Regional Gardening Reports :: National Gardening Association

In the Garden:
Middle South
April, 2010
Regional Report

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Building good soil is important in any garden, but is especially critical in new neighborhoods where the topsoil has been removed and the subsoil is compacted from heavy equipment.

Good Soil Equals Less Toil

Nine years ago when I moved to my current home, I was eager to begin a garden. Like many new neighborhoods, however, the valuable layer of topsoil had been removed and sold to offset the cost of land development. The exposed subsoil, a heavy red clay, was nutrient-rich but lacked organic matter. It's tightly packed particles offered little opportunity for air or water movement and it's pH level was too acidic for many of the ornamentals I wanted to grow.

Poor soils can only produce poor plants, so I knew establishing a good foundation would have to be my first priority. It's been a long and arduous process and I've learned much along the way.

The time and effort it takes to build good soil can be considerable, but I've found it means less expense and toil in the long run. Gardens with healthy soil require less irrigation and fertilization, and the plants they produce are not as susceptible to harmful insects and diseases.

Simply put, gardening is a happier and more fulfilling experience when plants thrive with less care.

Soil Basics
The best soils have three things: an optimum ratio of solids, air and moisture; the nutrients vital for plant growth; and a pH that facilitates their uptake.

The ideal soil ratio is 45% mineral matter, 5% organic matter, 25% water and 25% air. Mineral matter is categorized as either sand (large-size particles), silt (medium-size particles), or clay (small-size particles). The relative proportion of mineral matter determines soil texture, while the arrangement of all solids and the pore spaces between them accounts for its structure. Together, soil structure and soil texture determine the amount of air and water available to roots.

Plants require six primary nutrients: carbon, hydrogen, oxygen, nitrogen, phosphorous and potassium. The first three are obtained from air and water. The remaining three, known as macronutients, are the key components of most fertilizers. Plants also need secondary nutrients: calcium, magnesium, and sulfur; and micronutrients: boron, copper, manganese, zinc, iron, molybdenum and chlorine; but in much smaller amounts.

Soil pH is measured on a scale of 0 (extreme acidity) to 14 (extreme alkalinity). Soils with a pH of 5.8 to 6.5 are considered best for most plant growth, as most nutrients are more soluble in slightly acidic conditions, and thus more accessible. This pH level also creates a favorable environment for helpful organisms, such as earthworms and microbes.

If pH falls below 5.8, however, the availability of macronutrients and secondary elements decreases and the availability of some micronutrients increases. In fact, if pH drops much below 5.5 the availability of manganese, iron, and aluminum may increase to a level that is toxic to some plants.

Why Soil Test?
Soil testing, which is available for a small fee through local Cooperative Extension Service centers, will provide an accurate measurement of nutrient levels and soil pH, as well as detailed instructions for improvement.

Lowdown on Lime
In gardens like mine, where the soil is too acidic, pelletized dolomitic limestone is the most commonly recommended neutralizer. While bringing pH into balance, dolomitic limestone also supplies two essential secondary nutrients- calcium and magnesium.

A healthy dose of calcium improves soil structure, aiding both moisture and nutrient retention. It also helps release phosphorus and potassium, two of the vital macro-nutrients. Magnesium, on the other hand, is the only mineral component of chlorophyll. Without it, plants cannot process sunlight to gather energy from the sun.

Facts about Fertilizer
Fertilizers, either organic or inorganic, are sometimes needed to provide nutrients, especially the three vital macro-nutrients which are not derived from air and water, namely, nitrogen, phosphorous and potassium.

Nitrogen helps plants make proteins, the building blocks of plant growth, and increases seed and fruit production; phosphorous stimulates root growth, helps bud and flower set, and increases seed size; while potassium helps make carbohydrates, regulates metabolic activities, improves overall vigor, and increases resistance to pests and disease.

Organic fertilizers are made from enriched matter of plants or animals, such as bone meal, cotton meal or composted manures. As they decompose, they release nutrients slowly over a period of months or years. Inorganic fertilizers, composed of synthetic chemicals and/or minerals, are higher in nutrients, more water-soluble, and can be taken up almost immediately, giving plants a quick boost.

For the long-term health of the garden, organic fertilizers offer several advantages. They help build a soil that is rich in organic matter and teeming with microbial life, and they often improve the structure of the soil, increasing the availability of water and air to plant roots.

Inorganic fertilizers offer some benefits too, especially in spring. Because they are more water-soluble, they are available to plants even when the soil is cool, before microbes begin to work.

Most fertilizers are sold labeled with an N-P-K ratio to indicate what proportion of each macro-nutrient the product contains. For example, if a 100 pound bag of inorganic fertilizer has a N-P-K ratio of 20-20-10, it contains 20 pounds of nitrate (nitrogen), 20 pounds of phosphate (phosphorous), 10 pounds of potash (potassium), and 50 pounds of filler.

Soil Amendments & Mulches
The addition of uncomposted organic matter, such as leaf mold or soil conditioners (usually a combination of ground bark and topsoil), can also increase the nutrient level of soils and improve their structural deficiencies. When added to heavy clay soil, they improve tilth, drainage, and air movement; when incorporated into sandy soil, they increase water retention.

Mulches, especially the pine needles and ground bark that are so useful for moisture retention and weed control, will offer similar benefits as they decompose. Be aware, however, all additions, and even some natural factors such as rain, will continually modify soil pH.

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