Composting, Fertilizer, & Mulch

Giving your Garden the right nutrients is key for any healthy garden. Here are some more information on how to make sure your garden stays health from the start.

Composting is the process of producing compost through aerobic decomposition of biodegradable organic matter. The decomposition is performed primarily by aerobes , although larger creatures such as ants, nematodes, and oligochaete worms also contribute. This decomposition occurs naturally in all but the most hostile environments, such as within landfills, extremely arid deserts or cold weather such as boreal winters or polar regions, which prevent the microbes and other decomposers from thriving.

Composting can be divided into the two areas of home composting and industrial composting . Both scales of composting use the same biological processes, however techniques and different factors must be taken into account.

Composting is the controlled decomposition of organic matter. Rather than allowing nature to take its slow course, a composter provides an optimal environment in which decomposers can thrive. To encourage the most active microbes, a compost pile needs the correct mix of the following ingredients:

• Carbon
• Nitrogen
• Oxygen (in the case of aerobic composting)
• Water

Decomposition happens even in the absence of some of these ingredients, but not as quickly or as pleasantly. (For example, vegetables in a plastic bag will decompose, but the absence of air encourages the growth of anaerobic microbes, which produce disagreeable odors. Degradation under anaerobic conditions is called anaerobic digestion.)

The goal of a composting system

The goal in a composting system is to provide a healthy environment and nutrition for the rapid decomposers, the bacteria. The most rapid composting occurs with the ideal carbon to nitrogen ratio of between 25 and 30 to 1 by dry chemical weight . In other words, the ingredients placed in the pile should contain 25 to 30 times as much carbon as nitrogen. For example, grass clippings average about 19 to 1 and dry autumn leaves average about 55 to 1. Mixing equal parts by volume approximates the ideal range. Commercial-grade composting operations pay strict attention to this ratio. For backyard composters, however, the charts of carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so many rule of thumb exist to guide composters in approximating this mixture.

Materials for composting

Given enough time, all biodegradable material will compost. However, not all compost feedstocks are appropriate for backyard composting. Most backyard systems will not reach high enough temperatures to kill pathogens and deter vermin, so pet droppings, non-vegetarian animal manure, meat scraps, and dairy products are best left to operators of high-rate, thermophylic composting systems.

Certain substances should not be composted by the average homeowner, as they require more sophisticated systems, competent management, and more efficient, cost-competitive, environmentally sound technology.

These substances include non-vegetarian animal manures and bedding, by-products of food production and processing, restaurant grease and cooking oils, and residuals from the treatment of wastewater and drinking water. Composting will also break down petroleum hydrocarbons and some toxic compounds for recycling and beneficial reuse. The use of composting for such purposes is most commonly referred to as a form of bioremediation.

High-carbon sources provide the cellulose needed by the composting bacteria for conversion to sugars and heat, while high-nitrogen sources provide the most concentrated protein, which allow the compost bacteria to thrive.

Some ingredients with higher carbon content:

• Dry, straw-type material, such as cereal straws
• Autumn leaves
• Sawdust and wood chips
• Some paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks )

Some ingredients with higher nitrogen content:

• Green plant material (fresh or wilted) such as crop residues, hay, grass clippings, weeds
• Animal manures (choose vegetarian horse manure, cow manure, llama manure, etc.)
• Fruit and vegetable trimmings
• Seaweeds
• Coffee grounds

Poultry manure provides lots of nitrogen but little carbon. Horse manure provides both. Sheep and cattle manure don’t drive the compost heap to as high a temperature as poultry or horse manure, so the heap takes longer to produce the finished product.

Mixing the materials as they are added increases the rate of decomposition, but it can be easier to place the materials in alternating layers, approximately 15 cm (6 in ) thick, to help estimate the quantities. Keeping carbon and nitrogen sources separated in the pile can slow down the process, but decomposition will occur in any event.

Greasy food waste and wastes from meat, dairy products, and eggs should not be used in household compost because they tend to attract unwanted vermin and they do not appropriately decompose in the time required. However, eggshells are a good source of nutrients for the compost pile and the soil although they typically take more than one year to decompose. If recycling of meat and dairy products is desirable, Bokashi is a suitable alternative, which uses fermentation. However, even in Bokashi, liquids like milk and oil should not be used. Manure from non-vegetarian animals should never be used, and neither should human or pet waste.

Composting techniques

There are a number of different techniques for composting, all employing the two primary methods of aerobic composting:

• Active (or hot ) composting allows aerobic bacteria to thrive, kills most pathogens and seeds, and rapidly produces usable compost. Aerobic bacteria produce less odour and fewer destructive greenhouse gases than their anaerobic counterparts. In addition, they are usually faster at breaking down material and the faster material is broken down, the faster compost is created for your garden.

Pasteurisation in a hot compost (such as the Compost Oven) will occur in any garden compost bin if the temperature reaches above 55 C (131°F) for three or more days. To achieve it, you need to keep your garden compost bin warm, insulated and damp since this encourages the cultivation of actinomycetes , a vital bacteria in the pasteurisation process.

The pasteurised soil naturally created through heat in the garden compost is valuable for the composting home gardener, since the pasteurisation process is otherwise both expensive and complicated, and adding chemicals to produce the pasteurisation effect makes the compost less healthy.

• Passive (or cold ) composting allows Nature to take its course in a more leisurely manner, while leaving many pathogens and seeds dormant in the pile.

Cold composting is the type of composting done in most domestic garden compost bins in which temperatures never reach above 30 C (86°F). Cold composting is characterised by individuals putting their kitchen scraps in the garden compost bin and leaving them untended. This “scrap bin,” having a very high moisture content and without aeration, is likely to turn anaerobic and generate foul odours, including significant adverse greenhouse gas emissions.

When composting this way, a gardener can improve the process by adding some wood chips or small pieces of bark, leaves, twigs or a combination of these materials distributed throughout the mixture. This material helps to improve drainage and airflow.

Such composting systems may be either enclosed (home container composting , industrial in-vessel composting ) or in exposed piles (industrial windrow composting ).

Home composting

Home composters use a range of techniques, varying from extremely passive composting (throw everything in a pile in a corner and leave it alone for a year or two) to extremely active (monitoring the temperature, turning the pile regularly, and adjusting the ingredients over time). Some composters use mineral powders to absorb smells, although a well-maintained pile seldom has bad odors.

Microbes and heating the pile

An effective compost pile is kept about as damp as a well wrung-out sponge. This provides the moisture that all life needs to survive. Bacteria and other microorganisms fall into a variety of groups in terms of what their ideal temperature is and how much heat they generate as they do their work. Mesophilic bacteria enjoy midrange temperatures, from about 20 to 40 C (70 to 110°F). As they decompose the organic matter they generate heat, and the inner part of a compost pile heats up the most.

The heap should be about 1 m (3 ft ) wide, 1 m (3 ft) tall, and as long as is practicable. This provides a suitable insulating mass to allow a good heat build-up as the material decays. The ideal temperature is around 60 C (140°F), which kills most pathogens and weed seeds while providing a suitable environment for thermophilic (heat-loving) bacteria, which are the fastest acting decomposers. The centre of the heap can get too warm, possibly hot enough to burn a bare hand. If this fails to happen, common reasons include the following:

• The heap is too wet, thus excluding the oxygen required by the compost bacteria
• The heap is too dry, so that the bacteria do not have the moisture needed to survive and reproduce
• There is insufficient protein (nitrogen-rich material)

The solution is to add material, if necessary, and/or to turn the pile to aerate it.

Depending on how quickly the compost is required, the heap can be turned one or more times to bring the outer layers to the inside of the heap and vice versa, as well as to aerate the mixture. Adding water at this time helps keep the pile damp. One guideline is to turn the pile when the high temperature has begun to drop, indicating that the food source for the fastest-acting bacteria (in the center of the pile) has been largely consumed. When turning the pile does not result in a temperature rise, there is no further advantage in turning the pile. When all the material has turned into dark brown or nearly black crumbly matter, it is ready to use.

Worm Composting

Recycling the organic waste of a household into compost allows us to return badly needed organic matter to the soil. In this way, we participate in nature’s cycle, and cut down on garbage going into burgeoning landfills. Worm composting or vermicomposting is a method for recycling food waste into a rich, dark, earth-smelling soil conditioner. The great advantage of worm composting is that this can be done indoors and outdoors, thus allowing year round composting. It also provides apartment dwellers with a means of composting. The worm then excretes a soil-nutrient material called worm castings. This is why wise farmers have historically wanted to have healthy worm populations living in their fields. Worms are at the bottom level of the food chain but are critical to healthy soil. In a nutshell, worm compost is made in a container filled with moistened bedding and redworms. Add your food waste for a period of time, and the worms and micro-organisms will eventually convert the entire contents into rich compost. Some good gardeners have developed a radical composting product, made through a brewing process which runs distilled water through Red Wiggler worm castings. The nutritious elements and microorganisms of the castings are captured in a concentrated liquid form, named worm tea. By using worm tea on your plants and gardens, you put healthy microorganisms back into the soil where they thrive and multiply, creating a much healthier growing environment for your plants.

Industrial composting

Industrial composting systems are being increasingly installed as an alternative form of waste management to landfill along with other advanced waste processing systems . The industrial composting or anaerobic digestion can be combined with mechanical sorting of mixed waste streams and is given the term mechanical biological treatment . Industrial composting helps prevent global warming by treatment of biodegradable waste before it enters landfill. Once this waste is landfilled it breaks down anaerobically producing landfill gas that contains methane, a potent greenhouse gas.

Most commercial and industrial composting operations use active composting techniques. This ensures a higher quality product and produces results in the shortest time (see compost windrow turner). The greatest control, and therefore the highest quality, is generally achieved by composting inside an enclosed vessel which is monitored and adjusted continuously for optimal temperature, air flow, moisture, and other parameters. See In-vessel (also en-vessel) .

Large scale composting systems are used by a few urban centers around the world. Co-composting is a technique which combines solid waste with de-watered bio-solids. The world’s largest co-composter is in Edmonton, Alberta, Canada, which turns 220,000 tonnes of residential solid waste and 22,500 dry tonnes of biosolids per year into 80,000 tonnes of compost using a facility 38,690 square metres in size (equivalent to 8 football fields). The aeration building alone is the largest stainless steel building in North America (the size of 14 NHL rinks).

Other ingredients

Some users like to put special materials and activators into their compost. Adding commercially available Effective Microorganisms TM helps to keep the balance between “good” and “bad” bacteria. A light dusting of agricultural lime (not on the animal manure layers) can curb excessive acidity that can slow down the fermentation. Seaweed meal can provide a ready source of trace elements. Finely pulverized rock ( rock flour or rock dust) can also provide needed minerals, as opposed to clay (which is trace mineral-poor and/or leached rock dust).

Animal manure should only be collected from vegetarian animals, such as horses, cows, sheep, llamas, etc. Pet waste, human waste, and non-vegetarian animal waste should not be used in the average compost heap.

Human waste can be collected by composting toilets (in this case, human feces). However, such compost is usually not used as a fertilizer for plants that are directly edible (e.g., salad crops). Most composting toilets do not allow for the thermophilic activity needed to completely kill off the pathogens and bacteria . However, if these high temperatures are reached, there is no danger of contamination, and the resulting compost can be safely used on food crops. Most composts heaps are unable to reach those temperatures. Composting toilets should only be used as a way to reduce waste in the environment, not as a fertilizer; in the case that they are used with crops, they should only use human waste for non-food crops, or with careful filteration, food based crops.

Fertilizer

Fertilizers ( British English fertilisers ) are compounds given to plants to promote growth; they are usually applied either via the soil, for uptake by plant roots, or by foliar feeding, for uptake through leaves. Fertilizers can be organic (composed of organic matter, i.e. carbon based), or inorganic (containing simple, inorganic chemicals). They can be naturally occurring compounds such as peat or mineral deposits, or manufactured through natural processes (such as composting), or chemical processes (such as the Haber process).

Fertilizers typically provide, in varying proportions, the three major plant nutrients ( nitrogen, phosphorus, and potassium ), the secondary plant nutrients ( calcium , sulfur , magnesium ), and sometimes trace elements (or micronutrients) with a role in plant nutrition: boron, chlorine, manganese, iron, zinc, copper, and molybdenum.

In the past, both organic and inorganic fertilizers were called “manures,” but this term is now mostly restricted to man-made manure.

Inorganic fertilizers (mineral fertilizer)

• Examples of naturally occurring inorganic fertilizers include Chilean sodium nitrate, mined “rock phosphate,” and limestone (a calcium source).

Macronutrients and micronutrients

Fertilizers can be divided into macronutrients or micronutrients based on their concentrations in plant dry matter. There are six macronutrients: nitrogen, phosphorus, and potassium, often termed “primary macronutrients” because their availability is usually managed with NPK fertilizers, and the “secondary macronutrients” — calcium, magnesium, and sulfur — which are required in roughly similar quantities but whose availability is often managed as part of liming and manuring practices rather than fertilizers. The macronutrients are consumed in larger quantities and normally present as a whole number or tenths of percentages in plant tissues (on a dry matter weight basis). There are many micronutrients, required in concentrations ranging from 5 to 100 parts per million (ppm) by mass. Plant micronutrients include iron (Fe), manganese (Mn), boron (B), copper (Cu), molybdenum (Mo), nickel (Ni), chlorine (Cl), and zinc (Zn).

Macronutrient fertilizers

Synthesized materials are also called artificial , and may be described as straight, where the product predominantly contains the three primary ingredients of nitrogen (N), phosphorus (P), and potassium (K), which are known as N-P-K fertilizers or compound fertilizers when elements are mixed intentionally. They are named or labeled according to the content of these three elements, which are macronutrients. The mass fraction (percent) nitrogen is reported directly. However, phosphorus is reported as phosphorus pentoxide (P 2 O 5 ), the anhydride of phosphoric acid, and potassium is reported as potash or potassium oxide (K 2 O), which is the anhydride of potassium hydroxide. Fertilizer composition is expressed in this fashion for historical reasons in the way it was analyzed (conversion to ash for P and K); this practice dates back to Justus von Liebig (see more below). Consequently, an 18-51-20 fertilizer would have 18% nitrogen as N, 51% phosphorus as P 2 O 5 , and 20% potassium as K 2 O, The other 11% is known as ballast and may or may not be valuable to the plants, depending on what is used as ballast. Although analyses are no longer carried out by ashing first, the naming convention remains. If nitrogen is the main element, they are often described as nitrogen fertilizers.

In general, the mass fraction (percentage) of elemental phosphorus, [P] = 0.436 x [P 2 O 5 ]

and the mass fraction (percentage) of elemental potassium, [K] = 0.83 x [K 2 O]

(These conversion factors are mandatory under the UK fertilizer-labelling regulations if elemental values are declared in addition to the N-P-K declaration.)

An 18−51−20 fertilizer therefore contains, by weight, 18% elemental nitrogen (N), 22% elemental phosphorus (P) and 16% elemental potassium (K).

Agricultural versus horticultural

In general, agricultural fertilizers contain only one or two macronutrients. Agricultural fertilizers are intended to be applied infrequently and normally prior to or along side seeding. Examples of agricultural fertilizers are granular triple superphosphate, potassium chloride, urea, and anhydrous ammonia. The commodity nature of fertilizer, combined with the high cost of shipping, leads to use of locally available materials or those from the closest/cheapest source, which may vary with factors affecting transportation by rail, ship, or truck. In other words, a particular nitrogen source may be very popular in one part of the country while another is very popular in another geographic region only due to factors unrelated to agronomic concerns.

Horticultural or specialty fertilizers, on the other hand, are formulated from many of the same compounds and some others to produce well-balanced fertilizers that also contain micronutrients. Some materials, such as ammonium nitrate, are used minimally in large scale production farming. The 18-51-20 example above is a horticultural fertilizer formulated with high phosphorus to promote bloom development in ornamental flowers. Horticultural fertilizers may be water-soluble (instant release) or relatively insoluble (controlled release). Controlled release fertilizers are also referred to as sustained release or timed release. Many controlled release fertilizers are intended to be applied approximately every 3-6 months, depending on watering, growth rates, and other conditions, whereas water-soluble fertilizers must be applied at least every 1-2 weeks and can be applied as often as every watering if sufficiently dilute. Unlike agricultural fertilizers, horticultural fertilizers are marketed directly to consumers and become part of retail product distribution lines.

Justus von Liebig

Chemist Justus von Liebig (in the 19th century) contributed greatly to the advancement of understanding of plant nutrition. His influential works first denounced the vitalist theory of humus, arguing first the importance of ammonia, and later the importance of inorganic minerals. Primarily his work succeeded in setting out questions for agricultural science to address over the next 50 years. His attempt at implementation of his theories commercially in England with artificial phosphate based fertilizer, which was much less expensive than the guano that was used at the time, failed because it was not able to be absorbed by crops.

Nitrogen fertilizer

Nitrogen fertilizer is often synthesized using the Haber-Bosch process, which produces ammonia. This ammonia is applied directly to the soil or used to produce other compounds, notably ammonium nitrate and urea, both dry, concentrated products that may be used as fertilizer materials or mixed with water to form a concentrated liquid nitrogen fertilizer, UAN. Ammonia can also be used in the Odda Process in combination with rock phosphate and potassium fertilizer to produce compound fertilizers such as 10-10-10 or 15-15-15.

The production of ammonia currently consumes about 5% of global natural gas consumption, equating to around 2% of world energy production. Natural gas is overwhelmingly used for the production of ammonia, but other energy sources, together with a hydrogen source, can be used for the production of nitrogen compounds suitable for fertilizers. The cost of natural gas makes up about 90% of the cost of producing ammonia. The price increases in natural gas in the past decade, among other factors such as increasing demand, have contributed to an increase in fertilizer price.

Health and sustainability issues

Inorganic fertilizers sometimes do not replace trace mineral elements in the soil which become gradually depleted by crops grown there. This has been linked to studies which have shown a marked fall (up to 75%) in the quantities of such minerals present in fruit and vegetables. One exception to this is in Western Australia where deficiencies of zinc, copper, manganese, iron and molybdenum were identified as limiting the growth of crops and pastures in the 1940s and 1950s. Soils in Western Australia are very old, highly weathered and deficient in many of the major nutrients and trace elements. Since this time these trace elements are routinely added to inorganic fertilizers used in Agriculture in this state.

In many countries there is the public perception that inorganic fertilizers “poison the soil” and result in “low quality” produce. However, there is very little (if any) scientific evidence to support these views. When used appropriately, inorganic fertilizers enhance plant growth, the accumulation of organic matter and the biological activity of the soil, while reducing the risk of water run-off, overgrazing and soil erosion. The nutritional value of plants for human and animal consumption is typically improved when inorganic fertilizers are used appropriately.

There are concerns though about arsenic, cadmium and uranium accumulating in fields treated with phosphate fertilizers. The phosphate minerals contain trace amounts of these elements and if no cleaning step is applied after mining the continous use of phosphate fertilizers leads tos a accumulation of these elements in the soil. Eventually these can build up to unacceptable levels and get into the produce. (See cadmium poisoning.)

Another problem with inorganic fertilizers is that they are presently produced in ways which cannot be continued indefinitely. Potassium and phosphorus come from mines (or from saline lakes such as the Dead Sea in the case of potassium fertilizers ) and resources are limited. Nitrogen is unlimited, but nitrogen fertilizers are presently made using fossil fuels such as natural gas . Theoretically fertilizers could be made from sea water or atmospheric nitrogen using renewable energy, but doing so would require huge investment and is not competitive with today’s unsustainable methods.

Organic fertilizers

• Examples of naturally occurring organic fertilizers include manure, slurry, worm castings, peat, seaweed, sewage, and guano. Green manure crops are also grown to add nutrients to the soil. Naturally occurring minerals such as mine rock phosphate, sulfate of potash and limestone are also considered Organic Fertilizers.
• Examples of manufactured organic fertilizers include compost, bloodmeal, bone meal and seaweed extracts. Other examples are natural enzyme digested proteins, fish meal, and feather meal.

The decomposing crop residue from prior years is another source of fertility. Though not strictly considered “fertilizer”, the distinction seems more a matter of words than reality.

Some ambiguity in the usage of the term ‘organic’ exists because some of synthetic fertilizers, such as urea and urea formaldehyde, are fully organic in the sense of organic chemistry. In fact, it would be difficult to chemically distinguish between urea of biological origin and that produced synthetically. On the other hand, some fertilizer materials commonly approved for organic agriculture, such as powdered limestone, mined “rock phosphate” and Chilean saltpeter, are inorganic in the use of the term by chemistry.

Although the density of nutrients in organic material is comparatively modest, they have some advantages. For one thing organic growers typically produce some or all of their fertilizer on-site, thus lowering operating costs considerably. Then there is the matter of how effective they are at promoting plant growth, chemical soil test results aside. The answers are encouraging. Since the majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and are slow release fertilizers their effectiveness can be greater than conventional nitrogen fertilzers.

Implicit in modern theories of organic agriculture is the idea that the pendulum has swung the other way to some extent in thinking about plant nutrition. While admitting the obvious success of Leibig ’s theory, they stress that there are serious limitations to the current methods of implementing it via chemical fertilization. They re-emphasize the role of humus and other organic components of soil, which are believed to play several important roles:

• Mobilizing existing soil nutrients, so that good growth is achieved with lower nutrient densities while wasting less
• Releasing nutrients at a slower, more consistent rate, helping to avoid a boom-and-bust pattern
• Helping to retain soil moisture, reducing the stress due to temporary moisture stress
• Improving the soil structure

Organics also have the advantage of avoiding certain long-term problems associated with the regular heavy use of artificial fertilizers:

• the possibility of “burning” plants with the concentrated chemicals (i.e. an over supply of some nutrients)
• the progressive decrease of real or perceived “soil health”, apparent in loss of structure, reduced ability to absorb precipitation, lightening of soil color, etc.
• the necessity of reapplying artificial fertilizers regularly (and perhaps in increasing quantities) to maintain fertility
• the cost (substantial and rising in recent years) and resulting lack of independence

Organic fertilizers also have their disadvantages:

• As acknowledged above, they are typically a dilute source of nutrients compared to inorganic fertilizers, and where significant amounts of nutrients are required for profitable yields, very large amounts of organic fertilizers must be applied. This results in prohibitive transportation and application costs, especially where the agriculture is practiced a long distance from the source of the organic fertilizer.
• The composition of organic fertilizers tends to be highly variable, so that accurate application of nutrients to match plant production is difficult. Hence, large-scale agriculture tends to rely on inorganic fertilizers while organic fertilizers are cost-effective on small-scale horticultural or domestic gardens.
• Improperly-processed organic fertilizers may contain pathogens harmful to humans or plants. Organic fertilizers are derived from natural sources, which may include animal feces or plant/animal matter contaminated with pathogens. However, proper composting of raw materials used in organic fertilizers will kill pathogens.

In practice a compromise between the use of artificial and organic fertilizers is common, typically by using inorganic fertilizers supplemented with the application of organics that are readily available such as the return of crop residues or the application of manure.

It is important to differentiate between what we mean by organic fertilizers and fertilizers approved for use in organic farming and organic gardening by organizations and authorities who provide organic certification services. Some approved fertilizers may be inorganic, naturally occurring chemical compounds, e.g. minerals.

Risks of fertilizer use

Excessive nitrogen fertilizer applications can lead to pest problems by increasing the birth rate, longevity and overall fitness of certain pests (Jahn 2004; Jahn et al. 2001a,b, 2005; Preap et al. 2002, 2001).

It is also possible to over-apply organic fertilizers. However: their nutrient content, their solubility, and their release rates are typically much lower than chemical fertilizers, partially because by their nature, most organic fertilizers also provide increased physical and biological storage mechanisms to soils.

The problem that we face of over-fertilization is primarily associated with the use of artificial fertilizers, because of the massive quantities applied and the destructive nature of chemical fertilizers on soil nutrient holding structures. The high solubilities of chemical fertilizers also exacerbate their tendency to degrade ecosystems.

Storage and application of some fertilizers in some weather or soil conditions can cause emissions of the greenhouse gas nitrous oxide (N 2 O). Ammonia gas (NH 3 ) may be emitted following application of inorganic fertilizers, or manure or slurry. Besides supplying nitrogen, ammonia can also increase soil acidity (lower pH , or “souring”).

For these reasons, it is recommended that knowledge of the nutrient content of the soil and nutrient requirements of the crop are carefully balanced with application of nutrients in inorganic fertiliser especially. This process is called nutrient budgeting. By careful monitoring of soil conditions, farmers can avoid wasting expensive fertilizers, and also avoid the potential costs of cleaning up any pollution created as a byproduct of their farming.

The concentration of up to 100 mg/kg of Cadmium in phosphate minerals (for example Nauru and the Christmas islands) increases the contamination of soil with Cadmium, for example in New Zealand. Uranium is another example for impurities of fertilizers.

Mulch

In agriculture and gardening, mulch is a protective cover placed over the soil, primarily to modify the effects of the local climate. A wide variety of natural and synthetic materials are used.

Mulch is used for various purposes:

• to adjust soil temperature by helping soil retain more heat in spring and fall, and by keeping soil cool and evening out temperature swings during hot and variable summer conditions
• to control weeds by blocking the sunlight necessary for germination
• to retain water by slowing evaporation
• to add organic matter and nutrients to the soil through the gradual breakdown of the mulch material
• to repel insects
• to incrementally improve growing conditions by reflecting sunlight upwards to the plants, and by providing a clean, dry surface for ground-lying fruit such as squash and melons.
• for erosion control - protects soil from rain and preserves moisture
• for sediment control - slows runoff velocity

A variety of materials are used as mulch:

• organic residues - grass clippings, leaves, hay, straw, shredded bark, sawdust, shells, wood chips, shredded newspaper, cardboard, wool, etc. Many of these materials also act as a direct composting system. There are many differing opinions on what to use.
• compost - This relies on fully composted material, where potential weed seed has been eliminated, or else the mulch will actually produce weed cover.
• rubber mulch - Environmentally safe & secure; made from 100% recycled rubber.
• plastic mulch - Crops grow through slits or holes in thin plastic sheeting. This method is predominant in large-scale vegetable growing, with millions of acres cultivated under plastic mulch worldwide each year (disposal of plastic mulch is cited as an environmental problem).
• organic sheet mulch - Various products developed as a biodegradable alternative to plastic mulch.
• rock and gravel can also be used a mulch. In northern climates the heat retained by rocks will extend the growing season.

The way a particular organic mulch decomposes, and reacts to wetting by rain and dew, determine in great degree its effectiveness. Organic mulches can rot rapidly rather than slowly break down, and it can mat into a barrier that blocks water and air, both conditions that can be detrimental to crops.

Living mulch may also be considered a type of mulch, or as a mulch-like cover crop. This technique involves undersowing a main crop with a fast-growing cover crop that will provide weed suppression and other benefits associated with mulch.

Mulching is an important part of any no-dig gardening regime, such as practiced within permaculture systems.

Application

Mulch is usually applied towards the beginning of the growing season, and may be reapplied as necessary. It serves initially to warm the soil by helping it retain heat. This allows early seeding and transplanting of certain crops, and encourages faster growth. As the season progresses, the mulch stabilizes temperature and moisture, and prevents sunlight from germinating weed seed.

Plastic mulch used in large-scale commercial production is laid down with a tractor -drawn or standalone plastic mulch layer. This is usually part of a sophisticated mechanical process, where raised beds are formed, plastic is rolled out on top, and seedlings are transplanted through it. Drip irrigation is often required, with drip tape laid under the plastic, as plastic mulch is impermeable to water.

In home gardens and smaller farming operations, organic mulch is usually spread by hand around emerged plants. For materials like straw and hay, a shredder may be used to chop up the material. Organic mulches are usually piled quite high, six inches or more, and settle over the season.

In some areas of the United States such as central Pennsylvania near Harrisburg and norther California, mulch is often referred to as “tanbark”, even by manufacturers and distributors. In these areas, the word “mulch” is used specifically to refer to very fine tanbark or peat moss.

Rubber mulch

Rubber mulch is a product made from recycled tires and other recyclable rubber products for use in playgrounds and landscapes.It can be 97% wire free for landscape use and 99.9% wire free for playgrounds. There is a company in Saint Paul, MN. that does make 100% wire free mulch (jjvrubbermulchusa.com) Rubber mulch is also becoming a product of choice used in playgrounds, and in horse arenas for footing material when mixed with sand. It can be found in nugget or shredded style. Tests have shown rubber mulch is superior in breaking falls to traditional bark mulches. It is being used in playgrounds and even military training pits across the country to help prevent injuries due to falls.

Sour mulch

Mulch should normally smell like freshly cut wood, but sometimes will develop a toxicity that will cause it to smell like vinegar, ammonia, sulfur or silage. This happens if the material is not rotated often enough and it forms pockets where no air is circulating. When this occurs, the decomposition process become anaerobic and produces these toxic materials in small quantities. Once exposed to the air, the process quickly reverts back to an aerobic decomposition, but these toxic materials will be present for a period of time. If the mulch is placed around plants before the toxicity has had a chance to dissipate, then the plants could very likely be severely damaged or killed depending on their hardiness. Plants that are predominantly low to the ground or freshly planted are the most susceptible.

If sour mulch is applied and there is plant kill, the best thing to do is just water the mulch heavily. Water will help the chemicals to dissipate more quickly and refresh the plants. By the time plant kill is noticed, most of the toxicity will have already disappeared anyway, so removing the offending mulch will have little effect. While testing after plant kill will not likely turn up anything since the toxicity will have dissipated, a simple pH check may reveal a highly acid content, perhaps in the 1.8 to 3.6 range instead of the normal 6.0 to 7.2 range. Finally, placing a bit of the offending mulch around another plant to check for plant kill will verify if the toxicity has departed. If the new plant is also killed, then sour mulch is probably not the problem.