Get the dirt on what's 'good' when buying soil, compost and mulch

·8 min read
The darker the soil the more organic matter it contains.
The darker the soil the more organic matter it contains.

Every gardener wants the best soil for their garden. How do you know if your soil is "good"? What do you add to make it good? When you go to a plant nursery or the gardening section of a big box store, you see bags of things advertised as garden soil or mulch or mushroom compost.

Which one do you use for your garden? And what about the garden mix from landscape companies or free stuff at the Solid Waste Facility? Can you plant in that?

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It is confusing. For instance, compost can be used as garden soil, or as mulch, or as an amendment to the native soil in your garden. And what’s the difference between mushroom compost and the compost you make in your own yard? Let’s start with soil – what is it and what makes it ‘good’?

Soil: What's in the ground

Soil is a primary component of the earth’s surface and provides a place for plants to root as well as a source of necessary materials for plant growth. Soil is composed of minerals, organic matter, microorganisms, air, and water. It is usually about 45 percent mineral material, five percent organic matter (or less in Florida soils), and 50 percent pore space, which is occupied by air or water.

The organic portion of the soil provides most of the nutrients which sustain plants and the microorganisms which assist them. The mineral portion of soil consists of particles graded by size, with sand the largest, silt smaller, and clay smallest.

The particles in the soil gather in clumps called aggregates, which create the soil structure. Organic matter assists by coating the mineral particles of the soil, helping them clump together. Florida soils are generally low in organic components and adding organic matter helps to bind soil particles into aggregates and improves soil structure. Improving soil structure provides many benefits for a gardener. The structure of the soil affects its ability to transmit and store water and nutrients and allows roots to reach deeper to access more resources.

A soil test can determine soil pH and the concentration of Phosphorus (P) and Potassium (K) in your soil—two of the three essential elements for plant growth. Nitrogen (N) is the other essential element but is not determined from soil samples. The soil test won’t tell you how much organic matter is in the soil or the nature of the soil structure.

Organic matter and microorganisms

Organic matter consists of decaying organic materials which when fully decomposed are called humus. Organic materials in your garden are anything that was alive and now exists on or in the soil. Common organic materials are plant residues, leaves, grass clippings and other yard waste, food waste, and animal manures. Organic matter in soils provides a variety of benefits such as:

Maintenance of stable soil pH: Soil organic matter moderates major changes in the soil pH. This helps to keep soil pH in the low neutral range (5.5 to 7), which is optimum for most garden plants.

Supplying energy to soil microorganisms: Organic matter is the main source of food for microorganisms. When fresh organic material (e.g., plant residues, compost, organic wastes) is added to the soil, microorganisms start the decomposition process. During this process, nutrients are released, soil aggregates are formed, and humus is created.

Maintenance of soil fertility: Organic matter must be regularly replenished as decomposition is completed and nutrients are used up by microorganisms and plants. Plants obtain P, K, and trace minerals from decomposing organic matter. It also feeds specialized soil bacteria which convert N from the atmosphere to a form available to plants.

Maintenance of soil structure: The presence of adequate amounts of organic matter in soils helps to coat soil particles (sand, silt, clay), aids aggregation, and improves soil structure.

Removal of harmful pollutants: Soil organic matter binds some harmful pollutants like residual pesticides and trace elements so they can’t escape from the soil and pollute our water bodies.

Adding compost and growing cover crops improves soil structure, increases the population of microorganisms, and enhances the overall health of the soil ecosystem.

Converting yard debris and kitchen waste into compost provides a useful product for gardens. Master Gardener Laurie Jones turns mulch at the VegHeadz Garden.
Converting yard debris and kitchen waste into compost provides a useful product for gardens. Master Gardener Laurie Jones turns mulch at the VegHeadz Garden.

Compost improves soil

Compost is a dark, crumbly material created when microorganisms break down organic materials such as leaves, grass clippings, animal manures, and kitchen waste. Compost is not completely decomposed (like humus); it will contain small pieces of debris like bits of twigs and leaves. The decomposition process is far enough along in compost so that nutrients are more readily available to plants.

Converting yard debris and kitchen waste into compost is an environmentally friendly way to reduce the amount going to solid waste facilities, and it provides useful and beneficial products for gardens. Compost is an excellent soil amendment that improves the health and structure of both sandy and clay soils. It can be incorporated into garden soil or added on top as mulch. It can be mixed with other materials for use as potting soil or brewed into compost "tea" for plants.

Mushroom Compost

This is the residual waste sold by mushroom farms when it no longer produces a commercially viable crop of mushrooms. It is generally some mix of grain straw, blood meal, animal manure, and lime, composted together.

Mulch can be moved aside, and seeds or seedlings planted, a method known as no-till gardening.
Mulch can be moved aside, and seeds or seedlings planted, a method known as no-till gardening.


Mulch controls weeds and more

Mulching is one of the best ways to improve your garden at low or no cost. Mulch helps control weeds, conserves moisture, moderates soil temperatures, improves soil fertility, and last, but not least, adds to the order and beauty of the garden.

Your garden soil should always be covered with closely spaced edible plants, companion plants, cover crops, or mulch. When soil is bare, pioneer plants – which we know as weeds – spring up; nutrients leach away; soil erodes; predator insects, spiders, and other garden helpers move on; and whole populations of beneficial microorganisms die. Your garden ecosystem must be reestablished when you plant your next crop, giving it a slow start each season.

Mulch is any material placed on the bare soil surface or around plants to moderate the soil environment. Mulches can be inorganic or organic, but in a vegetable garden, organic mulches are preferred, since these are simply additional organic materials added to the top of the soil. Microorganisms soon begin the decomposition process, and the mulch eventually becomes part of the soil’s organic matter.

As organic mulches decompose they improve soil structure and release nutrients. Some examples of organic mulches include wood chips, pine straw or bark, hay, oak or other tree leaves, compost, and cover crops which are growing, or which have been cut and dropped. Materials like newspaper and cardboard can also be utilized as mulches and will decompose along with other organic material.

Rake up leaves and pine straw

The best mulches are those which are readily available (and free), such as leaves and pine straw. Some tree services will put you on a list to receive free wood chips if they are cutting or trimming trees in your area. You may need to accept a full load if you choose this option. Neighbors who rake their yards in the fall are also good sources for additional mulch.

Free mulch is often available at the Leon County solid waste facility in two sizes – wood chips and “fines” which are ground up much smaller. Occasionally there will be partially composted material available also. Visit the Leon County website at leoncountyfl.gov.

The question is often asked whether these mulch materials can be incorporated into your garden soil as organic material or whether the fines can be used as garden soil. All these materials are suitable for composting, and as mulches will provide organic matter to your garden as they decompose on top of the soil.

In the decomposition process, microbes use large amounts of nitrogen, and when carbon-rich un-composted materials are incorporated into the soil, the possibility of nitrogen depletion is a factor. This is usually not a problem when the materials are left on the surface of the soil to decompose. For the same reason, planting in un-composted fines could be a problem.

Bagged amendments and garden mixes

All of these materials are beneficial to your garden to a greater or lesser degree. No general statement can be made about which ones are best, but some best practices include reading the labels to see what materials are incorporated into the bags and noticing the concentration of NPK in each type which will be noted on the bag as a percentage number such as 10-10-10 or 5-2-3.

In general, you would want to avoid materials that are high only in nitrogen (the first number) or low in the last number (potassium). The middle number (phosphorus) can be as low as zero since Florida soils usually contain enough phosphorus.

Bulk garden mixes are usually good for raised beds or amending your native soil and are generally some mix of composted animal manure and other materials. As a rule of thumb, the darker the mix, the more organic matter is incorporated, and the better it is for your garden.

Janis Piotrowski
Janis Piotrowski

Janis Piotrowski is a Master Gardener Volunteer with UF/IFAS Extension Leon County, an Equal Opportunity Institution. She hosts a blog about gardening and sustainable living in North Florida at https://northfloridavegheadz.blogspot.com. For gardening questions, email the extension office at AskAMasterGardener@ifas.ufl.edu.

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