Compost & Soil
Compost is not only a soil amendment, it is the soil amendment. Soil with a serious pH imbalance should be treated with lime or sulfur or some other pH-specific amendment, but for most garden soil problems the amendment of choice is always compost. Assuming that you’ve got average soil with average problems and you can only add one thing. Compost would be the thing to choose. Other amendments will solve particular problems more quickly or completely, but compost is the best all-around soil conditioner available.
To stretch the conditioning metaphor to the breaking point, one could think of most soil amendments as an exercise for one body part — the biceps, or the hamstrings. But biceps curls are not a conditioning program. No one would do biceps curls and only biceps curls and expect the result to be a healthy mind in a healthy body.
Cross-country skiing, on the other hand, works just about everything — arms, legs, glutes, abs — all of it. If you’ve got a serious weakness or injury in one part of the body, you may need to work or cure it before setting out, but as a whole-body exercise that will benefit muscles, cardio-vascular health, and so on, nothing beats it.
If you want the whole works, you need to work the whole. This is what compost does for soil. It improves soil physically, biologically and chemically. (These roles actually overlap, but it’s helpful to consider them separately.)
Compost has the unique ability to affect all these factors positively. In increasing organic material, including humus, it raises the CEC index making nutrients less likely to leach away and stabilizing soil pH (chemical affects). It also adds micro-organisms which perform complex functions (biological affects) and it improves soil structure, reducing drainage problems in both clay and sandy soils (physical affects).
Physical Effects: Soil Structure and Nutrients
Improves Soil Structure
Compost performs the seemingly contradictory functions of improving drainage in clay soils and water retention in sandy soils because in both cases what it really improves is soil structure.
Good soil structure — what used to be called “good tilth” — is the basis for any good garden or farm. In good soil, different molecules tend to glom onto each other, forming what are called aggregates: small, irregularly shaped particles or clumps. This clumping of material opens up spaces or channels between the aggregates, space which allows air to circulate and water to drain.
These channels also provide easy paths for plant roots to follow. Plants in loose, friable soil develop deeper and more complex root systems than do those in heavy soils. Since some nutrients such as phosphorus tend to stay where they are, roots must come to them. A large root system means that the plant can access more of these key nutrients which might otherwise remain out of reach.
Soils heavy in either clay or sand have poor structure for two reasons: they tend to be low in organic matter and their mineral particles have a fairly consistent size. Good soil, by contrast, is diverse in both content and in the size of its constituent particles. The mineral particles should have a range of sizes and the earth should be a complex mix of many ingredients including rock particles, organic material, and a dense population of microbes, insects and invertebrates.
Soil with good structure will hold water better than sandy soils do and drain better than clay soils do. Neither clay nor sand absorbs water well; water simply floods between the large particles in sand, and it can be trapped by the tiny particles in clay. The organic material in soil absorbs water far more willingly than sand but releases it more readily than clay.
These physical effects have important ramifications. Compost in sandy soil ensures that the soil holds water long enough to dissolve nutrients — an essential role since plants can only use nutrients when they are dissolved. In clay, compost allows dissolved nutrients to circulate, making them more available for root uptake.
Soil structure is important for soil biology as well. In a wonderful paper on this topic, Drs. Jill Clapperton and Megan Ryan point out that soil with good structure “makes a better soil habitat that attracts more soil animals.” These animals, however tiny, play their own essential roles discussed below under Biological Effects: Beneficial Microbes and Others.
Organic material is one of the keys to good structure — and good structure is the key to everything.
Adds (a few) Nutrients
In spite of the insistence that compost is not an organic fertilizer, it is incumbent upon any responsible writer to admit, however reluctantly, that compost does contain some nutrients.
A finished compost may contain small amounts of the primary nutrients, usually 1-2% nitrogen, 0.6.-0.9% phosphorus, and 0.2-0.5% potassium. It may also contain low levels of secondary nutrients: (calcium (Ca), magnesium (Mg) and sulfur (S).
More importantly, it provides vital micronutrients such as iron, manganese, copper and zinc which are essential to plant health in minute quantities and which are often missing from synthetic fertilizers and are overlooked by gardeners.
The percentage of nutrients in compost is higher than that in the feedstocks it’s made from. This is because compost loses bulk as it decays. But its nutrient content remains constant. It’s as though the compost heap shrinks around an unchanging core of nutrients. This is true of heaps made in composters with lids. However, a compost pile that’s open to the weather will lose some of the more soluble nutrients to leaching. Nitrogen is especially vulnerable.
Despite this concentrating factor, and the fact that compost contributes important levels of micronutrients, compost does not contain high enough levels of the primary nutrients to qualify as a fertilizer.
Biological Effects: Beneficial Microbes and Others
Adds Beneficial Microbes
Think of composting as the act of growing microorganisms. – Compost Fundamentals, Washington State University.
Beneficial microbes is a term that turns up again and again in compost literature. It is not a buzz-word. The fact that compost contains a host of living creatures and nurtures others sets it apart from most other soil amendments. The tiniest of these creatures, the microscopic ones, play perhaps the most important roles.
Consider absorbency, discussed above in the section on soil structure. Compost shares this property with other organic materials such as peat moss and coconut coir, but compost also improves soil structure in ways that they cannot. For instance, the micro-organisms in compost produce mucus which helps bring soil particles together, aiding in aggregation. Also, the long hyphae of some fungi and actinomycetes that flourish in organic soils also help in soil aggregation. Since peat moss and coconut coir are essentially sterile materials, they do not provide this kind of boost to the soil.
Micro-organisms improve soil-structure because they help soil to aggregate. But they also play a number of other roles. Some help to reduce plant diseases while others establish the mycorrhizal fungi that allow plant roots to access nutrients far below the reach of their roots.
Mycorrhizal fungi make up 80-90% of plant systems, forming a close symbiotic relationship with roots. There are two main types, external ectomycorrhizae, which form a dense web around tree roots, and internal endomycorrhizae, which actually penetrate the roots of most other plants. All extend or enhance the plant’s ability to reach distant nutrients. In return, plants provide the mycorrhizae with carbohydrates for energy. It’s a classic symbiotic system, benefiting both partners.
One of the most important mycorrhizae for our purposes are arbuscular mycorrhizal fungi (AM), which infect (that’s the technical term) the roots of many plants, forming long, slender, branching systems of threads stretching from plant roots into the soil below. These systems, which resemble roots, function like a second set of roots, transporting nutrients from several meters below the plant not just to the root zone but directly to the roots themselves. Consisting frequently of one-cell-wide threads, AM fungi can penetrate much smaller spaces than can even the smallest root threads, accessing nutrients that roots cannot.
This is particularly important in allowing plants to reach phosphorus and other immobile nutrients which are available only in the extremely small volume of soil immediately surrounding the roots — or in the fungi that extend those roots.
Mycorrhizae are not one of the most important fungi populating compost in the bin. But adding compost helps boost their populations in the soil.
Still other microbes play a major role in converting nutrients to forms available to plants. Nitrogen, one of the three primary nutrients, abounds in earth and air in numerous forms. Pure nitrogen gas (N2) makes up over 70% or our atmosphere, but most plants can only make use of it as ammonia (NH4) or nitrate (NO3).
It is soil microbes that do the essential work of converting other forms of nitrogen into these usable ones. A paper by the Mississippi State University Extension Service, “Nitrogen Fertility,” helps to clarify the importance of organic material (such as compost) in this conversion:
Mineralization is the process of converting organic N to plant available inorganic forms. It is a gradual breaking down of large molecules to smaller molecules by a succession of soil microorganisms. … Energy for this process is obtained from carbon in the material being used, so introduction of fresh plant materials stimulates breakdown activity.
Notice the last clause in that quotation: “Introduction of fresh plant materials stimulates breakdown activity.” Compost, in other words, helps fuel the microbes that convert nitrogen into the soluble forms plants can use.
Reduces Plant Diseases
Soil bacteria and fungi nourished by compost can also help reduce incidence of a wide range of plant diseases. While quite obviously good in itself, this is also important because it can in turn reduce the need for various fungicides and other chemicals, many of which are toxic to greater or lesser extents to humans, animals and the soil biota themselves.
The word fungus doesn’t have positive connotations in our culture. We tend to forget that penicillin is a mold. And most of us probably never knew that many antibiotics were originally cultured from soil or compost. Chloramphenicol and tetracycline, two of the most successful wide-spectrum antibiotics, were both derived from soil cultures, as was the precursor of erythromycin.
|Penicillin, a recent book on the first antibiotic by Robert Bud, Head of Information and Research at London’s Science Museum, paints a picture of medical and pharmaceutical investigators in the 1940s and 50s madly collecting soil samples from around the world and testing them for anti-microbial agents.
Crazed as this search may have been, it was not crazy. Chloramphenicol, a wide-spectrum antibiotic still used today in many developing countries, was a true world traveler, being derived from a compost in Venezuela kept by an expatriate Basque until it was sent off to New Haven for analysis in 1944. The first tetracycline, named chlortetracycline, made a much shorter journey: from a hay field in Missouri to Lederle Laboratories on the east coast in 1946. But erythromycin, derived from a soil sample collected in the Philippines, traveled half-way around the world.
As Bud tells it, this exhaustive if exhausting search was inspired by the success of Selman Waksman of Rutgers University, who is often credited with coining the term “antibiotic.” In looking to soil for microbes that would cure diseases, Waksman had been following in the footsteps of pioneering microbiologist Rene Dubos. Dubos’ most telling discoveries had been too toxic for human ingestion. In the early 1940s, Waksman’s lab had discovered a whole range of antibiotics — the actinomycetes — all produced by a particular class of soil biota. Perhaps the most important of these was streptomycin, the first effective treatment for tuberculosis, discovered by Waksman’s student Albert Schatz.
Perhaps these reminders will make it easier for us to believe it when we read that soil biota produce antibiotics which fight soil-borne diseases. Compost has been used to fight avocado root-rot both in California and in Brazil, while its potential in fighting tomato diseases has been widely reported.
But production of antibiotics is only one way that soil microbes fight disease. A major British review (PDF) of the literature identified four different mechanisms by which micro-organisms are currently believed to suppress soil-borne diseases. Beneficial micro-organisms can:
1. Produce antibiotics;
2. Out-compete other organisms for nutrients;
3. Activate disease-resistant genes in plants;
4. Parasitize pathogens.
One branch of research has focused on a class of soil fungi called Trichoderma. These fungi occur naturally in most soils, but they are also easily cultured, which means that they can be used to inoculate composts, mulches and other media. Particular species of Trichoderma can be especially effective in fighting certain diseases. Spreading material inoculated with Trichoderma ensures that it will be present in the necessary numbers. This is only one branch of many on-going investigations.
Nourishes Larger Soil Organisms
There’s a long list of insects, worms and other creatures that benefit from compost that in turn improve soil structure. These creatures all help to aerate soil as they move through it and are part of the complex soil web that makes plant life possible. When they die, they also contribute their bodies to the organic matter in the soil.
Giant Gippsland Earthworm
At the top of this list reside earthworms which themselves produce the best of composts. Most of us are familiar only with the common earthworm, or nightcrawler, but dozens of other species exist, including the Giant Gippsland Earthworm which reaches lengths of 2-3 meters. Australian farmers claim they can hear these worms moving through the earth.
Chemical Effects: pH and CEC
Balances and Buffers pH
If compost’s first miracle is to improve both clay and sandy soils, its second must be its ability to balance pH, to make alkaline soils more acidic and acidic ones more alkaline. This can seem especially mind-boggling because, unlike most pH treatments, compost’s pH is near neutral. Its ability to balance pH results directly from the fact that it boosts cation exchange capacity (CEC) of soil. But first, a quick review of pH itself.
On the pH scale that runs from 1 (highly acidic) to 14 (highly alkaline), most plants can grow at anything near neutral (7), though many have their own preferences for lower or higher pH. Blueberries, strawberries, potatoes, rhododendrons and azaleas, for example, prefer acidic soils. Brassicas (cauliflower, broccoli, Brussels sprouts, kale, turnips and cabbages) prefer slightly alkaline soils.
Soil pH affects whether nutrients present in the soil can actually be taken up by plant roots. Nitrogen is most available at a neutral pH because the microbes that convert nitrogen into the usable forms of ammonia and nitrate operate best at near-neutral pH levels.
Phosphorus, designated by the P in the NPK formulas that appear on most fertilizers, is another of the three primary nutrients necessary to all plants. It is most easily available at pH values between 6 and 7. In alkaline soils (pH values above 7), it bonds with iron and aluminum. In acidic soils (pH values below 6) it bonds with calcium, forming chemicals which are insoluble and therefore unavailable to plants.
Other nutrients are also affected by soil pH. Plants require seven micronutrients, elements essential to plant health but needed only in minute quantities. Five of these — manganese, iron, copper, and zinc and boron — become less available in alkaline (higher pH) soils. Molybdenum, on the other hand, becomes less available in acidic soils.
Though we speak freely of soil pH, what’s actually being measured by a soil test kit is the pH of the soil solution because pH only has meaning in relation to water and to minerals in contact with it. Technically, pH measures the level of free hydrogen ions (positively charged ions) in a water solution. The more hydrogen ions, the more acidic the solution and the lower the pH.
Most pH treatments are themselves either quite alkaline or acidic. They will only shift the pH in one direction. Loosely speaking, an acidic amendment provides free hydrogen ions, while an alkaline one absorbs them. More precisely, they interact with other soil chemicals in ways that either release or attach hydrogen ions. These soil amendments work more quickly than compost can. If a quick or drastic shift in pH is called for, these are the way to go.
Balances Soil pH
Compost, by contrast, has a nearly neutral pH. The composting process itself produces various acids. But by the time it has cured, its pH should be around 6.5. Most soil amendments designed to adjust soil pH have very simple molecules. However, compost consists of large, complex, and diverse compounds that provide both negatively-charged attachment points and numerous hydrogen atoms. Which of these comes into play depends on the pH of the soil in which the compost is placed.
Acidic soil suffers from an overabundance of positively-charged hydrogen ions. When compost is added, its many negatively charged attachment sites attract and bind the hydrogen. When enough hydrogen ions are taken out of solution, the pH level of the soil rises.
In alkaline soil, compost’s complex, hydrogen-rich molecules provide a source for hydrogen ions. Many get stripped away, leaving their electrons behind them — which means that they have become positively charged ions. When enough ions are released into the soil solution, the pH falls. The negatively charged sites on the compost molecules (the ones that used to be occupied by hydrogen atoms) are now available to bind other positively charged particles which includes various soil nutrients.
Buffers Soil pH
The statement that compost buffers pH means that it protects soil pH from sudden changes. The extra attachment sites it provides as it raises CEC will help absorb any chemical addition — including that of amendments meant to change pH. Larger quantities of such amendments must therefore be used in compost-rich soil.
In this case, as in all the others above, it is compost’s chemical complexity that makes it able to do what it does. That complexity creates the many attachment sites in compost and assures that it contains plenty of hydrogen atoms. Because of these particular chemical attributes, compost can buffer soils against sudden changes in pH, and it can render both acidic and alkaline soils more nearly neutral.
Boosts Cation Exchange Capacity (CEC)
The cation exchange capacity or CEC of soil is, roughly put, a measure of how well a soil retains nutrients and therefore how available its nutrients are to plants. A low CEC indicates that the soil has a low capacity for retaining nutrients, meaning that applied fertilizers quickly leach away. A high CEC (over 50) indicates a greater capacity to retain nutrients, meaning that fertilizers can remain viable for long periods.
Low here means 5 milliequivalents per 100 grams (meq/100g), which is what’s found in some sandy soils. Pure organic matter may have a CEC of 150. A soil with a lot of organic matter in it may measure 50 to 100. CEC for loams range from 10 to 25, while clay loams may have a CEC as high as 40. Humus has a CEC of 200.
More technically, the CEC measures the capacity of a soil to allow the movement (or exchange) of positively charged ions (cations) between soil particles and the soil solution, the water around the particles; water that should be rich in dissolved minerals.
It may seem counter-intuitive that facilitating cation exchange (including that of nutrients) would correlate with a soil’s ability to hold onto or bind nutrients. How can enhancing mobility lead to greater stability? In order to understand how these things can be true, it helps to understand some of the basic chemistry involved.
Many plant nutrients (potassium, calcium, magnesium and ammonium, a plant-available form of nitrogen) exist in soil as cations (pronounced cat-ions), which are positively charged ions — molecules that have lost one or more of their negatively charged electrons leaving the molecule with a net positive charge. Clays and organic matter, especially humus, tend to accumulate extra electrons, forming negatively charged particles called anions. These large anions attract and hold cations, preventing them from washing away with irrigation or rain water.
The amount of clay and organic material in a soil gives a rough indication of its ability to hold positively charged nutrients. This is why clays, in general, are more fertile than sandy soils: they are able to retain nutrients better than do sandy soils, though their CECs do vary. (There are exceptions. A few clays, such as the kaolinite common in the Carolinas, have very low CECs.)
However, to speak of clays and humus as binding nutrients gives a false picture to the extent that it implies immobility. Atoms, molecules and electrons are actually in constant motion, attaching to one site, being stripped away as a more attractive site becomes available, reattaching elsewhere, and so on. The “exchange” in the term Cation Exchange Capacity acknowledges this mobility. The negatively charged attachment sites on humus and clays don’t simply latch onto a passing cation and hang on forever. Indeed, they might be better termed exchange sites, as they are the points at which an unending chain of cations attach and detach.
In a soil with a high CEC, a particular attachment site on a particular particle of clay or humus may entertain many tenants — millions in one second. But this seeming instability is deceptive, for the presence of many possible attachment sites in a small area keeps cations in the general region which means that they remain available to plant roots. The individual molecules are dashing about, bumping, binding, dis-associating and re-attaching countless times in a second. But their relative concentration in the area remains constant.
In a soil with low CEC, the cations have little or nothing to bind to so they tend to give in to the pull of gravity and leach away with water. That pull is always present, but it has less effect on a molecule in a crowded environment full of attractive sites. The cation in high CEC soil bounces back and forth and up and down, attaching and reattaching to negative sites on clay or organic matter. It moves in all directions. With low CEC the motion is primarily vertical and in only one direction: down.
No matter how much fertilizer you dump on low CEC soil, you will get only a brief nutrient boost after which many of the nutrients will simply wash out of the soil leaving your plants bereft and your local waters polluted. Fertilizer in high CEC soil, however, will hang around longer, providing greater benefit to plants.
Cation Exchange Capacity says nothing about the nutrient content of a soil. It only indicates how well the soil can retain the nutrients that are present. Nor does CEC alone indicate whether or not nutrients in the soil are actually available to plants. At least two factors discussed in this section, pH and micro-organisms, also play a major role in making nutrients available — or not.
These factors — physical, biological and chemical — interact in complex ways. But compost has the unique ability to affect all positively. In increasing organic material, including humus, it raises the CEC index thus stabilizing pH. It also adds micro-organisms and improves drainage problems in both clay and sandy soils. Compost provides the optimal environment for improving a soil’s ability to retain nutrients and therefore to make them available to plants.