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.

Considering the pros and cons of waste management technologies?

Evaluations of waste management technologies can be riddled with inaccurate, incomplete, and outdated information – and the full dollar value of compost use is rarely included.

Elected officials making decisions on behalf of taxpayers may be experts in their respective fields.  But most lack knowledge in many areas specific to municipal governance, especially waste management technologies.  Consequently, staff and consultants are often asked to do some research and provide a report of findings, including recommendations.

A couple of articles released last month aimed a spotlight on inherent weaknesses in a process that relies on published research and interpretive reports for decision-making.

One piece focused on a study out of North Carolina State University.  It concluded the best use for compost was as landfill cover.  The other, from the University of Washington’s Dr. Sally Brown, said those research assumptions were off.  Many benefits of compost use weren’t considered.

No matter which viewpoint seems right to those who read the articles, the fact that there are two different takes on “best use” for compost – both from very reputable sources – focuses attention on one of the biggest struggles engineers and consultants face as they attempt to develop meaningful recommendations for policy-crafters and lawmakers.

A never-ending information stream floats about in cyberspace.  Available facts and bits of data are of sufficient quantity and quality to support almost any position one chooses to promulgate.  Adding or subtracting just one factoid in the mix of observations can result in a very different conclusion.

And the sad-but-true fact is that too many studies involving waste management systems fail to include the full range of economic and environmental benefits of composting.  Conspicuous by their absence are those elusive “dollars saved” numbers resulting from compost use.

Level playing fields for waste management technologies are hard to find  

Decisions related to organic waste management options present unique challenges.  Credible research comparing all four of the modern commercial technologies in the same study is rare.  Landfill gas-to-energy, thermal waste-to-energy, anaerobic digestion, and high-rate composting – finding a level playing field for technology comparisons feels like the impossible dream.

A researcher must wade through an ocean of irrelevant and often conflicting studies to find the few that fit the bill.  Volumes and types of materials may differ from one study to the next.  The specific parameters and amount of data collected won’t match up.  One report might focus on energy generation while skipping over input costs.  Others fail to include industrial composting and/or anaerobic digestion along with landfills and incineration or base conclusions on data that is now decades old.

Unfortunately, staff/consultants hired by municipal governments to gather information for these kinds of reports must rely on this mishmash of published data.  Rarely (if ever) does that consultant or in-house specialist have the budget to conduct new economic research comparing four different technologies at field scale using identical waste streams under real life conditions.

Hunting for needles in haystacks

A literature review may require the researcher to sort through a hodgepodge papers and websites to find information.  The hunt can include bench-scale studies, computer modeling outcomes, masters theses, field trials, magazine articles, and published budgets from public record projects. 

From this jumble comes the reviewer’s analysis, report of findings, and recommendations.  But proprietary information like construction and operating costs from privately-owned composting facilities is rarely available in the public arena.  As a result, a consultant’s report may not reflect an accurate picture of a technology’s true potential or the latest innovations.

In the real world, research scientists are limited by budgets, people power, time, personal knowledge, and the expectations of funders.  And while that may be the nature of the beast, the resulting scientific paper merely represents a snapshot of conditions and available data as they existed within the framework and specific timeline of the investigation – nothing more.

Scientists understand this.  But the elected officials using those study results to guide their decisions may not, taking those studies as gospel.  They don’t see inconsistencies or information gaps.  They don’t ask the right questions.

But if all of those studies ignored compost use, do any of their only-halfway-there conclusions really matter?

Getting waste management comparisons to the finish line

Just to see if it could be done, we took a stab at stitching together a balanced comparison of organics management options. Pulled the most recent data we could find from multiple studies.  Adjusted dollars for inflation.  Converted all energy input/output to a common unit of measure.  Cobbled together bits and pieces from a slew of research papers, municipal budgets, and other web resources.  

Tipping fee revenue was assumed for all technologies.  Commercial composting was compared instead of municipal operations because [1] we had a pretty good idea of the costs for building and operating a big, industrial composting plant and [2] many of the published costs we’ve seen over the years for municipal construction and/or operations for similarly-sized or smaller facilities were far higher than our own experience.

Based on a 100,000 TPY operation, annual revenue calculations included tipping fees and sale of products like energy and compost, minus debt amortization for facility construction (without interest) and operating costs per ton processed.

Admittedly, the resulting numbers were very, very crude.  But the grand total of those figures?  All options netted about the same dollars per ton.

Yeah.  We were surprised, too.  However, what none of the studies calculated – including our own investigation – were all the additional economic benefits to be had through compost use. 

Compost use tips the scale in favor of organics recycling

Sadly, the oversights of these reports were not unusual.  The full dollar value of compost use was missing from almost every published economic evaluation of waste management technologies for organics.  The absence of this highly relevant data represents a glaring hole in the big picture, one that can negatively impact an entire region for decades.

Mostly, these benefits represent dollars saved, which are much more difficult to identify and calculate than dollars spent.  But that doesn’t mean those dollar values should be ignored:

  • There is a dollar value for carbon sequestration through compost use.  
  • There are dollar savings in water treatment costs when runoff is cleaner because of compost’s filtration abilities.  
  • Construction projects save when they use compost-based controls for erosion.  
  • Turfgrass managers save when there is compost beneath players’ feet.  There is also a reduction in the severity of sports injuries … more avoided dollars.
  • When compost use is specified as part of a communitywide stormwater program, stormwater systems and their construction costs can shrink.  
  • During times when synthetic fertilizer costs are high, the NPK content of compost can represent a real bargain.  There are also avoided costs related to transatlantic shipping and synthetics’ reliance on natural gas.
  • Compost helps soil combat weeds and control some plant diseases, reducing chemical use on lawns and sports fields.

When a government body responsible for the general well-being of hundreds (or millions) of people is not provided with all the facts, their decision-making suffers.

Granted, it’s devilishly hard to assign dollar values to some of these benefits.  This article by Dr. Brown demonstrates how involved the calculation of even one aspect of compost’s advantages can be.

Yet, the greatest value of organics recycling comes not from composting, but from compost use.  Ignoring this fact serves no one and significantly undervalues composting as an option for mainstream waste management.

If a municipality wants to be assured of choosing the very best technology option for its organic waste stream, issuing agencies and departments will include those important “dollars saved” calculations on the list of deliverables required of their consultant or engineer.

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.  

FAQ: Do I have to rake fall leaves?

Nature drops fall leaves for a reason, and it’s not to give sightseers an excuse to tour the countryside.  Those red, yellow, and gold gems will eventually decay to help fertilize the soil for the coming season.   So, no, leaf raking is not a necessity. 

Know, however, that the fall leaf drop can wreak havoc on stormwater systems.  One should, at the very least, make the effort to keep those leaves well away from stormwater inlets and  flow pathways.

Use a mulching mower to break up the leaf mat and accelerate biodegradation once that colorful blanket starts to fade.

If you can’t get through October or November without grabbing a rake, rough chop some of those leaves and use them to mulch planting beds and gardens.

The remainder can go to composting, of course.  Add them to your backyard compost pile, or prep them for curbside collection following your municipality’s guidelines.  And, please, do remove plastics, metal, glass, and other contaminants before moving those leaves to the curb. 

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.  

How to make topsoil

When you order topsoil, do you really know what you’re getting?  

In some developed areas,  most of the topsoil has been scraped away or eroded.  What passes as topsoil is really subsoil – nearly dead dirt.  It will not function like good soil.

The good news?  You can make your own, be assured of its quality, and likely pay less than having topsoil trucked in.  Here’s how:

FOR EXCAVATED SOIL:  Mix the native soil with compost at a ratio of about 1 bucket or shovelful of compost to every 2 of soil.  A 30 percent compost content is recommended for raised beds and containers.  

FOR IN-SITU SOIL:  Work 2-3 inches of compost into the top 6-8 inches of native soil.

Compost is a very “forgiving” material.  It’s hard to use too much  (though you shouldn’t use it instead of topsoil),  and as little as 1/8 inch can be enough to give your soil a boost.

Whatever the amount, be sure to blend well so the compost is evenly distributed.

How can you tell if a soil is good or bad?  

The ideal soil for growing things will be a mix of sand, clay, and organic matter.   If having your soil tested, be sure the report will include these parameters.

Forging ahead without the soil test? The first part of this article describes various soil types and provides simple methods of identification.  

If you need to add sand or clay in addition to compost, ask your landscape supply yard for a custom blend.

According to this article,  most soil scientists agree that 50% pore space, 45% mineral matter (sand, silt, clay), and 5% organic matter make up an ideal ratio.  A typical compost is 50%-60% organic matter (dry weight).