Annual Flowers: Preparing the Soil

Incorporating soil amendments
The best amendments for clay soils are pine bark (less than 1/2 inch in diameter), composted leaf mold, or small pea gravel (less than 3/8 inch in diameter). Be careful when selecting leaf mold, making certain that the material is fully composted and not merely “aged.” Partially decomposed materials will compete with plants for nutrients, especially nitrogen and sulfur, resulting in nutrient deficiencies and poor plant growth. Peat moss, sand, hardwood bark, wood chips, and pine straw are not recommended because they do not adequately improve the physical properties of a clay soil. Organic matter content must be increased to 25 percent, by volume, to be effective as a soil conditioner. For example, to achieve approximately 8 inches of amended soil, a minimum of 2 inches of material should be incorporated into the top 6 inches of soil. This also raises the bed which improves drainage and makes bedding plants more visible. Incorporating over 50 percent organic matter may have a negative effect on plant growth, while incorporating less than 25 percent is a waste of time and material.

Amendments such as pine bark, composted leaf mold, or peat moss will improve water retention in sandy soils. These amendments need to be added at a minimum of 25 percent by volume and a maximum of 50 percent.

Soil testing and fertilizing
After any amendments to improve aeration and drainage have been incorporated into the bed, have the soil analyzed to determine fertilizer needs and pH. A soil test is the only way to determine if phosphorus, potassium, calcium, or magnesium must be added or if pH should be adjusted. Overapplication or application of unneeded fertilizer could result in salt injury to plants and cause nutrient imbalances. It is also environmentally unsound since runoff of excess nutrients degrades water quality. If soil test results indicate that nutrient or pH adjustments are needed, the materials should be mixed into the soil uniformly since bedding plants have a very limited root system.

The soil pH for bedding plants should be between 5.5 and 6.5. Lime should be thoroughly tilled into the soil prior to planting. If the pH must be lowered, elemental sulfur can be incorporated into the soil. If only a small decrease in pH is required, acid-forming fertilizer such as ammonium nitrate can be used to provide nitrogen and to slightly lower the soil pH.

If a soil test is not made, incorporate 2 to 3 pounds of 10-10-10 fertilizer per 100 square feet of bed area. An easy conversion to remember is that 1 pound of fertilizer is equal to about 2 cups (or 96 teaspoons). Use 2 pounds (4 cups) of dry fertilizer per 100 square feet or about 2 teaspoons per square foot. An alternative to 10-10-10 is a slow-release fertilizer such as 16-4-8 or 12-4-8.

It is impossible to tell how much calcium and phosphorus are required without a soil test. However, because most North Carolina soils are low in phosphorus, it is usually safe to add 2 to 4 pounds of triple superphosphate (0-46-0) per 1,000 square feet. Phosphorus and calcium move very slowly through the soil profile so they should be incorporated into the top 6 to 10 inches.

It is not a good idea to guess at the amount of calcium to apply, first, because it is possible to get too much calcium in the soil which can lead to potassium and magnesium deficiency; and second, because some sources of calcium such as calcitic and dolomitic limestone raise soil pH so high that some nutrients become unavailable to plants. If a soil test shows that calcium is low but a pH change is undesirable, gypsum can be incorporated into the soil prior to planting.

Magnesium may be deficient, especially in low pH soils. If a soil test shows that magnesium levels and soil pH are low, dolomitic limestone can be used to raise the pH and to supply the needed magnesium. To add magnesium without changing the pH, apply Epsom salts. The general rate for Epsom salts is 1 pound per 100 square feet.

Prepared by: Erv Evans, Consumer Horticulturist