Dr. Gary Bachman is an Assistant Extension Professor of Horticulture at the Mississippi State University Coastal Research and Extension Center in Biloxi.

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The Perfect Garden Soil
by Gary Bachman       #Soil

















Planting a dianthus in soil that has been amended with the great soil recipe.

The problem most of us have to deal with is a soil that is less than ideal, especially in suburban residential neighborhoods. The lots have been cleared of vegetation and the layer of topsoil has been removed. When the home is finished, the builder brings in not the topsoil that was removed, but some other soil guaranteed to be of lesser quality. Remember, the plant roots do not grow through the soil, but around the soil particles. Without great soil there is little chance of having impressive plant growth.

So what does the homeowner have to do to get that perfect soil and the outstanding plant growth?

To help explain the characteristics of a good garden soil, we need to have a little horticulture history lesson. Englishman John Lindley in the 1840s postulated that physical properties were more important than chemical properties. This is not to imply that pH and fertilization are not important, but just to highlight the need for good physical characteristics of the planting soil. Lindley’s list of the requirements of a good garden soil are:

Typical example of a new home soil profile. The dark layer at the top is from the sod that was installed. The layer below is what the builder brought in to level the lot.

1. Have good water holding capacity so sufficient water is available to the plant.
2. Be well aerated; the roots have a requirement for the movement into and out of the soil of oxygen and carbon dioxide.
3. Possess the ability to retain enough nutrients for plant use.
4. Include good percolation characteristics thus allowing irrigation and rainfall to move through the soil profile without causing a water-logged condition.
5. Have high organic matter which improves the previous four characteristics and encourages a healthy microorganism population in the soil.

The John Innes Horticultural Research Institute in the 1930s set out to try and standardize soil preparation to help the gardening public improve garden growing conditions. A mixture of one part native soil, one part composted organic matter, one part coarse sand and one part peat moss gave the best results and also met Lindley’s requirements.

While the John Innes soil recipe is very good for in-ground and raised-bed growing, because of the native soil component it is not well suited for use in containers and window boxes. The fine structure of the native soil tends to cause drainage problems. I like to keep the garden as simple as possible and do not want to have to mix different growing media for different growing styles.

I like to use a soil recipe that is suitable for use in-ground, in raised beds and in container growing conditions. One recipe, three growing styles. I would like to offer the recipe I use for a great soil:

•  6 parts peat moss
•  4 parts vermiculite or perlite
•  3 parts coarse sand
•  3 parts compost (your choice)

This soil recipe can be incorporated into the current bad soil by applying a 6-inch layer and working or tilling in. It is also great for creating raised beds on top of the less-than-ideal soil; in effect you would be creating a large surface container. And of course, use it in any of your favorite containers or window boxes. One mix, three uses, what could be easier?

Close-up of perlite. As the pumice is heated and expands it creates particles of various sizes that aid in loosening bad soil. • Example of vermiculite. Notice the accordion structure of the mica particles after heating and the various particle sizes. These layers will compress if handled too extensively. • The finished product of mixing a great soil. It is a mixture of peat, sand, compost and perlite/vermiculite.

Characteristics of soil amending materials for creating that perfect garden soil
Peat moss:* Peat moss is partially decomposed plant material that forms in cold, anaerobic bog conditions. A vast majority of peat moss used in the United States is imported from Canada. Peat moss has great water-holding capacity, has a slow rate of decomposition and has longevity of several years.

*Editor’s note: There is some controversy about the use of peat moss and the fact that it’s a mined product that is not quickly renewed. Please read up on peat moss and make an educated decision about whether or not to use this product or one of the many suggested alternatives, such as compost.

Vermiculite: Vermiculite is a natural mica material that is mined out of the earth and is widely available. When heated it expands in layers like an accordion and is able to hold an amazing amount of water between the layers. It helps to maintain water-holding capacity and lightens heavy soils. Handling is important – if the accordion layers are compressed they will not re-expand and any positive characteristics will be lost.

Perlite: Perlite is a natural pumice material that is mined out of the earth. It is subjected to high temperature and “pops.” It is lightweight and is used like vermiculite to loosen tight soils thereby increasing aeration, though has little moisture retention. It tends to be a little gritty and may float to the soil surface in heavy rain events. Perlite particles can be crushed from rough handling.

Coarse, sharp sand:The addition of sand helps to moderate soil particle size and improve drainage.

Compost: Compost is organic materials that have decomposed through microbial actions. It is produced from many different organic waste materials. It can be purchased commercially or produced by the homeowner in the backyard. No matter how it is obtained, compost is very beneficial. Compost increases the populations of microorganisms, acts as a slow-release source of nutrients and helps to loosen tight soils.

Now that you have the perfect garden soil recipe, there are steps to take to protect it. Avoid walking on or compacting the soil because this squeezes out all of the aeration and porosity. Be sure to cultivate or dig only when the soil is dry, never wet, as this encourages the soil to be clumpy and lumpy and not loose and crumbly. Make a yearly amendment of composted materials, as this adds life and nutrients.

Naturally good garden soil may be difficult to find, but with proper preparation and the right recipe, that bad soil can become that perfect garden soil. Remember the golden rule of gardening, “If you treat your soil well, it will treat your plants well.”


A version of this article appeared in an April 2010 print version of State-by-State Gardening.
Photography courtesy of Gary Bachman, Ph.D.


Posted: 03/30/18   RSS | Print


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