soil organic matter levels

  • Grow perennial pasture
    A period under perennial, grass-dominant pasture is an effective way of increasing organic matter in farm soils. Short-lived annual grasses are a source of dead roots; perennial grasses are a source of leaf matter. Even short periods (1–2 years) under pasture can improve soil structure, even though the actual increase in organic matter may be small.
  • Grow cereal crops
    Cereal crops leave significant amounts of organic matter in their dead roots and stubbles after harvest.
  • Grow green manure crops
    Green manure crops provide protective cover until they are ploughed into the soil. Initially they provide a large increase in organic matter levels, but they break down rapidly to give only a small increase in long-term organic matter levels; also, the ploughing operation can do more harm than the good done by the organic matter.
  • Spread manure
    Bulky organic manures will increase organic matter, but frequent and heavy applications are needed to produce significant changes.
  • Use organic fertilisers
    Organic fertilisers applied in large amounts can boost organic matter levels but are generally less cost-effective as supplies of nutrients than inorganic fertilisers. Applied in small quantities, they are unlikely to have a significant effect on organic matter levels.
  • Keep cultivation to a minimum
    Cultivation breaks down the stable aggregates, exposing humus in the aggregates to air and faster decomposition. Direct drill techniques allow you to sow seed while leaving stubble residues on top of the soil, and leaving aggregates intact.
  • Concentrate organic matter
    An alternative to increasing inputs is to make more effective use of what is already there. Retain all organic additions, whether roots, stubble or manure, close to the surface. The stability of soil structure is related to the concentration of organic matter at the surface, not the total quantity present in the soil.

Problems with incorporation

Incorporation of organic matter can present some problems.

  • It is difficult to incorporate large quantities by cultivation.
  • Green manure crops break down quickly and provide only a small increase in soil organic matter levels. Ploughing hastens the breakdown of humus and may counteract the small benefit from the crop itself.
  • If organic matter is incorporated when the soil is wet, the soil may compact so that there is not enough oxygen available for microroganisms to decompose the organic matter. This may affect crop growth and nitrogen supply.
  • Chemicals released from organic matter may reduce the rate of plant growth for a short time or have a toxic effect on young seedlings.
  • Incorporating straw can also lead to a temporary shortage of available nitrogen for the planted crop, as the microorganisms will draw on the limited nitrogen in the decomposing straw.

Composting With Coffee Grounds – Used Coffee Grounds For Gardening

Whether you make your cup of coffee daily or you have noticed your local coffee house has started to put out bags of used coffee, you may be wondering about composting with coffee grounds. Are coffee grounds as fertilizer a good idea? And how do coffee grounds used for gardens help or hurt? Keep reading to learn more about coffee grounds and gardening.

Composting Coffee Grounds

Composting with coffee is a great way to make use of something that would otherwise end up taking up space in a landfill. Composting coffee grounds helps to add nitrogen to your compost pile.

Composting coffee grounds is as easy as throwing the used coffee grounds onto your compost pile. Used coffee filters can be composted as well.

If you will be adding used coffee grounds to your compost pile, keep in mind that they are considered green compost material and will need to be balanced with the addition of some brown compost material.

Coffee Grounds as Fertilizer

Used coffee grounds for gardening does not end with compost. Many people choose to place coffee grounds straight onto the soil and use it as a fertilizer. The thing to keep in mind is while coffee grounds add nitrogen to your compost, they will not immediately add nitrogen to your soil.

The benefit of using coffee grounds as a fertilizer is that it adds organic material to the soil, which improves drainage, water retention and aeration in the soil. The used coffee grounds will

also help microorganisms beneficial to plant growth thrive as well as attract earthworms.

Many people feel that coffee grounds lower the pH (or raise the acid level) of soil, which is good for acid loving plants. But this is only true for unwashed coffee grounds. “Fresh coffee grounds are acidic. Used coffee grounds are neutral.” If you rinse your used coffee grounds, they will have a near neutral pH of 6.5 and will not affect the acid levels of the soil.

To use coffee grounds as fertilizer, work the coffee grounds into the soil around your plants. Leftover diluted coffee works well like this too.

Other Uses for Used Coffee Grounds in Gardens

Coffee grounds can also be used in your garden for other things.

  • Many gardeners like to use used coffee grounds as a mulch for their plants.
  • Other used for coffee grounds include using it to keep slugs and snails away from plants. The theory is that the caffeine in the coffee grounds negatively affects these pests and so they avoid soil where the coffee grounds are found.
  • Some people also claim that coffee grounds on the soil is a cat repellent and will keep cats from using your flower and veggie beds as a litter box.
  • You can also use coffee grounds as worm food if you do vermicomposting with a worm bin. Worms are very fond of coffee grounds.

Using Fresh Coffee Grounds

We get lots of questions about using fresh coffee grounds in the garden. While it’s not always recommended, it shouldn’t be a problem in some situations.

  • For instance, you can sprinkle fresh coffee grounds around acid-loving plants like azaleas, hydrangeas, blueberries, and lilies. Many vegetables like slightly acidic soil, but tomatoes typically don’t respond well to the addition of coffee grounds. Root crops, like radishes and carrots, on the other hand, respond favorably – especially when mixed with the soil at planting time.
  • The use of fresh coffee grounds are thought to suppress weeds too, having some allelopathic properties, of which adversely affects tomato plants. Another reason why it should be used with care. That being said, some fungal pathogens may be suppressed as well.
  • Sprinkling dry, fresh grounds around plants (and on top of soil) helps deter some pests same as with used coffee grounds. While it doesn’t fully eliminate them, it does seem to help with keeping cats, rabbits and slugs at bay, minimizing their damage in the garden. As previously mentioned, this is thought to be due to the caffeine content.
  • In lieu of the caffeine found in fresh, unbrewed coffee grounds, which can have an adverse effect on plants, you may want to used decaffeinated coffee or only apply fresh grounds minimally to avoid any issues.

Coffee grounds and gardening go together naturally. Whether you are composting with coffee grounds or using used coffee grounds around the yard, you will find that coffee can give your garden as much of a pick me up as it does for you.

It’s one of the most common gardening tips going: apply spent coffee grounds around your garden for amazing results. A quick internet search for “coffee grounds + plants” will draw up close to four million hits, with consistent claims they can add essential minerals to the soil, boost populations of friendly soil bacteria and even reduce the pH of growing media for acid-loving plants like rhododendrons. In fact, on a trip to an achingly eco organic coffee shop in San Francisco last year I saw big barrels of used coffee grounds with scoops and brown bags, free to customers to collect, under a sign detailing their many horticultural virtues. What a great idea!

Always keen to try out a quirky horticultural tip, and being a bit of a caffeine fiend, I decided to put the theory to the test this summer on two identical vegetable beds containing a mix of tomatoes, lettuce, herbs and flowers. Now, this was hardly a rigorous scientific trial, just a rough-and-ready experiment to satisfy passing curiosity. I’d just dump my daily coffee grounds on the surface as a mulch once they had cooled (the way books and blogs suggest), creating a beautiful dark inch-thick layer of coffee compost by the end of the summer.

And the results? Well, here’s the deal. The crop yield and growth of pretty much everything in the coffee bed became noticeably worse within about two weeks of application. Plant growth slowed, some developed leaf yellowing, others defoliated and died. Seedling germination in some cases was almost completely inhibited. While some species looked OK, none of the plants in the coffee group proved better than my basic control. But it’s just adding organic matter. What went wrong?

So I had a look at the scientific literature, and frankly I kicked myself. Coffee grounds are of course a rich source of caffeine – in fact they can be richer than coffee itself, depending on brewing technique. One of the key functions of caffeine in the plants that produce it is allelopathy – the ability to reduce competition from surrounding species by suppressing their growth. Caffeine is packed into coffee seeds for the very function of suppressing the germination of other seeds.

There is a stack of studies to suggest it also stalls root growth in young plants, preventing their uptake of water and nutrients. Yet others have shown it has antibacterial effects (so much for boosting soil bacteria). And guess what? It isn’t even always very acidic. OK, its effects have varied widely depending on plant species, but it’s never shown colossal benefits that could outweigh the risks. I love a quirky piece of hort advice, and some are repeated so often you assume they are true, but often they call them old wives’ tales for a reason.

Email James at [email protected] or follow him on Twitter @Botanygeek

Use Diluted Coffee to Fertilize Plants

You know that last bit of coffee that always seems to be left in the carafe? Don’t just pour it down the drain — you can use it to fertilize your container-grown plants. Coffee grounds (and brewed coffee) are a source of nitrogen for plants, which is the nutrient that produces healthy green growth and strong stems. Coffee also contains calcium and magnesium — both of which are beneficial to plant health.

To use coffee as a plant fertilizer, you’ll need to dilute it. It should look like weak tea — see the photo for an example. If you aim for about 1/4 coffee and 3/4 water in your mixture (depending on how strongly you brew your coffee), that’s about right, but you don’t have to be fussy about it. You can use coffee fertilizer on your potted plants, houseplants, or in your vegetable garden. Coffee and coffee grounds can be acidic, but since we’re diluting it so much, that’s not really a problem unless you’re watering the same plant with it every day.


A good rule of thumb is to feed and water your plants once a week with a weak coffee solution. They’ll appreciate the additional nutrients, as well as the water.

Organic Matter: What It Is and Why It’s So Important

Follow the appropriateness of the season, consider well the nature and conditions of the soil, then and only then least labor will bring best success. Rely on one’s own idea and not on the orders of nature, then every effort will be futile.


Figure 2.1. A nematode feeds on a fungus, part of a living system of checks and balances. Photo by Harold Jensen.

Figure 2.2. Partially decomposed fresh residues removed from soil. Fragments of stems, roots, and fungal hyphae are all readily used by soil organisms.

As we will discuss at the end of this chapter, organic matter has an overwhelming effect on almost all soil properties, although it is generally present in relatively small amounts. A typical agricultural soil has 1% to 6% organic matter. It consists of three distinctly different parts—living organisms, fresh residues, and well-decomposed residues. These three parts of soil organic matter have been described as the living, the dead, and the very dead. This three-way classification may seem simple and unscientific, but it is very useful.

The living part of soil organic matter includes a wide variety of microorganisms, such as bacteria, viruses, fungi, protozoa, and algae. It even includes plant roots and the insects, earthworms, and larger animals, such as moles, woodchucks, and rabbits that spend some of their time in the soil. The living portion represents about 15% of the total soil organic matter. Microorganisms, earthworms, and insects feed on plant residues and manures for energy and nutrition, and in the process they mix organic matter into the mineral soil. In addition, they recycle plant nutrients. Sticky substances on the skin of earthworms and other substances produced by fungi help bind particles together. This helps to stabilize the soil aggregates, clumps of particles that make up good soil structure. Organisms such as earthworms and some fungi also help to stabilize the soil’s structure (for example, by producing channels that allow water to infiltrate) and, thereby, improve soil water status and aeration. Plant roots also interact in significant ways with the various microorganisms and animals living in the soil. Another important aspect of soil organisms is that they are in a constant struggle with each other (figure 2.1). Further discussion of the interactions between soil organisms and roots, and among the various soil organisms, is provided in chapter 4.

A multitude of microorganisms, earthworms, and insects get their energy and nutrients by breaking down organic residues in soils. At the same time, much of the energy stored in residues is used by organisms to make new chemicals as well as new cells. How does energy get stored inside organic residues in the first place? Green plants use the energy of sunlight to link carbon atoms together into larger molecules. This process, known as photosynthesis, is used by plants to store energy for respiration and growth.

The fresh residues, or “dead” organic matter, consist of recently deceased microorganisms, insects, earthworms, old plant roots, crop residues, and recently added manures. In some cases, just looking at them is enough to identify the origin of the fresh residues (figure 2.2). This part of soil organic matter is the active, or easily decomposed, fraction. This active fraction of soil organic matter is the main supply of food for various organisms—microorganisms, insects, and earthworms— living in the soil. As organic materials are decomposed by the “living,” they release many of the nutrients needed by plants. Organic chemical compounds produced during the decomposition of fresh residues also help to bind soil particles together and give the soil good structure.

Organic molecules directly released from cells of fresh residues, such as proteins, amino acids, sugars, and starches, are also considered part of this fresh organic matter. These molecules generally do not last long in the soil because so many microorganisms use them as food.


It is believed that the unusually productive “dark earth” soils of the Brazilian Amazon region were produced and stabilized by incorporation of vast amounts of charcoal over the years of occupation and use. Black carbon, produced by wildfires as well as human activity and found in many soils around the world, is a result of burning biomass at around 700 to 900°F under low oxygen conditions. This incomplete combustion results in about half or more of the carbon in the original material being retained as char. The char, also containing ash, tends to have high amounts of negative charge (cation exchange capacity), has a liming effect on soil, retains some nutrients from the wood or other residue that was burned, stimulates microorganism populations, and is very stable in soils. Although many times increases in yield have been reported following biochar application— probably a result of increased nutrient availability or increased pH—sometimes yields suffer. Legumes do particularly well with biochar additions, while grasses are frequently nitrogen deficient, indicating that nitrogen may be deficient for a period following application.

Note: The effects of biochar on raising soil pH and immediately increasing calcium, potassium, magnesium, etc., are probably a result of the ash rather than the black carbon itself. These effects can also be obtained by using more completely burned material, which contains more ash and little black carbon.

The well-decomposed organic material in soil, the “very dead,” is called humus. Some use the term humus to describe all soil organic matter; some use it to describe just the part you can’t see without a microscope. We’ll use the term to refer only to the well-decomposed part of soil organic matter. Because it is so stable and complex, the average age of humus in soils is usually more than 1,000 years. The already well-decomposed humus is not a food for organisms, but its very small size and chemical properties make it an important part of the soil. Humus holds on to some essential nutrients, storing them for slow release to plants. Humus also can surround certain potentially harmful chemicals and prevent them from causing damage to plants. Good amounts of soil humus can both lessen drainage and compaction problems that occur in clay soils and improve water retention in sandy soils by enhancing aggregation, which reduces soil density, and by holding on to and releasing water.

Another type of organic matter, one that has gained a lot of attention lately, is usually referred to as black carbon. Almost all soils contain some small pieces of charcoal, the result of past fires, of natural or human origin. Some, such as the black soils of Saskatchewan, Canada, may have relatively high amounts of char. However, the interest in charcoal in soils has come about mainly through the study of the soils called dark earths (terra preta de indio) that are on sites of long-occupied villages in the Amazon region of South America that were depopulated during the colonial era. These dark earths contain 10–20% black carbon in the surface foot of soil, giving them a much darker color than the surrounding soils. The soil charcoal was the result of centuries of cooking fires and in-field burning of crop residues and other organic materials. The manner in which the burning occurred—slow burns, perhaps because of the wet conditions common in the Amazon— produces a lot of char material and not as much ash as occurs with more complete burning at higher temperatures. These soils were intensively used in the past but have been abandoned for centuries. Still, they are much more fertile than the surrounding soils—partially due to the high inputs of nutrients in animal and plant residue—and yield better crops than surrounding soils typical of the tropical forest. Part of this higher fertility— the ability to supply plants with nutrients with very low amounts of leaching loss—has been attributed to the large amount of black carbon and the high amount of biological activity in the soils. Charcoal is a very stable form of carbon and apparently helps maintain relatively high cation exchange capacity as well as biological activity. People are beginning to experiment with adding large amounts of charcoal to soils—but we’d suggest waiting for results of the experiments before making large investments in this practice. The quantity needed to make a major difference to a soil is apparently huge— many tons per acre—and may limit the usefulness of this practice to small plots of land.

Normal organic matter decomposition that takes place in soil is a process that is similar to the burning of wood in a stove. When burning wood reaches a certain temperature, the carbon in the wood combines with oxygen from the air and forms carbon dioxide. As this occurs, the energy stored in the carbon-containing chemicals in the wood is released as heat in a process called oxidation. The biological world, including humans, animals, and microorganisms, also makes use of the energy inside carbon-containing molecules. This process of converting sugars, starches, and other compounds into a directly usable form of energy is also a type of oxidation. We usually call it respiration. Oxygen is used, and carbon dioxide and heat are given off in the process.

Soil carbon is sometimes used as a synonym for organic matter. Because carbon is the main building block of all organic molecules, the amount in a soil is strongly related to the total amount of all the organic matter—the living organisms plus fresh residues plus well-decomposed residues. When people talk about soil carbon instead of organic matter, they are usually referring to organic carbon. The amount of organic matter in soils is about twice the organic carbon level. However, in many soils in glaciated areas and semiarid regions it is common to have another form of carbon in soils—limestone, either as round concretions or dispersed evenly throughout the soil. Lime is calcium carbonate, which contains calcium, carbon, and oxygen. This is an inorganic carbon form. Even in humid climates, when limestone is found very close to the surface, some may be present in the soil.

Healthy Soils: Sources | Top | Why Soil Organic Matter Is So Important

Organic Materials

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Organic materials are defined in modern chemistry as carbon-based compounds, originally derived from living organisms but now including lab-synthesized versions as well. Most are combinations of a few of the lightest elements, particularly hydrogen, carbon, nitrogen, and oxygen. Organic materials include the wood from which furniture is made, feathers, leather, and synthetic materials such as petroleum-based plastics. In spite of this variety they share some general characteristics. For example, many organic materials undergo fading, yellowing, or embrittlement in response to prolonged exposure to light or other forms of radiation, caused by breakdown of the covalent bonding structure shared by many carbon-containing compounds.
Organic materials are further divided into three categories based on their source. Many conservation decisions are based on understanding the different structures and behaviors of these forms:


Cellulosic Materials. Plant materials are – or were – living matter made of cellulose and lignin. Examples include grass, wood, roots, bark, leaves, even flowers. There are approximately 350,000 species of plants in existence. As of 2004, roughly 288,000 have been identified, including almost 259,000 flowering species. The variety of material that has been used for cultural heritage objects almost matches the number of plants available. Asian lacquer is another organic material, derived from plant sources.

Proteinaceous materials have an animal origin. An astonishing array of animal-based materials have been manipulated by man, for use in tools, decorative objects and fine art. Common categories include Leather and Skin, parchment, gut, hides, fur and hair, wool and silk, feathers and quills, baleen, and tortoiseshell.
Ivory, bone, antler, and shell may also contain protein components.
Organic Polymers are derived from fossil fuels or other oils.

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Organic matter

This topic has 16 questions:

  1. What is organic matter?
  2. What are the 4 main steps in degradation of organic matter?
  3. What is the impact of incorporating organic matter into the soil?
  4. What is humus?
  5. How does organic matter improve soil health?
  6. How can we increase the amount of organic matter content in soil?
  7. How does soil erosion affect organic matter in soil?
  8. How does tillage affect organic matter in soil?
  9. What is light fraction organic matter?
  10. How can I extract light fraction organic matter from my soil?


  1. What is compost?
  2. How are organisms involved in composting?
  3. What happens during the composting process?
  4. What is compost maturity and why is it important?
  5. What are the main types of compost and what are their benefits?
  6. How does composted grape marc affect soil microorganisms?

Question 1. What is organic matter?

Organic matter is anything that contains carbon compounds that were formed by living organisms. It covers a wide range of things like lawn clippings, leaves, stems, branches, moss, algae, lichens any parts of animals, manure, droppings, sewage sludge, sawdust, insects, earthworms and microbes.

There are 3 main components of organic matter in soils:

  • dead forms of organic material – mostly dead plant parts
  • living parts of plants – mostly roots
  • living microbes and soil animals

By far the largest component is the dead matter – it constitutes about 85% of all organic matter in soils. Living roots make up about another 10% and the microbes and soil animals make up the last few percent.

Organic matter in soil

Soil sand – no organic matter

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Question 2. What are the 4 main steps in degradation of organic matter?

In answering this question, we concentrate mainly on dead plant matter as this constitutes the major part of organic matter in soil (apart from living roots). However, the broad principles also apply to the degradation of animal and microbial matter.

Living organisms are made up of thousands of different compounds, so when they die there are thousands of compounds in the soil to be decomposed. As these compounds are decomposed, the organic matter in soil is transformed and gradually changes so that eventually it is no longer recognisable as part of the original plant. These stages are:

  1. Breakdown of compounds that are easy to decompose – like sugars, starches and proteins.
  2. Breakdown of compounds that take several years to decompose like cellulose (an insoluble carbohydrate found in plants), lignins (a very complicated structure that is part of wood).
  3. Breakdown of compounds that can take up to 10 years to decompose – like some waxes and the phenols. This also includes compounds that have formed stable combinations and are located deep inside soil aggregates and are therefore not accessible to soil organisms.
  4. Compounds that take tens, hundreds or thousands of years to decompose include humus-like substances which are the result of integration of compounds from breakdown products of plants and those generated by microorganisms humus-like substances.

Compounds in the first group are quick and easy for fungi and bacteria to decompose, so the carbon and energy they provide is readily available. Most of the microbes living in the soil have the enzymes needed to decompose these simple compounds. This type of decomposition is the first stage during the degradation of organic matter. Mites and small soil animals often help this stage by breaking up the organic matter into smaller pieces exposing more of the material to colonisation by bacteria and fungi.

The second stage involves the microbes decomposing more complicated compounds – the second group listed above. Many, but not all, fungi and bacteria can decompose these compounds. The compounds take longer to decompose because they are much larger and they are made up of more complicated units than the compounds in the first group. Specific enzymes, not commonly produced by most microorganisms are required to break down these compounds.

Hyphae asociated with organic matter
(photo van Vliet and Gupta).

It’s important to realise that decomposition will only take place if conditions are suitable. The rate of breakdown is greatly affected by the conditions in soil. There must be some moisture available, soil temperatures must be suitable (usually between 10 and 35°C) and the soil must not be too acidic or alkaline. Decomposition also occurs at higher temperatures, as in composts, or under waterlogged conditions. The types of organisms involved in breaking down the organic matter will depend on the conditions.

More on Organic Matter Breakdown

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Question 3. What is the impact of incorporating organic matter into the soil?

Incorporating organic matter into soil can have several impacts because it disturbs the physical, chemical and biological balances in the soil. It can:

  • change the amount of nitrogen that is available to plants
  • change the amount of other nutrients available
  • change the way the soil sticks together (soil aggregation)
  • change the number and type of organisms present in the soil

All of these changes are related to the way organic matter is decomposed when it is incorporated into soil and to the particular type of organic matter used.

When organic matter is incorporated into soil, the larger organisms like mites and soil animals break it into smaller pieces. Then the fungi and bacteria start to decompose it (they secrete enzymes to break up the chemical compounds it is made of). When the enzymes break the molecules into smaller sections, the bacteria and fungi can use some of energy or nutrients released for their own growth. For example, when an enzyme stimulates the breakdown of a protein, the microbe may be able to use the carbon, nitrogen and sulphur (if there is some) for its own physiological processes.

If there are nutrients that the microbes do not use, they will be available for other soil organisms or plants to take up and use. When microbes die, their cells are degraded and nutrients contained within them become available to plants and other soil organisms.

Microbes can access nitrogen in the soil more easily than plants can, so the plants sometimes miss out. This means that if there is not enough nitrogen for all the organisms, the plants will probably be nitrogen deficient. This is why incorporating organic matter into soils can change the amount of nitrogen (and other nutrients) that is available to plants. These will be a short-term effect that happens when soils do not have high levels of organic matter and soil microbes.

If the number of fungi and bacteria associated with the breakdown of organic matter increases, there may be some improvements to the soil structure. Adding organic matter can also increase the activity of earthworms, which in turn can also improve soil aggregation. If organic matter is retained in the soil, the number of microbes in the soil increases. This is because the microbes can use the organic matter as a source of energy and so they can grow and multiply.

More on Organic Matter Breakdown

More on Aggregates

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Question 4. What is humus?

Humus is composed of organic compounds that are highly complex in structure. Like other organic matter, humus originates from dead organic material. The compounds that make up humus are highly complex organic compounds that have resisted decomposition and have accumulated in the soil. Humus organic matter is so altered that it can no longer be recognised as plant material.

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Question 5. How does organic matter improve soil fertility?

The amount of organic matter in the soil is an important factor controlling the potential sustainability of a system. Soil organic matter plays a key role in supplying plants with the nutrients they require (especially nitrogen, sulphur and phosphorus). Organic matter also helps to improve soil structure, binds pollutants, and influences soil buffering capacity.

More on Fertility

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Question 6. How can we increase the amount of organic matter content in soil?

To maintain or improve organic matter levels we need inputs of new organic matter (from plant debris and/or animals) to exceed losses of organic matter by decomposition or erosion. Farming systems have traditionally ‘mined’ the soil for nutrients, causing soil organic matter levels to decline. This decline continues until management practices are improved.

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Question 7. How does soil erosion affect organic matter in soil?

When soil is lost by erosion organic matter, microorganisms and nutrients are also lost. Most animal waste and plant material (except deep roots) are returned to the soil at or near its surface. Soil organic matter therefore accumulates at the soil surface. The organic matter is food for microorganisms so they also concentrate in the surface few centimetres of soil. The loss of even a thin layer of soil during wind or water erosion results in a disproportionately large reduction in organic matter and microorganisms. This problem is increased in soils in the south west of Western Australia because they usually have poor soil structure and minimal plant cover during summer. Decreasing soil disturbance, creating wind breaks, maintaining crop cover or increasing clay content are all practices that can help to minimise soil loss.

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Question 8. How does tillage affect organic matter in soil?

Large losses of soil organic matter can be attributed to cultivation. Organic matter that is inside aggregates or coated with soil particles is protected by decomposition because microorganisms are unable to come into physical contact with it. Tillage disturbs the soil and brings “protected” organic matter in to physical contact with microorganisms, which then decompose it. No-till systems can overcome this problem. No-till systems also decrease soil erosion. Organic matter and microorganisms are restricted to the top layers of the soil. This increases the potential for losing organic matter if management practices cause soil loss.

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Question 9. What is light fraction organic matter?

Light fraction organic matter is the recently added and partially decomposed organic matter. It is called “light” fraction organic matter because it floats on the surface of water. This distinguishes it from organic matter that has been in soil longer, which is denser and sinks in water.

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Question 10. How can I extract light fraction organic matter from my soil?

A simple method to extract light fraction organic matter.

  1. Collect a sample of surface soil with a piece of PVC pipe (5 cm long).
  2. Crush soil to destroy aggregates and remove organic matter larger than 2 cm.
  3. Put the remaining soil into a 2 L drink bottle ans add water to approximately two thirds of the way up the height of the bottle.
  4. Shake vigorously for 1 minute.
  5. Lay bottle on its side and slowly roll back and forwards like a rolling pin for one minute.
  6. Slowly tilt the bottle upright, fill with water and allow to settle overnight.
  7. After settling overnight, the water should be clearer as the mineral particles and denser organic matter have settled. The small pieces of organic matter floating on the surface are light fraction organic matter.
  8. Cut the bottom out of a plastic conainter and secure a handkerchief over one end with an elastic band.
  9. Pour the light fraction organic matter floating on the surface through the handkerchief filter.

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Question 11. What is compost?

Compost is the semi-stabilised product obtained after organic materials have undergone biological degradation under controlled conditions. Basically, compost is organic matter. Compost can be made from any organic material such as garden waste, food scraps, manure, sewage effluent, sawmill waste, leaves and cardboard. Composts vary greatly depending on their maturity, the organic materials used to make the compost, the type of composting (aerobic/anaerobic) and length of the composting.

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Question 12. How are organisms involved in composting?

The composting process relies on a variety on organisms that feed on the organic matter. Therefore factors that influence these organisms can affect the final compost product. By understanding the biology of composting, it is possible to manipulate processes, maximise the rate of decomposition and meet quality standards for final product.

Many of the organisms involved in composting need aerobic conditions. How aerobic the conditions are is influenced by factors such as such as moisture, temperature, frequency of turning and the size of the pile. Also the quality of the organic matter at the start of the composting process also affects the organisms, especialy the pH and carbon to nitrogen ratio of the organic materials.

More on Organisms

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Question 13. What happens during the composting process?

During aerobic composting the material undergoes several phases each involving a separate set of organisms. The two main phases are:

  1. The initial mesophilic phase involves microorganisms consuming readily available compounds. Heat is produced as a by-product of this activity, the amount of which will depend on the size of the pile and food available. This heat is important for the elimination of pathogens and weed seeds. In smaller piles (less than 2 m²), much of the heat is lost to the atmosphere hence the required temperatures may not be reached.
  2. The thermophylic phase begins around 45°C and this stage has the highest rate of decomposition. Above 65-70°C, many organisms die off and the pile will require turning and maybe watering to encourage organisms to build up again. One way in which to test the maturity of compost is to test temperatures, a fully composted material will not readily reheat.

More on Organic Matter Breakdown

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Question 14. What is compost “maturity” and why is it important?

A mature compost is one in which the rate of decomposition has decreased. The maturity of a compost is important because applying immature compost to soil can lead to the immobilisation of nitrogen in the soil. This process could make nitrogen less available to plants. Most compost producers should be able to advise you on the maturity and qualities of their products. One way you can test the maturity of a compost yourself is by giving it suitable conditions to continue decomposing and observing whether is heats up. If it does not heat up, it is likely to be mature.

More on Immobilisation

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Question 15. What are the main types of compost and what are their benefits?

In general, composts can have many benefits for soil fertility including modifying pH, absorbing pollutants and excess nutrients, controlling erosion, buffering temperature, suppressing weeds and providing a source of essential nutrients (major and micronutrients).

Composts can be applied to soil in two main ways and composts for each use have different properties and benefits for soil fertility.

  1. Composts can be used as mulches spread on the soil surface. Compost mulch products help to decrease irrigation needs because the water holding capacity of the soil is increased and evaporation is decreased. Composted material also helps buffer soil temperatures and can keep weeds down and reduce wind/water erosion. Mulch composts tend to have larger particle sizes than compost that is suitable for incorporation into soil. The mulch composts also contain more woody material. They have proven beneficial in commercial vineyards and orchards, at rehabilitation sites and in gardens.
  2. Composts for incorporation into soil can vary greatly in their qualities. Some are high in nutrient values and can be used to supply plant nutrients. However, immature composts can lead to problems such as nitrogen draw-down (or nitrogen immobilisation). Therefore, it is important to be careful where and when you apply them. Incorporated composts build organic matter within the soil bringing with them improvements to soil quality.

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Question 16. How does composted grape marc affect soil microorganisms?

In viticultural regions, the harvest waste (or grape marc) poses a significant management problem for growers. It is because of this that many growers have been informally composting grape marc and applying it under-vine as an organic mulch.

A field trial near Busselton in Western Australia measured changes in soil biological fertility following the application of grape marc on the ground under vines. The grape marc was applied at 60 tonnes/ha and at 120 tonnes/ha. The quantity of microorganisms in the soil (microbial biomass) were assessed at a later time.

The study found that 8 weeks after application of grape marc, the quantity of soil organisms (microbial biomass) had almost doubled and at 16 weeks it was three times greater. Both quantities of grape marc had the same effect. The soil microbial biomass plays a significant role in organic matter turnover and nutrient cycling. Therefore, increases in microbial biomass may lead to improved nutrient cycling and increased soil biological fertility.

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PHOTO: Jessica Walliserby Jessica Walliser March 26, 2015

Spring is a busy time for gardeners. We spend hours cleaning out beds, cutting back perennials, pruning fruit trees, trimming shrubs, weeding and mulching. The list seems endless, but one of the most important spring chores gardeners undertake is improving the soil by adding organic matter to improve soil structure and increase water-holding capacity. It also adds both macro- and trace nutrients and improves overall soil health by feeding the beneficial microorganisms living there. Whether you choose to use it as a top-dressing or till it into the soil, just a few inches of organic matter added once or twice a year supplies all the nutrients plants need for optimum growth.

Organic matter comes in many forms, but they’re not all created equal. Here’s some information to help you determine which type(s) of organic matter is best suited to your garden.

1. Compost

Either homemade or commercially produced

Average pH:

Nutritional Content:

Compost is typically well-balanced and contains a great blend of all nutrients.

Notes for Use:

Good-quality compost should smell earthy and be a rich, dark brown. Check with any commercial source to ensure that bio-solids (sludge) were not used. If the product smells like urine, it’s likely the nitrogen content is too high. It’s always best to make your own compost to ensure it is balanced and well-rotted, though you can find quality commercial composts.

2. Mushroom Soil/Compost

Although fairly high in organic matter, mushroom soil or mushroom compost has low nutrient levels; however, the nutrients are slowly released over time so they’re constantly available.

A byproduct of mushroom production, this compost contains ingredients like horse manure and shredded corn cobs. It can be fairly high in soluble salts but also contains a substantial amount of organic matter. Because of its high pH, I don’t recommend adding it every year.

3. Sphagnum Peat Moss

Very low in all nutrients.

Peat moss helps loosen compacted soils, but can alter the pH. It’s weed free but adds very few nutrients to the soil. It’s a great amendment for acid-loving evergreens.

4. Leaf Mold/Humus

Leaf mold and humus have moderate but balanced nutrient levels, and also contain many minor nutrients.

Primarily composed of municipally collected leaves, these products are high in many trace nutrients, as well. They’ve also got great water holding capacity.

5. Manures

depends on type

The nutrient content of manure is variable but generally very high in all nutrients. The type of bedding used with the animal can also affect the nutrient content.

All manures are not created equal. Horse and cow manures are more mild, while chicken and sheep are highly concentrated. Manures contain many weed seeds and should be composted for at least 90 days before use.

What Is Organic Material: Examples Of Organic Material For Gardening

Whether you’re planning to use all-purpose fertilizer from the garden center or you’re going to grow your plants completely chemical-free, your soil needs organic matter before you ever put in a seed or seedling. The most important part of planning a garden is getting the soil ready for planting. Without the right nutrients and conditioners in the ground, your plants will never thrive.

What is Organic Material?

What is organic material? Basically, anything that occurs in nature can be considered organic material, although not all of it is useful as a gardening addition. If you read organic gardening information, you’ll find that almost every plant and animal by-product can be used in one form or another, and most of them can be added to composting.

Using organic material for gardening helps sandy soil to retain moisture while it allows clay soil to drain more efficiently. It breaks down to feed organisms, such as earthworms, as well as feeding the plants around it.

The types of organic matter needed in your soil will depend on the conditions you’re working with.

Organic Material for Gardening

Compost is considered by many organic gardeners as the most perfect of soil additives. It’s known in gardening circles as black gold because of the many purposes it can fulfill. Organic materials are piled in layers in a compost bin or a heap, then soil and moisture are added and the materials are allowed to decompose. The result is a rich, dark sort of loam that enriches and conditions any garden soil.

Examples of organic material that do well in compost piles are kitchen scraps, grass clippings, torn newspapers, dead leaves and even animal manure. Once the ingredients all break down, this additive is dug into the soil and mixed with the garden dirt.

Not all composts are made alike, and the value of any particular pile depends on the original materials that were added to it, but in general more variety of materials makes for a better end product. Lots of variety adds trace elements to your soil as well as conditioning it, making it even more valuable in your garden.

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