Last Updated on June 20, 2021 by Grow with Bovees
Using compost generously is the single most important thing you can do for the overall health and prosperity of your landscape.
The simple addition of compost to your garden soil, around trees and shrubs, or on your lawn will pay huge dividends in the form of healthy, disease-free plants.
And there is also the satisfaction of recycling organic material, the feeling of well-being that comes from physical effort, and the delight found in being outdoors, digging in the soil.
The benefits of compost are many, and composting also has many environmental benefits.
When you mix compost into your soil, the return on your investment from your yard and garden is so valuable that many gardeners refer to compost as “black gold.”
It revitalizes older plants and invigorates new, young plants. Because there are so many ways to use it, invariably, there is never enough.
Limited amounts of compost usually force you to set priorities for its use in your yard. Decide in advance whether it’s more important to use your supply on a new garden bed where the soil has never been improved, mulch new vegetable seedlings, or top-dress part of the lawn that is struggling.
Or you can consider several ways to stretch your compost supply by mixing it with materials acquired from commercial or municipal sources.
Compost Benefits Soil
To make the most of your supply of compost, it’s helpful to understand all of its benefits. Compost is essentially a soil conditioner. Adding compost to yard and garden soil changes it fundamentally: Compost alters its texture and infuses it with life to better support plants of all kinds; and compost helps to restore compacted, sterile soil, so it comes to closely resemble the rich natural soil found on the forest floor.
Compost Improves Soil Texture
Because it’s light and fibrous, compost aerates any soil that it’s added to. Whether you mix it into the soil yourself or let earthworms pull it down from the surface, the net result is more air spaces around the soil particles, if you are growing herbs, they will appreciate compost added to the herb potting mix.
It’s the nature of these soil particles— especially their size—that determines the texture of your soil.
If your soil is clay, the particles are so fine that they pack together tightly. There is little space for air around them, and the soil feels dense and heavy. The particles stick together when they are moist, and the soil feels gummy.
If your soil is sandy, the particles are coarse, larger, and lighter. There is excessive space around them, and water tends to drain through them very quickly.
The amount of space around soil particles affects how well plant roots—especially fine, newly formed plant roots—can grow and move through the soil in search of essential air, nutrients, and moisture.
It also affects how much air, nutrients, and moisture the soil can make available to these plant roots. Plants grow well in sandy soil, but the soil doesn’t hold water and nutrients well.
Plants struggle in clay soil, but the soil holds nutrients and moisture better.
Of course, the ideal soil texture is something in between these two extremes—loam.
Loamy soils have ideal size particles. They are coarse enough to allow space to store air and water, yet they are also fine enough to prevent moisture from draining away. Loamy soils have humus in them, which helps them hold moisture, yet drain well.
Not everyone is blessed with loamy soil.
Compost improves soil by increasing its capacity for holding moisture and draining well. Of course, fresh organic material will decompose eventually in the soil and provide humus, too, but it inevitably depletes the soil of some of its nitrogen as it completes the decomposition process.
With the addition of compost, inferior clay or sandy soils become friable, or lighter and more crumbly. As they are better able to retain air and moisture, their tilth—a combination of soil’s texture and its ability to retain moisture—improves.
Making Compost Reduces Landfill
If you turn all your garden waste into compost, you will be doing a favor for the environment by reducing the amount of rubbish that is placed into landfill. You can also include a fair amount of food scraps and kitchen wast that you can collect in a counter top compost pail to add to you compost pile, thereby reducing landfill at the same time.
Compost Increases Microbial Activity
At every stage of its production, compost is home to a host of organisms, both large and small, that are responsible for decomposing the organic raw materials.
When you integrate compost into the soil, it still harbors lots of these living creatures. They make the difference between live, fertile soil and essentially dead, sterile soil.
And they can’t live if they don’t have humus to sustain them.
Breaking down chemical compounds bound in the soil particles into nutrients and then converting those nutrients into a form that plant roots can take up is the main contribution of microorganisms.
Some are bacteria that convert nitrogen from the air and “fix” it into the soil so it’s available to plants. Still, other bacteria—more plentiful in compost from simple piles—are organisms that manufacture antibiotics, which attack disease pathogens in the soil.
Still, other bacteria are expert at releasing minerals from rock particles in the soil. There are also beneficial insects that prey on pest insects and their eggs in the soil.
Microorganisms also play an important role in providing food and air to plants. Plants can use nutrients only when they are in a liquid or gaseous state.
The bacteria, fungi, and other microbes in compost reinforce the microbial population already residing in the soil’s existing humus. The conversion of nitrogen, phosphorus, potassium, calcium, boron, and many trace minerals and nutrients into accessible gases and liquids is thus accelerated by microorganisms introduced with compost.
Compost Provides Some Nutrition
Although compost enriches the soil, it is not a fertilizer.
Compost breaks down over time into the basic nutrients used by plants to make food, but not in sufficient quantity and variety to make it a reliable substitute for a general, balanced fertilizer.
The nutrient content of compost inevitably changes from batch to batch because the raw materials (ultimately the C/N ratio) from which the pile is built vary.
The number of times the pile is turned, the internal heat it achieves, and the speed with which the compost is used are all factors that affect the presence and makeup of nutrients in a given batch of compost.
Also, the nutrients compost provides are only indirectly available to plants. Like organic fertilizers, compost depends on organisms in the soil to process its nutrients into liquid or gas form. Compost therefore contributes different amounts of nutrients depending on the organisms found in different soils around your property.
When compost is freshly harvested and has been protected from rain so its nutrients have not leached from the pile, it does contain some valuable soil additives.
Fresh compost contributes a measure of nitrogen, phosphorus, potassium, and many trace minerals to any soil. Typically, about half of the nutrients are released for plant use during the first year after compost is incorporated into the soil. Home made compost will also benefit the structure of store bought bagged compost products after the first year or so of use.
Half of the remaining nutrients are released during the second year, and so on.
If compost has been exposed to the elements or stored for the winter, it retains some of the micronutrients even though major nutrients have been released in the form of gases. So, while it doesn’t substitute for fertilizer, any compost will improve the overall fertility in soil and remedy a range of mineral deficiencies.
Compost Adjusts Soil Chemistry
Different groups of plants require certain levels of soil acidity or alkalinity to enable their roots to access nutrients in the soil. The levels of acidity and alkalinity are measured in terms of pH, which is expressed as a number on a scale of 1.0 to 9.0, with 7.0 being neutral. If soil tests below 7.0, it is more acid; above that, it is more alkaline.
Acid-loving plants such as rhododendrons, azaleas, and hollies prefer soil to have a pH around 4.5 to 6.5. In contrast, rose of Sharon, butterfly bush, and sedums will accept more alkaline soil that tests closer to 7.5 to 8.0.
However, most plants commonly found in residential landscapes do fine in an essentially neutral environment, their soil registering 6.0 to 7.2 or so.
Because the assortment of raw organic materials that end up in typical backyard compost piles usually decomposes into neutral humus, compost does not directly influence soil pH levels.
However, it has been recognized by generations of observant gardeners, and more recently by scientists, that compost definitely influences soil pH indirectly in many circumstances.
How it does this is not yet fully understood, but researchers have determined that if growing in soil with a high percentage of compost in it (2% to 5%), plants develop the capacity to modify the soil chemistry to suit their particular pH needs in the area where their roots are growing.
This is not likely to occur with young, unestablished plants or with annual flowers and vegetables that are in the soil for only a few months. In soil laced with compost over several years, however, some trees and shrubs have exhibited the ability to modify pH levels.
Compost Buffers Extremes of Soil Temperature
Soil temperature is critical to the health and timely growth of plants.
Adding rich, dark compost to your soil is one way to alter any soil’s response to the extremes of temperature, and to protect your plants or germinating seeds.
If the soil temperature exceeds 85 °F (29.44 °C), for example, root growth ceases until the temperature drops.
Used as a mulch in the heat of summer, compost insulates the soil against the sun and allows soil temperatures to stay 6° to 15 °F cooler, enabling plants to continue to grow and produce.
If the ground is too cold, microbial life is stressed and seeds won’t germinate. (In feet, soil temperature is even more critical to the germination of seeds than is ambient air temperature.)
For example, no matter when they are planted in late winter, peas will germinate only when the soil warms to 45 °F (7.22 °C). And the root systems of most plants don’t grow until the soil warms to at least 65 °F (18.33 °C) or more.
Mixed into the top layers where plants grow, compost darkens the soil and absorbs heat, which stimulates plants to start growing earlier in the season.
Compost Benefits Plants
Whether you’re growing annuals, bulbs, perennials, shrubs, trees, or vegetables, they will grow more vigorously and produce fruit, cones, seeds, and flowers more abundantly when they are in healthy, compost-laden soil.
The single most common cause of plant problems is stress.
Plants that are improperly planted, don’t have the right light exposure, or are in poor soil are vulnerable because their systems are strained by coping with these difficulties—their natural defenses are thus weakened.
However, if the soil in which they are growing is improved by compost, the stress is reduced and their vigor improves. They are better able to adjust to problems, and their innate resistance to insect and disease attacks is also improved.
Compost Reduces Insect Pests
In addition to reducing plant stress in general, soil containing compost harbors many beneficial organisms that prey on pest insect eggs and larvae that would otherwise plague plants. Spiders and ants are particularly fierce predators of pest insect eggs.
Life in healthy soil maintains a desirable balance among all populations of insects—both predator and prey—so the basic biological diversity is maintained.
Compost also adds fatty acids to the soil; these are effective at controlling certain pests, such as root-knot nematodes.
Compost Fights Plant Diseases
Any ordinary soil contains viruses, bacteria, and fungal spores. They are part of its life.
Some of these organisms are part of its decomposition team, while others threaten plant health. The compost you add to the soil contains plenty of microorganisms that combat other microorganisms threatening to plant health.
Some of these beneficial organisms in compost also suppress fungal diseases, such as certain rots and damping off, which invade plant roots.
These disease-fighting organisms are more abundant in the lower-temperature compost produced in a simple pile.
If you have a managed pile, take compost from the outer edges, which doesn’t get as hot, for use in disease control.
When used as a mulch, compost also helps control disease in plants. The spores of fungal diseases like powdery mildew are often spread to plants by raindrops that splash up from the soil onto the leaves.
A layer of soft, spongy compost under plants absorbs the raindrops, thereby virtually eliminating the spread of fungal diseases.
Compost Discourages Weeds
Finally, compost has a role in discouraging weeds. Like any organic material, it makes an excellent mulch, even when it’s not completely broken down. Spread in a 3″ or 4″ layer on the soil over plant roots and between plants, it covers weed seeds that need sunlight to germinate.
Compost made by the managed method—where extremely high temperatures kill any weed seeds in the organic raw materials from the pile—is best for this purpose.