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). 

How much compost for my garden?

Compost makes a great addition to any garden plan.  But how much compost do you need?

A new plot in sand may require wheelbarrows of the stuff.  But if you are digging up a patch of lawn that has seen repeated compost applications over the years, the soil beneath the sod should be in pretty good shape.  A sprinkle might be all that’s needed.

How can you tell if the soil is good?  

The best method is soil testing.  (Contact your county Cooperative Extension Service for more information).  But you can use visual clues, too.  

Weeds like purslane, crabgrass, and dandelion are signs of a troubled soil.  

Stick a spade in the ground and turn over a shovelful of soil.  If it’s sticky and looks like modeling clay or dry and resembles beach sand, you’ve got big problems.  Fortunately, your soil is probably somewhere between these two extremes. 

Is it dark brown and loose?  Are there earthworms?  That’s what you want to see.  

How much compost do you need for a garden?

If building raised beds or container gardening, the soil blend should be about 30 percent compost.  When breaking new ground, incorporate 2 to 3 inches into the top 6 to 8 inches of soil.  

If your soil is very hard,  and you are planning deep rooted vegetables like tomatoes,  consider digging a little deeper.  Maintain the compost-to-soil ratio at about one part compost to two parts soil.

For an established garden with decent soil, just rake an inch or two into the surface before planting.   A 1/8 to 1/4 inch layer of compost sprinkled on the surface as needed throughout the growing season can revitalize flagging rows or containers.  The compost will feed your plants when you water. 

Three to 4 inches of compost can also be used as mulch during the growing season or as blankets when putting beds to sleep for the winter.  However, don’t pile compost up against tree trunks and stems of woody ornamentals.   

Our compost calculator can help you determine how much to buy.       

How much does compost weigh?

Depending on moisture level, figure 2 to 2.5 cubic yards of compost per ton.  A one cubic foot bag of compost will weigh about 40 pounds (1 cubic yard = 27 cubic feet).

A product shipped at 30 percent moisture will weigh less than one at 60 percent when it crosses the weigh scale, resulting in more cubic yards per ton than the wetter material when delivered.  

This may be good for keeping transportation costs low. But it also means the microbes responsible for aerobic degradation of the composting mass might die of thirst.  Weights that are too high could be indicative of low oxygen levels resulting from compaction and/or too much moisture — again, not good for the beneficial microbial populations.

An ideal compost will be 40-50 percent moisture.

Are compost and fertilizer the same?

Compost and fertilizer are not the same. But compost does have fertilizer value.

Wikipedia describes fertilizer as any material of natural or synthetic origin that is applied to soil or to plant tissues to supply one or more plant nutrients essential to the growth of plants.”

Compost’s nitrogen, phosphorus, and/or potassium (a.k.a. NPK) values are low compared to a synthetic fertilizer.  Some may add ingredients like urea to hike these macronutrient numbers.

That said, compost’s NPK value does have dollar value. The nutrients delivered by a compost product should be a factor in any input decisions involving synthetic fertilizer purchases.  Compost also adds a slew of micronutrients not typically found in common synthetics and improves nutrient uptake.

Compost feeds the soil. In turn, the soil takes care of the plants, offering a smorgasbord of nutrients, pest and disease resistance, and more.   But those nutrients are slow-release, feeding plants over time.  The benefits of a single compost application can stretch over multiple seasons.

Fertilizer’s sole purpose is feeding plants.  The primary function of most synthetic fertilizers is adding N, P, and/or K.  Application gives an immediate burst of nutrition.

Do you need fertilizer if you use compost?

For the home gardener, probably not, especially if that gardener is a long time compost user.

But for a commercial grower?  Maybe.  If the crop likes a punch of nitrogen (for example) at a certain point in the growth cycle, the addition of a synthetic fertilizer may be warranted.

However, the smart grower will carefully weigh the cost of any input against the expected return on investment. Sometimes, a lower yield will still net higher profits if input costs for synthetic fertilizers and pest control products are reduced or eliminated as a crop management expense.

Also, keep in mind that compost-amended soil reduces rainwater and irrigation runoff, which means more nutrients are retained in the soil.   This will impact synthetic fertilizer input requirement, as well.

The Compost Connoisseur 

Compost maturity and stability are not the same 

A mature compost is usually stable, but a stable compost may not be mature.  Yet, both products have their uses.  Though the term“maturity” and “stability” are often used interchangeably to describe compost, they should not be.  

Confused? 

Look at a red and green tomato.  Both are stable and edible.  But the green tomato won’t be mature until it turns red.  This work-in-progress tomato is a bit on the tart side with firmer flesh that holds up when fried.  The mature red one is sweeter, softer, and makes a great sauce. 

As distinct products, mature and immature composts have their specific characteristics and uses, too.  But like red and green tomatoes, they’re definitely not the same. 

Compost maturity and stability 

MATURITY  All organicwill eventually decay until nothing remains but atoms.  The trick is to reach a degradation phase where the easy stuff is gone, leaving only dark, slow-to-degrade, earthy-smelling material behind.  That’s a mature compost. 

Between the raw waste and finished compost, however, are a series of degradation steps that aren’t that beneficial to plants.  In an immature state, compost can release compounds harmful to plants, fight with plants for oxygen, and pull nitrogen out of the soil.  

Compost maturity is best determined by testing, which is a good reason to insist on seeing a recent lab report for the compost under consideration.  Maturity indicators on lab reports include: 

  • C:N ratios  
  • Germination rates  
  • Oxygen uptake  

Maturity assumptions based on curing time are also recognized within the industry, but may not be as reliable as testing. 

STABILITY  If a compost passes the maturity test, it is a stable, market-ready product.  In a mature compost, microbiological activity slows because all the “easy” food has been consumed.  

But there are conditions within the composting mass that can cause product to enter a stable state without reaching maturity. 

Compost that has been dried to remove moisture, for example, makes it lighter for shipping, but can exhibit reduced biological activity, as well.  The same thing happens if the pile is deprived of oxygen. 

Unfortunately, once moisture or air has been reintroduced, microbial colonies can reestablish and return to active feeding.  Pathogens can rebloom and odors resurface as the composting process resumes. 

Germination tests remain one of the best indicators of mature stability.  If the compost exhibits no indications of phytotoxicity in conjunction with good pH ranges and slowed microbial activity, then the product has probably passed into the mature range. 

If trying to evaluate stability while standing next to a pile in a landscape supply yard, look for: 

  • A light, porous, evenly-textured product that encourages good air flow 
  • A compost with sufficient moisture to stick together when squeezed in the palm of the hand without crumbling or dripping water 
  • A pleasant, earthy scent 

Selecting the right product 

In the absence of testing information, the easiest way to gauge a product’s maturity is to smell it.  Compost that smells like soil has likely reached a stable, mature state and is ready for use anywhere and by anyone. 

Product that still retains some pungency isn’t stable or mature.  It’s not quite ready for unrestricted use.  But, provided it has met minimum quality standards for pathogen and vector reductions (as specified by regulations), the compost can be applied in rural areas away from sensitive noses where its higher NPK value is much appreciated by farmers. 

Time and nature will finish the job of product maturation and stabilization. 

Is it really a good idea to make compostable waste go away and never come back? 

Each year, taxpayers collectively spend millions of dollars to burn or bury compostables.  Much like a tribe of ubiquitous Gollums, they just want garbage — the biodegradable and putrefying fraction of the municipal solid waste stream – to go away and never come back. 

The desire to make disagreeable discards disappear into fiery furnaces or burial mounds is understandable.  But is it wise?  Is it fiscally responsible?  Is it really a good idea to make organic waste go away and never come back? 

Nature recycles everything 

Rocks weather and erode, creating sediment. With heat, pressure, and time, that sediment becomes rock again.  Plants and animals feed and drink from the earth, die, and decompose to replenish the soil that will sustain future generations of flora and fauna.  Water drops from the sky as rain, filters down to aquifers, upwells and evaporates back to the clouds to fall once more. 

In a fantasy land, it may be possible to keep using resources without a thought to replenishment.  But in the real world, organic waste – the decaying residuals of once-living things – must be recycled back to the soil to maintain life-critical soil functions.   

Some seem to think the destruction of organics to make energy is more important than rebuilding soil.  But pushing an organic-waste-to-energy agenda by sacrificing the soil makes no sense. Humans managed to survive for millennia without electricity and centralized energy systems.  Without soil’s life-essential contribution to food and clean water, people face extinction in weeks.  

So, which is more important, energy or soil? 

Make energy and rebuild soil?   

Organic waste from developed societies includes all types of vegetation, food, manures … even compostable plastics.  When turned into a quality compost, these once-lost resources can be used by anyone anywhere to replenish depleted soil.   

Happily, making energy and building healthy soil does not have to be an either/or proposition.  It is possible to extract energy from organic waste without destroying the beneficial properties that make it valuable to soil.   The organic waste streams from these processes can then be used as feedstocks in the manufacture of compost products. 

Unhappily, energy production from biomass is one of the most expensive ways to make energy.  Even solar and wind power can be more cost-effective. 

Furthermore, bioenergy technologies based on anaerobic digestion of organics are still too pricey to be practical in many places.  Where they do exist, the waste stream (digestate) is not always put to highest and best use (i.e. composted).  Instead, residuals may be landfilled or relegated to low-dollar-value reuse. 

But one day, as more communities opt to restore natural soil replenishment cycles and energy generation technologies become more efficient, extracting energy from biomass, followed by composting and compost use, can become the system of choice for organic waste management. 

In the meantime … 

The importance of healthy soil 

Where humans live, topsoil has been scraped away or eroded.  Nutrients are used up.  Compaction has destroyed the pore spaces essential to the transport of air, water, and microbes.  Without a regular infusion of new organic matter to correct these deficiencies, soil dies.   

There are lots of processes for generating energy, but there’s only one way to replenish disturbed soils in developed areas – feed them a good, wholesome diet derived from organic waste converted into compost.   

From farms to lawns to sports fields, soils require periodic applications of compost.  There’s no other way to easily and economically provide soil with everything it requires to retain water, nurture vegetation, and create the type of environment soil microbes need to support nutrient uptake, contribute to disease resistance, and degrade pollutants. 

The best news? In many metropolitan areas, efficient, high-rate composting – the type needed to successfully manage big, urban waste streams – costs no more than landfilling or incineration.  Often, recycling at a modern, industrial composting operation can be more affordable than traditional disposal.   

Composting makes organic wastes go away, but they come back as enriching soil amendments.  Biodegradables need to keep recycling, just like they have since the beginning of time. 

Breaking the natural soil cycle by incinerating or burying compostable waste is a bad idea that should go away and never come back.

VIEW THE SLIDESHARE:  Addicted to convenience

Compost is soil’s superhero

Sure, compost adds nutrients. But that might be this soil amendment’s least important function. 

Quite often, articles will mention compost as a replacement for some or all of the nutrients that might be provided to plants through applications of synthetic (man-made) fertilizers.   

That’s certainly true.  Compost delivers the macronutrients nitrogen, phosphorus, and potassium (NPK), plus a slew of plant-essential micronutrients that are missing from most synthesized fertilizer products.  Compost provides plants with a wholesome, well-rounded meal, not the nutritional equivalent of junk food. 

But what these fertilizer-focused articles rarely mention is the fact that the real value in compost use is not related to feeding plants, but to feeding soil … and soil does require a wholesome diet to function as a true soil and not a dead substrate. 

Compost feeds soil

Providing plant nutrients is just one of many soil functions.  Worms and other creatures that live in healthy soils help to physically break down food sources, then microbes take over to convert that food into plant-available form. 

Both physical and microbial conversion depend on a soil environment that can support those lifeforms.  If the soil is chronically too wet, too dry, too compacted  yada, yada  then it can’t support a healthy soil ecosystem.  That plot of ground may not be soil at all, but lifeless dirt. 

To countermand the impacts of human activity, disturbed soils require regular program of replenishment that includes organic matter and microbes.  Compost provides both.  Compost feeds soil.

Then, when it rains, soil retains that water, reducing runoff.  When runoff is reduced, so is erosion, sedimentation, and water pollution.  Because soil microbial activity also degrades pollutants, any stormwater that does run off is cleaner.  

That same microbial activity can help neutralize some soil-borne diseases, too. 

Improving plant nutrition, aiding in disease control, reducing water pollution, and retaining water are all important soil functions. 

But wait, there’s more. 

Compost as a carbon sink 

The build-up of greenhouse gases in the earth’s atmosphere is cause for concern.  As more greenhouse gases flood the atmosphere, temperatures increase. 

This rise in global temperatures influences many things, erratic and extreme weather being one of the most visible.  Subsequent climate shifts can impact people, crops, and livestock for hundreds of years. 

When used to amend soils, compost sequesters carbon.  This means the soil will act as a carbon “sink,” capturing and holding carbon in stasis – but only as long as the soil remains undisturbed.  When the soil is tilled, that carbon is released. 

Extensive use of compost for perennial crops and other long-term application(grasslands, tree farms, utility easements, etc.) can positively impact atmospheric conditions by reducing greenhouse gases.   

At the same time, the addition of compost rebuilds a topsoil layer that has been eroded or scraped away by farming, development, and other human activity.  Since topsoil loss has been identified as a significant threat to planetary health, second only to population growthits restoration is a global priority.   

At a time when nearly a third of the world’s arable land has become unproductive in just a few decades, compost really can be that superhero swooping in to save topsoil, save water, save the atmosphere, and save the planet. 

Comparing costs per gallon retained 

Soil amendment is one of the least expensive ways to collect and manage stormwater 

Manage water where it falls.” 

This sound advice is the foundation of the Milwaukee Metropolitan Sewerage District’s Regional Green Infrastructure Plana program that identified soil amendment as one of the least expensive ways to manage stormwater.  At 28 cents per gallon, improving soil is second only to native plantings in lowest cost per gallon retained. 

Green roofs?  $4.72 per gallon.  Those fancy-schmancy deep storage tunnels?  $2.42 per gallon.  At $1.59 per gallon, even pretty little rain gardens cost more than five times that of simple soil amendment. 

Milwaukee is not alone in promoting soil amendment as a first line of defense for stormwater management  For example: 

  • Denver and GreenleyColorado, require compost use for new landscaping, as does Leander, Texas. 
  • Some state Departments of Transportation (DOTs) now routinely specify compost.  A few years ago, the Texas DOT said it was the largest single market for compost in the U.S. 

In an urban environment, opportunities for soil amendment abound.  City parks, athletic fields, planters, urban lawns, highway medians and easements, foundation backfill – anywhere there’s soil, there’s opportunity for inexpensive water retention. 

Every 1 percent increase in soil organic matter (SOM) content adds an additional 16,000 gallons of water-holding capacity per acre foot.  A site managed to maintain soil organic matter at only 2 percent can hold all the water of a typical rain event (1 inch or less), which is 27,154 gallons per acre.     

In fact, at 5 percent SOM, the soil can retain the water equivalent of nearly 3-inches of rainfall.  In some regions, this equal95 percent of all storm events. 

Soil amendment may not solve all rainfall issues, especially in downtown areas.  But managing water where it falls can be the most sensible, efficient, environmentally- and economically-prudent strategy for “first line of defense” stormwater management.   

Food waste mandates are only the halfway mark 

Compost use gets organics recycling to the finish line 

Unlike a decade ago, when food waste mandates were few and far between, there is a flurry of activity these days focused on diverting food waste and other residential/commercial biodegradables from landfills and incineration. 

From the U.S. to Italy to northern India, the movement toward more sustainable management of organic waste from households and businesses is real and gaining momentum. 

But while laudable, there’s a big piece missing from some of these programs — mandated compost use.  Just making compost isn’t recycling.  The product must be used – returned to the soil – to be recycled.  That’s what makes the system “sustainable.” 

Landfilling organics isn’t sustainable because they’re buried.  Any thermal or other waste-to-energy (WTE) technology that destroys organics isn’t sustainable, either, no matter how hard technology providers try to paint them as such.  The feedstock – municipal waste – may be considered a sustainable source, but the management system is not. 

A possible exception is biochar, carbon-rich, charcoal waste material produced by pyrolysis that is sometimes used as a soil amendment.  However, not all biochar is right for this type of reuse.  It doesn’t offer as many benefits as compost, and — since the use of biochar is relatively new — there is a lack of research related to its long-term use.  While pure biochar is made from organics, of specific concern is contamination resulting from WTE biochar processes that use unsorted municipal solid waste as feedstock.  

But whether biochar or compost, the truth bears repeating — recycled organics must be used to feed the soil for a sustainable system to exist.  This is the only way to close the recycling loop for organics. 

Going the distance with food waste mandates

Football players don’t move the ball to the 50-yard line and then stand around waiting for the pigskin to get itself into the end zone. 

Establishing a curbside or drop-off program for source-separated organics is a good first step … but it’s only half the distance to the goal.   

The finish line for organics recycling is compost use.  Anything a community can do to encourage that use is important.  But sometimes, it takes more than education and outreach to get the ball rolling. 

When governmental entities write ordinances and project specifications requiring compost use, good things happen.  By creating early markets for quality compost productseveryone from green industry pros to stormwater managers to homeowners can clearly see the benefits of amending soil. 

This demonstration leads to voluntary compost use through the manufacture of quality products and product sales to high-value markets.  Product sales, not giveaway programs, is what will keep composting facilities – public or private – economically sound. 

Any community considering organics recycling needs to think about the end game.  To ignore the ultimate goal is to win the battle, but lose the war for organics recycling.  

READ:  Food waste diversion — it’s time to pursue alternatives that make environmental and economic sense