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Compost vs Mulch, What’s the Difference?

Sometimes, as anyone learning a new language can tell you, even words that have different dictionary meanings can take on new meanings. For gardeners the terms compost and mulch are two terms that are sometimes used interchangeably.. The difference between the compost and mulch can be quite confusing, especially for the novice gardener, since their uses can overlap. Yet, for the sake of your plant’s and soil’s health, there are differences.

Technically speaking, compost is organic matter that has been decomposed, while mulch is a layer of organic or inorganic matter placed on top of the soil as a protective cover. A commenter, RobertZ6, on defined compost as a ‘What’, while mulch is a ‘Where.’

So what are some of the main functions of the two and how do they differ:

Compost: Compost is a biologically active material resulting from decomposition of organic matter. Bacteria, fungi, soil insects and others help with this decomposition. In general, there are two different methods of making compost: 1) the “fast” method consisting of the ratio of two parts brown/dry material to one part green/juicy material, plus moisture, plus aeration/turning the pile; and 2) the “slow” method where leaves, grass, vegetable refuse, etc. are allowed to build up in a pile and slowly over time decomposition takes place and compost results.

Jane and Cindy at Work Making Compost-Fast Method!

Jane and Cindy at Work Making Compost-Fast Method!

Compost is often considered to be a soil conditioner, rather than a fertilizer since the actually nutrient value of compost can be so variable. Though the fertilizing component of compost is small, says that “compost can aid plants in many ways quite independent of its nutrient content. Because it improves soil structure, adds beneficial microbes, and boosts cation exchange capacity (CEC), compost improves the mobility of air, water and nutrients in the soil, all of which make nutrients more readily available to plants.” Compost that is fully decomposed/”finished” is called humus. It is dark brown, crumbly, with no distinguishable features, and has a sweet, pleasant smell.

Mulch: Mulch can be either organic or inorganic and is spread over the top of the soil to cover it. Examples of organic mulch include leaves, straw, and wood chips. Inorganic mulches include rubber, gravel, and landscape fabric. The purposes of mulch include suppressing weeds, moderating soil temperature, conserving water, maintaining a porous surface, and helping to prevent erosion. The use of organic mulch can improve the soil structure as it gradually decomposes over time.

Chopped Up Native Tree Trimmings and Leaves Can Be Used as Mulch

Chopped Up Native Tree Trimmings and Leaves Can Be Used as Mulch

So… where does the confusion in terms and usage for some gardeners lie? Two main questions come to mind:

1) Can compost can be used as mulch?   The answer to this question is “yes.” It is possible to add a layer of only compost to the top of the soil and use it as mulch. In fact, in the case of using organic mulch such as wood chips in a bed, the layer of wood chips closest to the soil will gradually break down into compost over time. Eventually it will be necessary to add more mulch to a bed or mulched pathway to account for this. One of the Master Gardeners says that she uses her unfinished, not completely decomposed, compost as mulch in her beds. Placing a 1-3 inch layer of this unfinished compost on top of the soil as mulch would enable the mulch to break down quicker into compost. Earthworms will gradually move the finished compost down into the soil.

Many gardeners however never have enough finished (or even unfinished) compost to use pure compost as mulch on top of the soil. Plus the cost of purchasing bags of compost to do this would be prohibitive for many. Therefore most gardeners choose to work their finished compost into the soil and top dress the soil with mulch.

2) Can mulch (for example shredded wood chips) be tilled into the soil? The answer to this is both yes and no.

No: Though it might seem as if this would “cut out the middle man” (i.e. the need to make finished compost), in general it is usually not recommended to incorporate shredded wood chips, or even un-composted leaves, into your soil.

To understand why, it is necessary to know a little about the way that decomposition of organic matter takes place and also the nutrients that plants need. To grossly over-simplify a very complex subject, plants need three primary nutrients (nitrogen (N), phosphorus (P), and potassium (K) plus thirteen secondary and micronutrients nutrients such as magnesium (Mg) and iron (Fe) to grow well. Most soils contain at least some of these nutrients. However many soils in Dallas County have a deficiency in nitrogen. A soil test can confirm whether this is true on your property.

This deficit of nitrogen in many Dallas County soils is important because in order to decompose organic material, the bacteria and fungi, which are the first organisms responsible for decomposition, also need nitrogen to feed on in order to live. When a large amount of un-decomposed material, such as wood chips, is incorporated into the soil, there is an increase in the number of bacteria and fungi needed to break it down. Since the bacteria and fungi need nitrogen as part of their diet, these microbes will start using up what is present in the soil. Since Dallas County soils are often deficit in nitrogen, sometimes not enough nitrogen is left in the soil to feed both this increase in soil microbes and the plants. This in turn can leave plants starved of nitrogen, one of their essential nutrients. Plants that are nitrogen deficient often have pale green or yellow leaves and exhibit poor growth.

              Yes: On the other hand, it is possible that the answer to the question of whether mulch can be tilled into the soil, can be answered “yes.” Though not generally recommended, one can till in mulch, even shredded wood chips or live oak leaves which do not readily decompose, into the soil if certain factors (time and/or supplemental nitrogen) are taken into consideration:

Time: If enough time is available to let a bed lie fallow/unplanted for a season or even one or two years, over time the mulch will gradually decompose into compost.  This can be compared to the “slow” method of making compost.

Supplemental nitrogen: An article in states that in an apple orchard, it was found that “a high-fiber diet of wood materials is exactly what many soils need. Rotted bits of wood persist as organic matter for a long time, enhancing the soil’s ability to retain nutrients and moisture, which results in bigger, better crops.” However a very high nitrogen source, in that case blood meal, 12-0-0, was also added at the same time as the apple trees were planted. The addition of supplemental nitrogen offset the bacteria’s and fungi’s taking it from the soil and provided nitrogen to the plants.

There is an ancient gardening technique called hugelkultur (which in German means “mound culture”) that makes use of using woody material, even logs, to make beds. According to those who use this technique, these raised beds retain moisture (supposedly needing only very infrequent watering,) improve soil fertility, and improve drainage. In this technique a mound of logs and twigs is built up, and finished compost, manure, kitchen scraps, etc. are packed into the spaces between the woody materials. A layer of top soil is placed on top of the mound and planted. There are several articles and videos on the web showing how to construct a hugelkultur bed.

A few years ago, several community gardens and individuals became interested in trying hugelkultur in the Dallas area. If you know of someone who has tried it, please share their results. We gardeners all learn from one another.

Though most of this article presents an extremely over-simplified explanation of the differences between compost and mulch and why it is important for your plant’s and soil’s health, hopefully the reasoning behind what is taking place will help you to understand the difference and help you grow the healthiest plants possible.


Pictures by Starla




Cardboard for Weed Control

At our old garden, we faced the  problem of all other gardens: weed invasion. At our new garden, we are making a concentrated effort to try to reduce the problem of weeds. You may have seen some of our Master Gardeners carrying cardboard from trash picks ups, we even get calls from friends donating “nice cardboard.”

Lisa Hauling Cardboard to The Raincatcher's Garden

Lisa Hauling Cardboard to The Raincatcher’s Garden

We prefer the plain brown stuff, stripped of packing labels and any plastic and broken down please.

We lay it down, overlapping seams, with 3-6 inches of mulch on top. Several layers of cardboard is permissible and  more mulch equals less weeds.  Some say to water the cardboard to make it more pliable. Of course, during this rainy year we have not had to do that.

Cardboard Peeking Out From Under Mulch, More Mulch to be Added

Cardboard Peeking Out From Under Mulch, More Mulch to be Added

And here’s a word about our mulch selection: you can see our mulch looks organic.  We use chopped up tree trimmings, not purchased mulch.  If you are buying mulch (we prefer free), don’t buy the colored mulch that has dye added.

Mulch Close-Up

Mulch Close-Up

Besides cardboard and mulch, what do you need?  Willing labor!

Thank You Judy, Abbe, and Michele!

Thank You Judy, Abbe, and Michele!

Our most recent mulch drop off came from Dallas Arborilogical Services. More is needed to build our beautiful, weed free garden. For drop off information, call the Dallas County Master Gardener hotline, 214 904 3053 and say The Raincatcher’s Garden sent you.


Pictures by Starla

One Way To Prepare A New Garden Bed

Dallas Garden Buzz wants to be the blog that helps you achieve the garden results we all long for in North Texas.  We don’t want to be just a pretty face but hope to guide you through the garden seasons with advice you can use to prepare and plan your own successful gardens.  From time to time, we may share experience from gardens other than The Demonstration Garden to illustrate gardening technique of lessons we have learned.

My Mom asked me to oversee the installation of a new garden patch at her house. I arrived early  just in time to see a plot of grass approximately 4 feet wide and 14 feet long being extracted.  The old grass went to the back part of the yard where grass had died out.

Mom's Yard "Before"

 Mom  wanted a  garden bed wide enough for flowers and a few tomatoes in the sunniest part of her yard.  She is also a Dallas County Master Gardener, so we agreed to use the Earth-Kind bed preparation for this new garden.  We have used this recipe for soil improvement at  The Demonstration Garden.

To coax the clay soil into submission for her dream garden, we added  3 inches of compost and 3 inches of expanded shale.  Picture the new garden bed as a cake, think about frosting it with a 3 inch layer of compost and a 3 inch layer of expanded shale. Now till it in so that you have changed the structure of the top 12 inches of soil in your garden.

Expanded Shale and Compost Blended Together

(To make the process easier, Dallasites can purchase a  blend of these two products  in bulk at places like Soil Building Systems and Living Earth Technology under the product names of Clay Slayer or  Clay Killer.)  Buy six inches of the product to till into your garden soil (3 inches of compost and 3 inches of expanded shale equals 6 inches of the two combined).

To figure the cubic yards needed we multiplied the 14 feet x 4 feet x .50 feet and divided by 27.  If you want to skip the math, use the cubic yard calculator on the Soil Building Systems website.  For Mom’s garden one yard of the blended product was purchased and tilled into the plot.

Rototilling Expanded Shale and Compost Into The New Garden Bed

Now that the clay has been amended into luscious, friable garden soil, the garden is ready for planting and will be topped off with 3 inches of mulch

Mom, your dream has come true!


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