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FAQ:  Can I use potting soil for raised beds?

Using potting soil for raised beds is not so much a question of whether you can, but whether you should.  A product designed for a raised bed, added to a very large container, might work.  But the inverse is not advised.

Like many things in life, “one size fits all” may not be the best soil strategy for container-grown, raised bed, and open ground/row crop plantings. 

Soil has a number of functions.  It acts as a substrate to support plants.  Air, water, and microbes move through its pore spaces.  Nutrients are stored until needed by those plants.

But the manner in which that soil is contained (or not) may require some adjustments to ensure optimum plant health.

Roots need wiggle room

Think about your feet in a pair of tight-fitting shoes.  There is not much space for your toes.  Air doesn’t circulate very well.  Your feet may sweat a bit.

A plant in a container can experience similar problems.

For one thing, containers need to be portable.  That dictates a lighter soil than might be found in the garden or raised bed.

The confined space physically restricts root development, too.  So the lighter soil makes it easier for the plant to send out roots.

A typical container is watertight with a small drainage hole at the bottom.  Lighter soil helps move water quickly from top to bottom to prevent waterlogging. 

But this rapid drainage may also lead to less water retained – one reason to make sure your potting mix includes the moisture-holding properties of compost.

Raised beds are little different.  They are generally much larger than a container and do not have an impermeable surface between the bed and the native soil.

A raised bed is like a comfortable pair of boots – more room to wiggle toes, but still a confined space.  

Rain and irrigation water have the space to spread laterally before percolating down through the bed.  Roots are able to do the same.

But media mixes for planting beds often have wood chips or other materials added to improve drainage.  Because growing space is still confined in a raised bed, the soil needs to have a lighter density than the adjacent lawn or garden.

That garden soil can be much heavier because roots have unlimited space to stretch out to seek water and nutrients.  In the landscape, it needs to have the ability to support big shrubs and trees, too.

But do note that “heavier” is used here as a density comparison to the lighter potting and container soils.  A heavy garden soil is not desirable, either.  It requires amendment to lighten things up.  Otherwise, roots will not develop properly.

Assuming that garden soil is a good density, it might be likened to going barefoot – plenty of room to wiggle the toes.  Or, in this case, plenty of space to spread roots.

Make your own or buy pre-made

If mixing up your own container or raised bed media, know that compost can be substituted 1:1 for peat moss in your favorite soil recipes. 

And if using your garden soil as a base, sterilize in the microwave or under plastic before adding the compost.  Sterilizing compost will kill the beneficial microorganisms that make compost such an ideal soil amendment.

For readers along the U.S. East Coast, McGill does not make a potting mix, but we do offer a bulk landscape mix for raised beds.  Ask for it at your local landscape supply yard.

FAQ:  How does compost help soil?

Most of compost’s benefit comes from its organic matter content, i.e., decaying plants and animals.  This organic matter contains the carbon, nutrients, and beneficial microbes that make compost the perfect amendment for so many soil types.  How does compost help soil?

Compost is about 50-60% organic matter on a dry weight basis.  A bit more than half of that is organic carbon.

This organic matter acts like a sponge, helping soil hold water and reducing runoff.  It also increases pore space to facilitate movement of air and water laterally through the soil, which creates transportation routes for microbes.

Compost is a soil conditioner.  Texture, moisture retention, and a host of other factors contribute to a well-conditioned soil.  A deep layer of light, friable soil encourages root development.  These root systems provide additional pathways for water and beneficial organisms. 

Healthy soil also provides a welcoming environment for those beneficial microbes, as well as other “critters” like earthworms.

The presence of active microbial populations improves nutrient uptake and contributes to the degradation of pollutants, too.  The combination of improved uptake and fewer nutrients lost through runoff results in a reduced requirement for synthetic fertilizers. 

For many growers, compost is the only amendment needed for a beautiful, abundant garden.

But don’t get carried away

Compost is a soil amendment – not soil.  Too much organic matter can be as bad as not enough.

Your target is a soil with a 5% organic matter content.  The best way to determine soil organic matter (SOM) percentage is with a soil test.

Soil testing is easy, inexpensive, and always a good move.  If you never tested your soil or can’t remember the last time you pulled a sample, this is the year.  Every three years is recommended.

You can buy a gizmo for a few dollars online or at the local garden center.  Make sure it tests for SOM along with moisture, salts, etc.

For about the same money, you can also use your local Cooperative Extension Service.  If you are not familiar with this excellent resource, we found this site which allows you to search for your county office by ZIP Code.

FAQ: When is the best time to add compost?

Anytime is a good time to add compost.  Fall, spring, mid-season – every growing space can benefit from the boost of soil microbes and organic matter.

Is one timing option better than another?

A quick scan of gardening articles and blogs seems to indicate a slight lean toward fall.  Putting growing spaces to bed for the winter under a layer of compost and leaves gives soil microbes plenty of time to prep the ground for spring planting.

But incorporating compost a couple of weeks prior to seeding or transplanting at the start of the growing season works well, too.

No time to wait those extra 14 days?  Go ahead and add compost to the soil at planting time.  Just make sure that compost is fully mature.  (It should smell “earthy,” like soil from the forest floor.)  An immature product could compete with seedlings for nutrients or even burn young plants.  An unpleasant, ammonia odor is a telltale sign of immaturity.

If the compost at hand is still a bit too fresh, incorporate some air by turning with a shovel.  Dumping a bag onto a tarp or into a wheelbarrow will add air, as well.  Let it sit a couple of days, then check progress.  Keep “fluffing” the compost until it’s ready for use.

And don’t forget to add a bit of compost to container mixes, backfill, and other non-crop uses.  Follow manufacturer instructions, especially about amounts to use.  Depending on the feedstocks that make up the blend, some compost products may be richer than others.  Here’s the link to McGill’s use recommendations.

Most plants will welcome a little mid-season pick-me-up, too.  Simply sprinkle a little compost on top of a container’s soil layer, use as a side-dressing for row crops, or add a dusting over lawns.  Water in or lightly rake.

True sustainability requires a system, not marketing-speak

Sticking a bird’s head on a spider does not transform that organism into a creature capable of flight.  Adding energy generation to incinerators and landfills doesn’t make them sustainable systems for organic waste management, either. 

“Sustainable” is one of those words that has been co-opted by Madison Avenue, slapped on everything from dog food to baby toys, and flung about willy-nilly like insults on nighttime reality TV.

It seems every product, process, and entity with even the smallest claim to the word uses it, because “sustainable” has finally caught the attention of the general public.

But the term, when applied to waste management choices, may be just as misleading as the words “natural” and “organic” on supermarket shelves.  What’s behind the label can still be the environmental equivalent of junk food. 

Admittedly,  people have become so adept at generating waste that the world has a never-ending supply have the stuff.  Ergo, any disposal or recycling technology could legitimately claim its feedstocks are sustainably sourced – even landfills without methane capture and plain, old incinerators.  

But that doesn’t make the total system sustainable or economically prudent or environmentally sound.

If pears are grown in compost in South America, shipped to Asia for processing, and transported back across an ocean to the U.S. for distribution and consumption, are those pears a sustainable choice?  

Using compost is better than not using compost.  But, c’mon, folks.  Did that pear earn the right to call itself sustainable?

Of course not.  Neither do disposal options that burn or bury compostables … even if they do result in energy generation.

Currently, only technologies that recycle or divert organics for use as a soil amendment (in farming, landscaping, turfgrass management, etc.) can claim true sustainability.  They close a loop, and when properly managed, do no environmental harm in the process.  

It remains to be seen whether some of the emerging re-uses for organic waste like building highways and formulating cleaning products will help or hurt the effort to recycle biodegradables back to the soil. 

Making new products from waste can be a swell idea.  But if those products can’t find their way to recycling at end-of-life, if the reclamation process renders them too toxic or otherwise inappropriate for composting, or if that reclamation generates a waste stream that cannot be efficiently returned to the soil, these types of reuse projects will likely – albeit indirectly – contribute to further soil depletion, more polluted runoff, increasing stormwater problems, and atmospheric carbon overload.

When government decision-makers are asked to evaluate new systems for organic waste management, marketing-speak has no place in a serious discussion.  One or two sustainable components does not make a sustainable system.

True sustainability cannot be conferred by feedstock source alone.   For organics, returning nutrients, organic matter, carbon, and beneficial microbes to the soil in an efficient, cost-effective manner makes composting and compost use a true sustainability choice – no marketing-speak required.

What’s the difference between compost and peat moss?

Compost is manufactured from recycled materials derived from plants and animals.  Peat moss forms naturally over many, many years – also from decaying plants and animals.  Both are rich in organic matter.  But it takes so many years for nature to form peat moss that the product is not considered “sustainable.”  Peat also tends to be too expensive to be used in large projects.  Fortunately, compost can be substituted 1:1 for peat in any media mix or soil recipe.  

Lab test lingo:  How much is 1 PPM?

Test results — compost analytical reports included — often convey constituent concentrations in parts per million (ppm) or milligrams per liter (mg/L).  Both state the fraction of the tested substance found per one million units of gas, liquid, or solid.

But what does that really mean?  Is 1 PPM a drop in the bucket or a thimble of water in an ocean?

Such infinitesimal amounts can be difficult to visualize, but here are a few examples found on the web that may help:

PPM

PPB

Sometimes, even smaller concentrations may be reported as parts per billion or micrograms per liter (μg/L).  When you see this term, correlate to:

Know the limits

One of the best analogies is 1 ppm equals one large mouthful in a lifetime of eating.  But it must be said:  just a small bite of the wrong thing can be one bite too many.

That’s why it’s important to always correlate reported concentrations  with the limits deemed safe by regulators and other jurisdictional entities.  Typically, for easy comparison, these ceilings will be reported in an adjacent column on the lab report.

FAQ: Is fall a good time to use compost?

Most definitely, yes.  In fact, some believe the fall season is the best time to add compost to lawns and gardens.  For grassy areas, sprinkle a little over the surface and rake in.  For planting beds, add compost and work into the top layer of soil.  Alternatively, just leave the compost to sit on the surface of the planting bed and allow Mother Nature to work her magic over the winter months.  Cover the surface with leaves or other mulch to help retain moisture.  When spring planting season rolls around, the soil will be ready for you.  Compost products will vary, so always follow the manufacturer’s recommendations about exact amounts to use for specific applications.  You can find McGill’s recommendations here.

FAQ: How does compost protect drinking water?

Primary sources of drinking water include wells, lakes, reservoirs, and rivers.  Compost will protect drinking water sources by breaking down pollutants and reducing erosion/siltation in runoff.  Microbial activity and absorption of rainfall energy are among the mechanisms at work.

Soil microbes break down many chemicals — like petroleum products – during feeding activity, severing molecular bonds and reducing complex compounds into simpler, more benign forms.  In fact, compost is used to remediate petroleum contaminated soils at airbases, underground storage tank removal sites, highway accidents, and similar clean-up projects.

Compost’s organic matter content cushions rain or irrigation water.  When water hits the ground, that energy is disbursed, and fewer particles are dislodged.  That same organic matter also absorbs more water, resulting in less runoff.

In addition, the use of compost reduces the need for chemical input on farms, turfgrass, and in the landscape, which also helps to protect drinking water sources.

FAQ: How do I sterilize soil?

When making your own potting soil from native soil or trucked in topsoil, it’s a good idea to sterilize that dirt to kill things like weed seeds and diseases before mixing with compost and other ingredients.  Large swaths of ground can be treated in-situ (in place) using plastic and the sun, but it takes time.  Fortunately,  small batches can also be treated using kitchen appliances.  Here’s a how-to article.  

What is composting and how does it work? 

Composting is the managed degradation of plant and animal matter under aerobic (with air) conditions.  The process mimics natural decay in a controlled environment to speed up the breakdown of these organics. Composting results in a safe and easy-to-use soil amendment — compost.

Insects and bacteria are examples of the types of creatures that feed on discards like food waste and leaves during composting.  The larger animals tend to use mechanical methods, while the microscopic rely on chemicals to degrade these materials.

This feeding activity reduces complex compounds into simple molecules that are benign and odor free. Compost is used to build and replenish soils, closing the recycling loop for organic matter.  

The only byproducts of composting are CO2 and water;  the process produces no waste requiring disposal.  The CO2 is considered “carbon neutral” since its release during composting is the same as if decomposed by nature.

Most municipal, commercial, and non-profit composting facilities rely on microbes to do the bulk of the organic decomposition.  There are mancomposting methods in use, although outdoor windrows are among the most common.  Earthworms are the primary agents of decomposition in the controlled process known as vermicomposting. 

However, some other processes that have the word “composting” attached to their name in the vernacular may not be true composting processes.

Bokashi composting, for example, is an anaerobic (without air) fermentation process. Anaerobic composting is another misnomer.  Because neither is aerobic, neither is true composting.   Both can biodegrade organics, however.  Unfortunately, anaerobic decomposition may generate unpleasant odors since anaerobes produce mercaptan during biodegradation. (Mercaptan is added to odorless natural gas to give the gas its distinctive rotten egg smell.)   

Composting digestate, the by-product of energy extraction using anaerobic digestion, increases both the market value and uses for this waste material if managed for quality compost production.  

While neglected composting piles have been known to “go anaerobic,” too, a well-managed composting process — one that keeps the piles aerated — will not generate unpleasant odors.  Any odors present in the incoming feedstocks will be quickly neutralized, too.