Soil, Compost, Mulch

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 materials at solid waste facilities? Can you plant in that?

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—Soil is a primary component of the earth’s surface and provides a place for plant roots, as well as a source of necessary materials for plant growth. Soil is composed of minerals, organic matter, microorganisms, air, and water— usually about 45% mineral material, 5% (or less in Florida soils) organic material, and 50% pore space, which is occupied by air and/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 garden. 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)—two of the three essential elements for plant growth—in your soil. Nitrogen (N) is the other essential element but it is not determined from soil samples. Florida soils usually have adequate P. The soil test won’t tell you how much organic matter is in the soil or the nature of the soil structure.

Organic matter—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 - 7), which is optimum for most garden plants.

Supplying energy to soil microorganisms: Organic matter is a source of food for microorganisms. When fresh organic matter (e.g., plant residues, composts, organic waste) 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.

Compost—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 clayey 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, bloodmeal, animal manure, and lime composted together.

Bagged amendments and garden mixes—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. But a soil test can confirm the amount of phosphorous in your soil.

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.

Mulch—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, and predator insects, spiders, and other garden helpers, like beneficial microorganisms, move on. Then your garden ecosystem must be reestablished when you plant your next crop, giving it a slow start each season.

Mulch is material placed on the soil to moderate the soil environment. Mulches can be organic or inorganic. 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. 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.

Free mulches—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. 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.

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 uncomposted 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 uncomposted "fine" mulch could be a problem.

An excellent source of more in-depth information on building soils is the Sustainable Ag Research and Education publication, “Building Soils for Better Crops,” available free at sare.org.

pH in the Garden

Have you ever wondered how those deodorants that look like a stick of marble work? They work because they change the pH on your skin surface to a more alkaline environment, making the area where it is applied less hospitable to odor-causing bacteria.

A soil test one fall revealed that the pH was quite high in the VegHeadz demonstration vegetable garden at the UF/IFAS Leon County Extension office. Plants did not thrive in our spring garden that year, and we had a lot of bug problems.

So what is pH and why is it important? pH stands for potential of hydrogen. Hydrogen ions directly affect the acidity or alkalinity of your soil. The concentration of hydrogen ions in a sample can be measured, revealing the acidity or alkalinity of a substance. The pH scale ranges from 1.0 to 14.0 with 7.0 as neutral. pH above 7.0 is alkaline or basic, while pH below 7.0 is acidic.

For reference, your blood has a pH ranging from 7.35 to 7.45, slightly alkaline. If blood pH exceeds that range in either direction, you will rapidly become ill, and will soon die, if it is not corrected.

Every living organism exists within a certain range of pH. Some organisms have a very narrow tolerance for changes in pH, such as our own bodies. Others, such as plants, can survive in a wider range of pH, but may only thrive within a prescribed range. Most plants in your yard and garden can survive in soil with a pH from 6.0 to 7.5. Acid loving plants like azaleas, camellias, potatoes, and blueberries prefer a lower pH.

Even with adequate nutrients, if the pH is out of the desired range, your plants cannot easily access the nutrients in the soil. Due to the complex interaction of pH and nutrients, it is difficult to determine whether the pH is incorrect or there is a nutrient deficiency when plants do not thrive. A soil test is the only reliable way to do so.

The level of acidity also affects the activity of soil microorganisms - fungi, bacteria, algae - which play an important role in converting nutritional elements from one form to another as well as breaking down organic matter to a form accessible to plants. They are also helpful in suppressing disease pathogens and degrading pesticides and other chemicals in the soil.

The interactions between soil organisms and plants is not yet fully understood, but maintaining soil at a desirable pH is well-known to benefit both the microorganisms and the plants. It also contributes to resistance to pests and plant diseases.

If a soil test reveals a pH between 5.5 and 7.0 in your yard or garden, there is no need to adjust pH. The majority of vegetables prefer this range.

Soil pH can be raised or lowered, but a better approach is to select plants that prefer the existing pH. Applying chemicals to change pH only has a temporary effect. For instance, the soil near your home may have a higher pH, reflecting alkalinity from masonry materials in the foundation or materials left there during construction. Alkaline materials will continue to leach into the soil throughout the life of the home, so it is better to select plants that prefer a more alkaline environment rather than attempting to lower the pH (Right Plant, Right Place). Organic mulch and soil amendments can slightly lower pH over the long term. Soil tests for your landscape should be done every two to three years and compared to previous results to determine if any major changes are taking place.

For individual plants and small vegetable gardens or container plants, changing the pH is possible. To raise the pH or make the soil less acidic, lime in various forms can be added.

To lower the pH, or make the soil more acidic, elemental sulfur, and/or peat moss, which has a pH around 4.0, can be incorporated to effect change over time. This should be done by following directions obtained from a soil test, as well as following the directions on the packaging of any amendments. A soil test before each planting season is ideal, particularly if you are amending the soil.

So what did we do at the VegHeadz demo garden to bring our pH down into a more desirable range? For each 25 foot row (about three feet wide), in a wheelbarrow, we combined a third garden soil from the row, a third mushroom compost, and a third peat moss with two cups of elemental sulfur. We mixed this well and added it back to the row. This only lowered the pH by two tenths of a point, possibly because mushroom compost tends to have an elevated pH. We did have a very nice garden that season with minimum pest and disease damage (except in our tomatoes). We would need to amend again if we wanted to lower the pH some more, and amendments only last for a year or two. We planned to do another soil test before we planted our following season’s crop.

For further reading, check out Soil pH in the Home Landscape or Garden from UF/IFAS.

The wrong pH for plants can lead to nutrient deficiencies. In this case, a high pH meant there was less iron available for the bean plant to uptake, leading to the deficiency. Photo by Howard F. Schwartz, Colorado State University, Bugwood.org.

Potting Mix Recipes

You can purchase pre-mixed potting soil at the store or make your own.

Our recipes are peat-free as peat is a non-renewable resource.

Seed Starting Mix Recipe

Use this recipe to create a starting medium for seeding indoors. Be sure to mix all ingredients well and moisten just before use. The mix should stay together when squeezed in your palm, but should not be dripping. To make a smaller batch, divide the ingredients in half.

Recipe for about 9-10 gallons of starting mix:

4 gallons fresh sifted worm castings or fresh sifted compost
4 gallons fine vermiculite

Amendments:

8 oz greensand or kelp meal for potassium
8 oz supplement with full spectrum of micro-nutrients (e.g. Azomite)
8 oz phosphate rock for phosphorus
8 oz alfalfa meal for early nitrogen

Container/Up-Potting Mix Recipe

Use this recipe to create a potting medium for container grown vegetables or for “up-potting” vegetable seedlings into bigger pots (prior to transplanting into the garden). Be sure to mix all ingredients well and moisten just before use. The mix should stay together when squeezed in your palm, but should not be dripping. To make a smaller batch, divide the ingredients by one-half.

Recipe for about 9-10 gallons of container mix:

1 gallon compost
1 gallon perlite
5 gallons pulverized pine bark
2 gallons worm castings

Amendments:

8 oz greensand or kelp meal for potassium
8 oz supplement with full spectrum of micro-nutrients (e.g. Azomite)
8 oz phosphate rock for phosphorus
8 oz alfalfa meal for early nitrogen
4 oz feather meal for mid-late nitrogen
2 oz oyster shell flour or crushed eggshells for calcium

Ammendments in the Organic Garden

Adding fresh compost several inches deep can supply the bulk of nutrients needed to grow healthy crops throughout the season in your garden. But if a soil test indicates you are low on particular nutrients, or if foliage color or plant vigor indicate your crops could benefit from a modest boost, it can be helpful to keep in mind some generalities about the three major nutrients plants need: nitrogen (N), phosphorus (P), and potassium (K).

For leafy vegetables, such as lettuce, mustards, and kale, a boost of N can improve foliage production and less P and K are needed.

For fruiting crops - such as tomatoes, cucumbers, and squash - as well as legumes, such as beans, take care not to add too much N. This could cause excessive leafy growth and negatively impact flower and fruit production.

P and K, on the other hand, can improve flower and fruit production.

For root vegetables, such as carrots, radishes, and onions, excessive N can stimulate extra foliage growth and less root growth. But a boost of P and K can help promote a strong healthy root system.

Amendments that supply Nitrogen (N):

Compost, composted animal manure (horse, cow, rabbit, chicken), bloodmeal (strong, be careful not to burn young plants, water soluble), alfalfa meal (also good source of P and K), cottonseed meal and soybean meal (cottonseed meal is slightly acidic and slow release), feather meal (moderate release rate), fish emulsion and fish meal (fast acting, especially in liquid form—apply with care as fast growing green crops attract pests), green manure legume cover crops such as alfalfa, clover, peas, etc.

Amendments that supply Phosphorus (P):

Compost, bone meal, pulverized rock phosphate, and green manure cover crops such as buckwheat and lupine. Buckwheat and lupine are good at mining P existing in the soil. Unlike N, which can be pulled from the atmosphere by microorganisms and supplied to legumes, minerals like P and K must exist in the soil to be accessed by cover crops. In order to utilize the minerals mined by the cover crop, cut it and drop in place to decompose and release the minerals back into the soil. (Additional P is usually not needed in Florida soils. A soil test can make that determination.)

Amendments that supply Potassium (K):

Compost, seaweed, wood ashes, greensand, granite, and deep-rooted cover crops such as grains and daikon-type radishes. Grains and daikon radishes are good at scavenging minerals deep in the soil.

Microorganisms in the Garden

The largest population on earth is never seen with the naked eye. They are everywhere—in our bodies, on our skin, in the air, in the water, and in the soil. They are referred to collectively as microorganisms. It is said that more microbes can be found in one gram of soil than there are people on the Earth.

Plants and microorganisms have evolved together. Bacteria were one of Earth's first inhabitants and they created an environment that allowed plants to develop. Without microorganisms, both then and today, life on earth would not exist.

A healthy population of microorganisms is the most important element for a successful garden. They are essential in nutrient cycling – providing nutrients to plants and breaking down organic matter to keep the supply constant. They promote plant growth and suppress diseases. They provide communication channels between plants and even help plants share resources.

When more fertilizer is applied than plants can absorb, it washes away and eventually makes its way to streams, lakes, rivers, and seas. In a healthy organic garden, microbes and plants form a symbiotic relationship to release essential elements like hydrogen, oxygen, carbon, nitrogen, phosphorus, potassium, iron, and micronutrients from organic matter and the atmosphere and make them available to plants. In return, plants spend as much as a third of their energy to produce compounds known as exudates and release them through their roots to nourish microorganisms.

The five types of soil microbes are bacteria, actinomycetes, fungi, protozoa, and nematodes. There are many, many thousands of unique varieties of microbes, most not yet identified specifically. Each type of microbe has its own niche in the soil.

Bacteria are the workhorses. Among their many services, they complete decomposition of organic material and release nutrients, promoting plant growth and increasing nutrient uptake, producing hormones essential for plant growth, and inducing disease resistance in plants.

Actinomycetes are actually classified as bacteria but they are unique enough to be mentioned separately. They help decompose less degradable organic materials found in soil, including chitin and cellulose, and produce several pigments which create the dark color of decomposed matter (humus). They can form associations with non-leguminous plants to help them fix nitrogen from the atmosphere.

The Steinernema scapterisci insect-parasitic nematode in the juvenile phase can infect and kill insects in the Orthoptera order, such as grasshoppers and crickets. Photo by David Cappaert, Bugwood.org.

Mycorrhizal fungi develop mutually beneficial symbiotic relationships with plant roots. Photo by Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org.

Fungi are the primary decomposers. Fungi known as mycorrhizae live in the root zone and connect with plants to exchange nutrients for sugars. They also help facilitate the uptake of nutrients and water for the plant.

Protozoa consume bacteria and in doing so mineralize organic soil nitrogen to produce ammonia. They can help plants add mass even when additional nutrients are not available. Their presence in the soil is influenced by the presence of living and dead plant roots.

Nematodes are microscopic worms. Harmful ones include root knot nematodes and others, but helpful nematodes prey on the bad ones and also provide nutrients to plants.

A balance of the different types of microbes is also necessary because some are predators and others are prey. Too many predators can deplete the helpful microbial prey. The application of fertilizers, herbicides, and pesticides can upset this balance and degrade the soil ecosystem.

We can help stabilize the environment by disturbing the soil as little as possible, providing a constant supply of organic materials as fuel, and keeping the soil covered with plants and mulch to keep populations of microbes thriving. A wide variety of plants is important to provide biodiversity as each variety attracts and sustains its own unique population of microbes, adding to the balance of the entire garden environment.

In a healthy forest, plants, animals, and microorganisms join together in a balanced ecosystem known as the soil food web. This is the ecosystem we want in our gardens and if one element is out of balance, the garden suffers. As opposed to landscape beds, which are relatively stable other than where we plant annuals, we are constantly disturbing the soil in our vegetable gardens, making it difficult to keep a consistent garden environment.

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