Types of organic matter

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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.

Organic Matter

The key to growing great plants and vegetables is the soil. A crumbly soil that is high in organic matter, retains moisture during dry spells and drains easily in wet weather is the ideal.

You can improve the structure of your soil by working in bulky organic matter, like garden compost and manures, and letting nature do the rest.

The magic is in the bacteria

The bacterium which is dormant in your soil comes alive when you provide a source of organic matter. This could be anything from multi-purpose compost, black gold or well-rotted farmyard manure.

The bacteria warms the soil as they work and transform the organic matter into valuable plant nutrients and humus. The humus then breaks down the sticky clay soil or binds together loose sand into soil crumbs, while providing a slow-releasing source of essential plant nutrients.

Under cultivation each year, the organic matter in soil will fall along with nutrient levels. As a result, they need to be replaced by adding new materials to the ground each season.

It just gets better and better…

After adding organic matter to your soil, it will become more workable and less compact as it settles. The improved structure will provide more tiny pockets in the soil to hold air and water, stimulating root growth and helping the plants thrive.

As the organic matter becomes settled, it begins to act as a sponge, holding the water the soil needs while allowing any excess water to drain through.

The increased feed source in the soil will attract worms to your flower beds and vegetable patches. The worms digest the material, converting it into valuable nutrient-rich worm casts which add to the soil’s fertility.

After a few seasons of cropping and maintaining organic matter levels, your soil will have significantly improved, creating a fantastic productive soil.

For many, gardening is not only an enjoyable activity but a great way to save some money. Unfortunately if you weren’t raised on a farm or with a large garden it can be difficult to have a successful garden withtout spending loads of money on fertilizer and soil amendments at the garden supply store. Thankfully it is possible to maintain a healthy garden without spending a dime! Try a combination of these methods to increase your soil’s nutrients and reap a better harvest.

Leaf Litter

Leaf litter is great because it can be used to make compost or can be applied straight to the garden as mulch. As a mulch it provides habitat for beneficial insects, blocks weeds, holds in moisture, and slowly breaks down adding nutrients to the soil.

Leaves can be gathered from your yard or woodlands. You may find piles that have collected behind fallen logs or stones after being carried by the wind. In gathering leaves remember to leave some for the natural ecosystem. It’s tempting but don’t strip any area of all it’s leaf litter. Even if you don’t own wooded property you can probably still find free leaves. Many cities and suburbs collect them and you can get bags of them for free, just ask around.

Grass Clippings

If you mow your lawn at all grass cllippings are deifintely worth getting a bagger for. They make great mulch to block out weeds, hold in moisture, and provide a lot of nitrogen. Grass clippings can also be soaked in water to create grass clipping tea. Watering plants with grass clipping tea provides a fast acting nitrogen boost.

Like with leaf litter, some areas may bag their grass clippings for collection and can be picked up for free.

Compost

Compost is surprisingly easy to make right in your backyard. The most important thing to remember when making compost is to have a mix of of “green” or high in nitrogen material and “brown” or high in carbon material. Examples of “green” material include grass clippings, vegetable scraps, and weeds. “Brown” material includes leaf litter, wood chips, straw, etc. The compost should be turned occasionally and watered to keep it moist as needed.

Compost can also be used to make compost tea the same way grass clipping tea is made. Many people choose to add powdered egg shells to compost tea for an extra boost of calcium.

Some cities and towns now offer free compost made from local plant waste like grass clippings and leaves. If you go this route you may want to have it tested for herbicides.

For more on making compost read, How to Make Compost from Mother Earth News.

Straw

Straw is a choice mulch but can be rather pricey to purchase. Look for bales leftover from Halloween decorations or consider growing a patch of wheat for a double duty crop!

Wood Chips

Wood chips can be used as mulch or can be added to compost piles. Both in compost and as a mulch they offer similar benefits to that of leaf litter but break down more slowly.

Other Plant Material

Any plant material that doesn’t contain weed seeds can be used as fertilizer. Examples include wheat chaff, weeds, corn stalks, etc. These can be applied directly to the garden or composted. Green plant material like freshly pulled weeds can be used in place of grass clipping in grass clipping tea.

Cover Crops

Cover croping may sound like something for a big farm but it’s actually very easy and effective to implement in a backyard garden. Some cover crops like vetch or clover are legumes and add nitrogen to the soil as they grow. Others like buckwheat add nutrients as they die and rot or are tilled under. Many cover crops come with added benefits like attracting pollinators.

Check out this post by Ira Wallace for more on Cover Crops.

Urine

It sounds wierd but urine is actually a great fertilizer if you’re not too squeamish. It can be collected and saved up then diluted (10 parts water to one part urine) and used to water plants for a nitrogen boost. Most people or more comfortable using this on fruit trees and shrubs than their annual vegetable crops.

Wood Ashes

If you have a wood stove or backyard campfires wood ashes make a great free garden amendment, addding potassium to the soil. They should be used in moderate amounts as they also act as a liming agent. They raise the soil’s pH making it less acidic. If this is helpful for your soil conditions it’s worth noting that they’re only about 1/3 as effective as commercial lime so you may need a larger amount.

Hugelkultur Beds

If you’re okay with a more involved project you may want to try building a hugelkultur bed for longtime fertility. Hugelkultur beds involve a pile of woody material which breaks down over time providing a long lasting nutrient source.

You can learn more about the benefits of hugelkultur and how to make a hugelkultur bed here

Manure

Manure can come from your own livestock or you may find it free from a local farm. Try checking with places that board horses as they typically don’t use it the way many farms do. If you’re sourcing it from anywhere besides your backyard be sure that the animals haven’t been fed plant material that was grown using herbicides as these can still be in the manure and will kill your garden.

It’s also worth noting that excessive use of manure can cause a phosphorus build-up which pollutes local water sources and can tie up other soil nutrients. This problem doesn’t occur with any plant based fertilizers so manure should be used sparingly.

If you’re unsure of where to start consider having your soil tested. Your local agriculture extension agency will be able to identify what your soil needs and advise you where to begin. Growing good, organic food shouldn’t be expensive. Experimenting with these tried and true methods can help you keep a frugal yet productive garden.

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.

—JIA SI XIE, 6TH CENTURY, CHINA

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.

BIOCHAR AS A SOIL AMENDMENT

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

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.

Decomposition

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.

Add organic matter to improve garden soils

CORVALLIS, Ore. – Adding organic matter is the best way to improve nearly all kinds of soils. If you’re unsure if your soil needs amendments, take note if it dries and cracks in summer, drains slowly or is difficult to dig whether wet or dry. Do your rhododendrons and other shrubs wilt in hot weather, even with added water?

Adding organic materials improves the ability of sandy soils to hold nutrients and water. For clay soil, organic additions improve drainage and aeration and help the soil dry out and warm up more quickly in the spring.

Good organic amendments for garden soils include wood by-products such as sawdust and bark mulch, peat moss, rotted manure, grass or wheat straw and compost. Inorganic amendments include pumice, perlite, vermiculite and sand.

Any composted material that has been reduced to humus is a good soil amendment. However, the breakdown of high-carbon organic matter in cattle and horse manure can take years. To speed the process, mix additional nitrogen into your garden – at least six pounds of ammonium nitrate or 10 pounds of ammonium sulfate per inch of organic matter, applied over a 1,000-square-foot area.

Peat moss, with its high humus content, is the ideal amendment for raised beds or small gardens because it is nearly weed-free. However, it is expensive to use in large gardens.

Inorganic amendments such as perlite, sand and vermiculite function primarily as wedges that separate soil particles, increasing soil porosity and aeration.

Sand does not hold water and nutrients very well and causes finer silt or clay soils to compact. Mix sand with an organic amendment such as peat moss or sawdust to improve the sand’s amending properties.

Thoroughly rototill any amendment into garden soil – when dry – to prevent layering. Rototilling organic amendments into gardens in the fall gives soil microorganisms an early start on converting organic matter to humus. Another rototilling in spring will thoroughly mix in the amendments.

An easy way to amend garden soils is to plant a green manure cover crop. An excellent winter cover crop for western Oregon is crimson clover. Plant 12 pounds of seed per 1,000 square feet. Plant no later than Oct. 1 and water the bed so the crop is established before cold weather sets in. When rototilled under in late April, crimson clover will produce 3-4 pounds of nitrogen per 1,000 square feet.

More information on improving garden soils is available in “Growing Your Own,” a practical guide to gardening for first-time gardeners. Copies of a printed version are at county Extension offices.

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.

What Does Organic Matter Do In Soil?

Of all the components of soil, organic matter is probably the most important and most misunderstood. Organic matter serves as a reservoir of nutrients and water in the soil, aids in reducing compaction and surface crusting, and increases water infiltration into the soil. Yet it’s often ignored and neglected. Let’s examine the contributions of soil organic matter and talk about how to maintain or increase it.

What is Organic Matter?

Many times we think of organic matter as the plant and animal residues we incorporate into the soil. We see a pile of leaves, manure, or plant parts and think, “Wow! I’m adding a lot of organic matter to the soil.” This stuff is actually organic material, not organic matter.

What’s the difference between organic material and organic matter? Organic material is anything that was alive and is now in or on the soil. For it to become organic matter, it must be decomposed into humus. Humus is organic material that has been converted by microorganisms to a resistant state of decomposition. Organic material is unstable in the soil, changing form and mass readily as it decomposes. As much as 90 percent of it disappears quickly because of decomposition.

Organic matter is stable in the soil. It has been decomposed until it is resistant to further decomposition. Usually, only about 5 percent of it mineralizes yearly. That rate increases if temperature, oxygen, and moisture conditions become favorable for decomposition, which often occurs with excessive tillage. It is the stable organic matter that is analyzed in the soil test.

How Much Organic Matter Is in the Soil?

An acre of soil measured to a depth of 6 inches weighs approximately 2,000,000 pounds, which means that 1 percent organic matter in the soil would weigh about 20,000 pounds per acre. Remember that it takes at least 10 pounds of organic material to decompose to 1 pound of organic matter, so it takes at least 200,000 pounds (100 tons) of organic material applied or returned to the soil to add 1 percent stable organic matter under favorable conditions.

In soils that formed under prairie vegetation, organic-matter levels are generally comparatively high because organic material was supplied from both the top growth and the roots. We don’t usually think of roots as supplying organic material, but a study in the Upper Great Plains showed that a mixed prairie had an above-ground (shoot) yield of 1.4 tons of organic material per acre, while the root yield was about 4 tons per acre. The plants were producing roots that were more than twice the weight of the shoots.

Soils that have developed under forest vegetation usually have comparably low organic-matter levels. There are at least two reasons for these levels:

  1. trees produce a much smaller root mass per acre than grass plants, and
  2. trees do not die back and decompose every year. Instead, much of the organic material in a forest is tied up in the tree instead of being returned to the soil.

Soils that formed under prairie vegetation usually have native organic matter levels at least twice as high as those formed under forest vegetation.

What Are the Benefits of Organic Matter?

  • Nutrient Supply
    Organic matter is a reservoir of nutrients that can be released to the soil. Each percent of organic matter in the soil releases 20 to 30 pounds of nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3 pounds of sulfur per year. The nutrient release occurs predominantly in the spring and summer, so summer crops benefit more from organic-matter mineralization than winter crops.
  • Water-Holding Capacity
    Organic matter behaves somewhat like a sponge, with the ability to absorb and hold up to 90 percent of its weight in water. A great advantage of the water-holding capacity of organic matter is that the matter will release most of the water that it absorbs to plants. In contrast, clay holds great quantities of water, but much of it is unavailable to plants.
  • Soil Structure Aggregation
    Organic matter causes soil to clump and form soil aggregates, which improves soil structure. With better soil structure, permeability (infiltration of water through the soil) improves, in turn improving the soil’s ability to take up and hold water.
  • Erosion Prevention
    This property of organic matter is not widely known. Data used in the universal soil loss equation indicate that increasing soil organic matter from 1 to 3 percent can reduce erosion 20 to 33 percent because of increased water infiltration and stable soil aggregate formation caused by organic matter.

How Can I Maintain or Improve Soil Organic Matter Levels?

Building soil organic matter is a long-term process but can be beneficial. Here are a few ways to do it.

  • Reduce or Eliminate Tillage
    Tillage improves the aeration of the soil and causes a flush of microbial action that speeds up the decomposition of organic matter. Tillage also often increases erosion. No-till practices can help build organic matter.
  • Reduce Erosion
    Most soil organic matter is in the topsoil. When soil erodes, organic matter goes with it. Saving soil and soil organic matter go hand in hand.
  • Soil-Test and Fertilize Properly
    You may not have considered this one. Proper fertilization encourages growth of plants, which increases root growth. Increased root growth can help build or maintain soil organic matter, even if you are removing much of the top growth.
  • Cover Crops
    Growing cover crops can help build or maintain soil organic matter. However, best results are achieved if growing cover crops is combined with tillage reduction and erosion control measures.

A good supply of soil organic matter is beneficial in crop or forage production. Consider the benefits of this valuable resource and how you can manage your operation to build, or at least maintain, the organic matter in your soil.

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Organic Materials

Organic materials are associated with the chemistry of compounds of carbons. Often organic materials are thought of as the byproducts living things — plants or animals. While carbon compounds are generally thought to be produced in living things, it was discovered in the nineteenth century that carbon compounds could be synthesized in the laboratory. This discovery opened a whole new door to the discovery of useful products, such as vulcanized rubber and plastic.

Plant Materials and Plastics

Organic materials include those products from plants and animals, as well as various plastics.

Plant Materials:
Structure
Plant Materials:
Chemistry
Plant Materials:
Technology
Plant Materials:
Integrated Pest Management
Plant Materials: Deterioration –
Environmental & Mechanical
Plant Materials: Deterioration –
Chemical & Biological
Plant Materials:
Preventative
Care
Plastics in
Museum Collections
Plastics:
Chemistry
Plastics: Properties
& Technology
Plastics:
History
Plastics:
Deterioration

Plastics:
Preventative Care

Animal Materials

Animal products may be divided into four types of materials: dermal, keratinous, skeletal, and other. Dermal materials are the most common animal products used and are identified as leather or skin. Keratinous materials include silk, wool, hair, feathers, and horn. Bone, ivory, teeth, and shell are categorized as skeletal materials. Other animal products include albumins and caseins.

Leather or skin, derived from the dermal layers of animals, is a relatively strong, flexible material used for a variety of purposes. Animal hides have been used for clothing, book bindings, parchment, belts, and shoes.

Animal products that are primarily keratinous are outgrowths of the dermal (skin) layer. These materials are strong and flexible, easily shaped by heat and pressure. Included are horn, baleen, quill, hooves, hair, and claws. Items made of keratinous materials include clothing decoration, adornments, combs, and baskets.

Bone, antler, ivory, teeth, and shell are skeletal materials and are highly inorganic with some organic elements like hydroxyapatite and collagen. These products are anisotropic (respond differently in different directions to the same stimulus) and very strong. In order to fabricate something from bone, antler, ivory, or teeth, the material must be carved, sawed, turned, or pierced.

Preserving Native
American Artifacts

Animal Materials:
Deterioration

Animal Materials:
Preventative Care

Animal Materials:
Skin & Leather
Animal Materials:
Hair, Hooves, Etc.
Animal Materials:
Bones, Shells, Etc.

Inorganic Materials

Stone

Stone is a purely inorganic material found in nature. Stone is a crystalline mineral or a range of compositions and is used to make jewelry, utensils, weapons, sculpture, and architectural elements.

Stone:
Origins & Properties
Stone: Fabrication & Technology Stone:
Deterioration
Stone:
Preventative Care

TYPES OF MATERIALS

Ceramics Glass Metals Wood Textiles Paper Paintings Other

Organic Materials

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Objects Specialty Group Conservation Wiki
ORGANIC MATERIALS
Contributors: Katherine Holbrow.
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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works.

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CAUTION: The Objects Group Wiki pages are published for the members of the Objects Specialty Group.
Publication does not endorse or recommend any treatments, methods, or techniques described herein.

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:

Cellulose

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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What is soil organic carbon?

How is soil organic carbon different to soil organic matter?

Soil organic carbon (SOC) refers only to the carbon component of organic compounds. Soil organic matter (SOM) is difficult to measure directly, so laboratories tend to measure and report SOC.

Soil organic carbon and carbon sequestration

Sequestering carbon in SOC is seen as one way to mitigate climate change by reducing atmospheric carbon dioxide. The argument is that small increases of SOC over very large areas in agricultural and pastoral lands will significantly reduce atmospheric carbon dioxide. For the reduction to be long-lasting, organic matter would have to be in the more stable or resistant fractions (Table 1). For more information see Soil organic carbon and carbon sequestration.

What is soil organic matter?

SOM is composed mainly of carbon, hydrogen and oxygen, and has small amounts of other elements, such as nitrogen, phosphorous, sulfur, potassium, calcium and magnesium contained in organic residues. It is divided into ‘living’ and ‘dead’ components and can range from very recent inputs, such as stubble, to largely decayed materials that are thousands of years old. About 10% of below-ground SOM, such as roots, fauna and microorganisms, is ‘living’ (Figure 1).

SOM exists as 4 distinct fractions which vary widely in size, turnover time and composition in the soil (Table 1):

  1. dissolved organic matter
  2. particulate organic matter
  3. humus
  4. resistant organic matter.

Figure 1 Most soil organic matter is dead or decaying, with living organisms making up about 10% of the soil organic matter pool

Table 1 The size, turnover time and composition of the 4 soil organic matter fractions

Fraction Size micrometres (µm) and millimetres (mm) Turnover time Composition
Dissolved organic matter <45µm (in solution) Minutes to days Soluble root exudates, simple sugars and decomposition by-products. It generally makes up less than 5% of total soil organic matter.
Particulate organic matter 53µm–2mm 2–50 years Fresh or decomposing plant and animal matter with identifiable cell structure. Makes up 2–25% of total soil organic matter.
Humus <53µm Decadal (10s to 100s of years) Older, decayed organic compounds that have resisted decomposition. Can make up more than 50% of total soil organic matter.
Resistant organic matter <53µm–2mm 100s to 1000s of years Relatively inert material, such as chemically resistant materials or organic remnants (e.g. charcoal). Can be up to 10% of soil organic matter.

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How much soil organic carbon is in WA soils?

Most WA soils are low in SOC (Viscarra Rossel et al. 2014). Low rainfall, warm conditions for much of the year, and sandy soils limit the build-up of stable SOC.

Typically, the organic carbon content of WA dryland agricultural soils is between 0.7% and 4% (Figure 2), although SOC can be as low as 0.3% for desert soils and as high as 14% for intensive dairy soils. Most organic matter is located near the soil surface. In south-west WA, about 60% of organic matter in the top 30cm of soil is located in the top 10cm.

Figure 2 Concentration (%) of soil organic carbon in the top 0–10cm of the soil. Areas of low confidence are white; grey areas represent native vegetation and reserves (Source: Griffin et al. 2013)

Estimating soil organic matter stock from soil organic carbon

1 Start with the measured total organic carbon %

About 58% of the mass of organic matter exists as carbon. We can estimate the percentage of SOM from the SOC% using the conversion factor 1.72 (derived from 100/58).

Organic matter (%) = total organic carbon (%) x 1.72

This conversion factor can vary in different soils, but 1.72 provides a reasonable estimate of SOM for most purposes.

2 Convert % to weight for a given depth and area

SOC stock in tonnes of carbon per hectare (tC/ha) = (soil organic carbon %) x (mass of soil in a given volume)

(0.013) x (1.2 x 0.1 x 10 000) = 15.6tC/ha.

Using the conversion factor of 1.72, the amount of SOM would be: 15.6 x 1.72 = 26.8 tonnes of organic matter.

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Soil organic matter cycling

Soil type, climate and management influence organic matter inputs to soil and its turnover or decomposition. Rainfall is a major driver of plant growth (biomass) and biological activity which results in the decomposition of organic matter that enters soil. The different fractions of SOM (dissolved, particulate, humus and resistant) turn over at vastly different rates (Figure 3). Furthermore, SOM cycles continuously between living, decomposing and stable fractions in the soil (Figure 4).

Figure 3 Different fractions of soil organic matter decompose in the soil over different time frames Figure 4 Organic matter transforms from one form to another as it decomposes and cycles into different soil organic fractions (figure adapted from University of Minnesota extension publication WW-07402)

  1. Inputs: plants and animals become part of the SOM as they die or create by-products.
  2. Transformation: soil organisms break-up and consume organic matter, creating different forms of organic residues. For example, fresh plant residues are broken into smaller pieces (<2mm) and become part of the particulate organic matter fraction. This material decomposes further and a smaller amount of more biologically stable material enters the humus pool.
  3. Nutrient release: nutrients and other compounds not required by microorganisms are released and are then available to plants.
  4. Stabilising organic matter: as the organic residues decompose, they become more resistant to further change.

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How much of the soil organic carbon that enters soil stays there?

Microorganisms digest up to 90% of the organic carbon that enters a soil in organic residues. In doing so, they respire the carbon back into the atmosphere as carbon dioxide. While up to 30% of organic inputs can eventually be converted to humus, depending on soil type and climate, in Australian agricultural soils this value is often significantly less. There are 3 main factors influencing the ability of a given soil type to retain SOC (Figure 5). Soils naturally higher in clay content generally retain more organic matter – and hence can retain more organic carbon – than sandy soils.

Figure 5 The influence of soil type, climate and management factors on the retention of soil organic matter in soils (Source: Ingram & Fernandes 2001)

SOM is primarily a result of inputs minus losses, and may be influenced by soil type, climate and management (Table 2). SOM increases when inputs are greater than losses, and vice versa. Inputs largely depend on plant biomass production, though may also be the result of amendments added to soils or by-products from animal production. Losses occur when organic matter decomposes and, in some cases, with soil erosion.

Table 2 Influences of soil, climate and management factors on soil organic matter accumulation in Western Australia

Factors Influence
Soil type
  • Naturally occurring clay in soil binds to organic matter, which helps to protect it from being broken down or limits access to it by microbes and other organisms.
  • Organic matter in coarse-textured sandy soils is not protected from microbial attack and is rapidly decomposed.
Climate
  • In comparable farming systems with similar soil type and management, soil organic matter increases with rainfall. This is because increasing rainfall supports greater plant growth, which results in more organic matter accumulating in the soil.
  • Organic matter decomposes more slowly as temperatures decrease. In Western Australia under moist conditions, each 10°C increase in temperature doubles the rate of organic matter decomposition (Hoyle et al. 2006). This means moist, warm conditions will often result in the most rapid decomposition of organic inputs.
Land and soil management
  • Maximising crop and pasture biomass via better water-use efficiency and agronomic management will increase organic matter inputs.
  • As a large proportion of organic matter is present in the top 0–10cm of soils, protecting the soil surface from erosion is essential for retaining soil organic matter.
  • Tillage of structured soils decreases soil organic matter stocks by exposing previously protected organic matter to microbial decomposition.
  • Adding off-farm organic residues, such as manures, straw and char, can increase soil organic matter. The agronomic benefits should be measured to establish economic viability.
  • Landscape can influence biomass production (inputs) associated with water availability.
  • Transfer of soil and organic matter down slope via erosion can increase soil organic matter stocks in lower parts of the landscape.
  • Soil constraints decrease plant growth and decomposition rates. This could slow the amount and transformation rate of organic matter moving into more stable fractions.
  • Microorganisms and particularly bacteria, grow poorly in strongly acidic or alkaline soils and consequently organic matter breaks down slowly in these soils.

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How do I measure or interpret soil organic carbon results?

Changes in stable SOC generally occur very slowly (over decades), and it is often hard to measure small changes against a relatively large background of soil carbon. Changes in SOC are largely determined by how much biomass is grown and retained above and below ground.

About 45% of organic matter is carbon and lighter textured soils retain less than 30% of this. For example, WA soils have measured between 20 and 160tC/ha. A typical Australian grain production system, yielding 2t/ha of wheat is likely to retain 0.1–0.5t of organic matter per hectare in the soil each year. This equates to a change in SOC in many instances of less than 1% of the total stock.

A larger change in total organic carbon stock, which may take several years or longer to occur, is required before a significant change could be measured with any degree of confidence. Given annual inputs of organic residues are likely to be less than 0.2tC/ha in a typical grain cropping system, the time required to detect a significant change in SOC is generally more than 10 years.

By monitoring SOC over time, changes caused by management can be estimated (Figure 6). However, fresh residues (Figure 1) can vary widely, depending on the crops or pastures grown each year.

Accurate measurement of changes in SOC requires:

  • a soil sampling strategy that captures the natural variation in soil carbon
  • a measure of SOC concentration
  • an estimate of bulk density of the soil to adjust for changes in soil mass at specified depth intervals.

Changes in SOC are most likely to be observed in the top 0–10cm of the soil.

Soil tests for organic carbon normally report a percentage total SOC. Using a measure of bulk density, the amount of carbon per hectare to a given depth of soil can be calculated as shown earlier.

Figure 6 Changes in SOC can be monitored over time with an accurate soil sampling program. Changes in total SOC and the distribution of organic carbon at different depths can help you understand how management practices are influencing SOC over time.

Hoyle, FC, Murphy, DV & Fillery, IRP 2006, ‘Temperature and stubble management influence microbial CO2-C evolution and gross N transformation rates’, Soil Biology and Biochemistry, vol. 38, pp. 71–80.

Viscarra Rossel, RA, Webster, R, Bui, EN & Baldock, JA 2014, ‘Baseline map of organic carbon in Australian soil to support national carbon accounting and monitoring under climate change’, Global Change Biology, vol. 20, pp. 2953–2970, doi:10.1111/gcb.12569

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