Soil Testing

Soil characteristics determine how well plants are able to use nutrients. A soil test indicates what nutrients are in the soil and what nutrients are needed. The Agronomic Division of the North Carolina Department of Agriculture (NCDA) provides a soil testing service. A soil analysis from NCDA lists the amount of nutrients in the soil and gives recommendations for improving the nutrients for landscape plants. A county Cooperative Extension Center can provide instructions on how to take soil samples and send them to NCDA. After the initial soil test, soil should be tested every three to five years to make sure the fertilizer program is on target.

Adjusting Soil Properties

The effectiveness of fertilizer applications depends on soil properties such as texture, organic matter content, drainage, and pH.

Soil particles are grouped by size and designated as sand, silt, or clay. Sand is the largest particle size, and clay is the smallest. Soil texture varies with the different proportions of sand, silt, and clay on each location. In general, soils with a greater portion of silt and clay retain more water and nutrients than those soils composed mostly of sand particles.

Soil organic matter also influences soil productivity. In general, organic matter increases both the water- and nutrient- holding capacity of a soil. Organic-matter additions to soil may also provide nutrients as they decompose, or they may increase the tilth of the soil by amending or modifying the soil structure to promote water infiltration and root penetration.

Soil drainage is critical to plant health. The soil's ability to hold water must be balanced with its ability to retain enough oxygen for plant growth. If soil becomes saturated for a prolonged period of time, the oxygen trapped in soil pore space can be depleted rapidly by the plant and soil organisms. When roots cannot get oxygen, the plant's ability to get nutrients and grow is impaired. Poor drainage causes more problems for landscape plants than any other factor.

Soil Acidity

Soil pH, a measure of acidity, has a significant impact on the plant's ability to use nutrients. The scale of pH ranges from 0 to 14. Seven is considered neutral; values below 7 are considered acidic, and values above 7 are considered alkaline.

North Carolina soils are highly weathered (leached) because of excessive rainfall and therefore are naturally acidic. This process has depleted the nutrient elements calcium (Ca) and magnesium (Mg) from naturally occurring minerals as well as those of previously applied agricultural limestone. Decay of plant residue, the addition of organic matter, and the widespread use of nitrogen fertilizer also increases soil acidity.

Most of North Carolina's landscape plants grow well in soil with a pH range from 6.0 to 7.0. Within this range, the essential nutrients are available to most plants, and soil organisms can carry out their beneficial functions. If the soil is too acidic (low pH), the pH can be raised by adding lime. If the soil is too alkaline (high pH), the pH can be lowered by adding sulfur.

A soil test analysis includes determination of the pH. The North Carolina Cooperative Extension Service publication Soil Facts: Soil Acidity and Proper Lime Use (AG-439-17), available from the county Extension Center, provides good information on adjusting pH and the reasons for it.

Types of Fertilizer

The analysis, or grade, of a fertilizer refers to the minimum amounts of nitrogen (N), phosphorus (in the form P₂ O₅), and potassium (in the form K₂O) in the fertilizer. The analysis is always printed on the fertilizer label. A fertilizer with a 10-10-10 analysis contains 10 percent nitrogen, 10 percent P₂O₅, and 10 percent K₂ O. For example, in 100 pounds of 4-8-12, there are 4 pounds of N, 8 pounds of P₂ O₅, and 12 pounds of K₂O. Conversion equations for the amounts of phosphorus and potassium are given for quantities of P₂O₅ and K₂O.

Fertilizers may be divided into two broad categories: natural and synthetic. Natural fertilizers generally originate from unprocessed organic sources such as plants or animals. Synthetic fertilizers are man-made or processed. Synthetic fertilizers can be organic (for example, urea) or inorganic (for example, superphosphate).

Natural fertilizers commonly misnamed "organic" also can contain inorganic ores such as rock phosphate. Most nutrients from living or once-living organisms are not readily available for plant growth, because they are bound in organic molecules such as proteins and amino acids and in structures such as cell walls. These nutrients are released only when microorganisms decompose the organic matter.

Slow-release fertilizers may be synthetic or natural. Because nutrients are released over an extended period of time, slow-release fertilizers do not have to be applied as frequently as other fertilizer types. Also, higher amounts of slow-release fertilizer can be added at each application without risking injury to plant roots. Slowly released nitrogen is used more efficiently because a higher percentage is absorbed by plants. The higher efficiency of slow-release fertilizers means less nitrogen is available to contribute to pollution of streams and subsurface water.

The primary disadvantage of slow-release fertilizers is higher initial cost. When an analysis is done to determine the cost of nitrogen that is actually absorbed by the plant, however, the unit cost is actually lower for slow-release materials.

Several categories of slow-release nitrogen fertilizers are available. Water-soluble or liquid fertilizers are applied either to the soil or foliage. Many water-soluble formulations are available for almost any specific need, from high-nitrogen plant starter fertilizers to minor element formulations.

Chelated iron is used extensively for prevention and control of iron deficiency in azalea, rhododendron, and other popular ornamentals.

Application Rates

Because of the variability in residual nitrogen in soil over time, nitrogen is not measured in soil tests. In North Carolina, the rate of application of nitrogen depends on the plant species and its stage of development. General recommendations may range from 2 to 6 pounds of nitrogen per 1,000 square feet (about 90 to 260 pounds per acre) per year.

To convert from actual nitrogen to fertilizer, divide the amount of actual nitrogen desired per 1,000 square feet by the fertilizer analysis or grade. With an 18-6-12 fertilizer, for example, to apply 3 pounds of nitrogen per 1,000 square feet would require 17 pounds of fertilizer (3 lbs/0.18 N = 17).

Nitrogen needs also vary with stage of plant growth:

• At planting time, nitrogen needs are relatively low, but sufficient nitrogen should be supplied to support the new root generation and top growth of the plant. High rates should be avoided because they will trigger excessive top growth that cannot be supported by a limited root system.
• Maximum growth of landscape plants is usually desired during the first few growing seasons after establishment. To achieve maximum growth, constant rates of nitrogen are needed.
• As the plant begins to mature and the rapid growth rate is no longer needed or desired, lower levels of nitrogen are sufficient to maintain plant vigor.
• Plants growing where the root zone is restricted (containers, parking lots, etc.) should not receive large amounts of nitrogen, because restricted roots cannot support excessive top growth.

Some plants, once established, may not need additional fertilizer to perform well. Silver maple, willow, ligustrum, and forsythia are good examples. The fibrous root systems of some ornamentals such as azalea, dogwood, hemlock, and rhododendron are easily damaged by fertilizers. Split applications of water-soluble nutrients or slow-release formulations are recommended for these plants.

Timing Fertilizer Applications

The best time to improve soil fertility is before planting. Nutrients can be mixed thoroughly into the soil where the plant roots will grow. Phosphorus is the most important element in this category, because it moves slowly. Lime or sulfur, if needed to correct pH levels, is most effective when incorporated the season before planting. This allows time for the chemical reactions that change pH to take place.

Nitrogen can also be added before planting. Inorganic forms, however, are not recommended at this time, because they leach easily, and at high levels, they may injure the fine roots of newly-planted ornamentals. The best sources of nitrogen in organic forms are composted materials or fish emulsion. Roots absorb nutrients most efficiently when they are growing actively in late winter/early spring or fall. The best time to apply fertilizer is about two weeks before these periods so that the nitrogen can move into the root zone in time to be available for plant use.

Application Methods

Five application methods are discussed below. Each serves a specific role depending on the site and health of the plant. Regardless of the method selected, the soil should be moist at the time of fertilizing to prevent fertilizer injury to the plant.

Liquid Injection. Through liquid injection into the soil, fertilizer solutions are placed in the root zone. This is an excellent method for correcting nutrient deficiencies.

Injection sites should be 2 to 3 feet apart, depending on pressure, and 6 to 9 inches deep. Fertilizing deeper than 9 inches may place the fertilizer below the absorbing roots and prevent the plant from using it. When using this method in summer or during periods of drought, water should be added to dry soil.

Drill-Hole or Punch-Bar. A major advantage of the drill-hole system is the opening of heavy, compacted soils, which allows air/moisture and fertilizer to move into the soil. Drill holes should be placed in concentric circles or in a grid system around the main stem of a tree beginning 3 to 4 feet from the main stem and extending beyond the drip line.

The holes should be spaced 2 feet apart and drilled 6 to 9 inches deep. The recommended rate of fertilizer for the area should be distributed uniformly among the holes and based on the root-zone space under the tree (and not the trunk diameter). The holes can be filled either with organic materials such as peat moss or compost, or inorganic materials such as gravel, sand, or calcined clay.

Surface Application. Applying fertilizer to the ground surface is as effective as most other methods. It is best to apply the fertilizer and then water in slowly. It is acceptable to place fertilizer on top of mulch in landscape beds.

Fertilizer stakes or spikes that are driven into the soil contain satisfactory fertilizer materials. Unfortunately, the spacing of spikes is such that very little fertilizer comes in contact with the root system. One or two stakes per inch of trunk diameter do not represent adequate fertilizer distribution, because lateral fertilizer movement is limited in soil.

Foliar Spraying. Spraying liquid or water-soluble fertilizer on the foliage is the most effective method of correcting deficiencies of minor elements, especially of iron and manganese. This method should not be used as a means of providing all the macronutrients required by plants. To correct chlorosis (a yellowing of plants caused by a nutrient deficiency) several applications during a growing season may be necessary.

Tree-Trunk Injection or Implants. The infusion of liquid or implants of fertilizer is often the best method for correcting iron and manganese problems in landscape trees. This method is especially useful in areas of adverse soil pH, high moisture, or where other means of application are not practical. The wound caused by holes made in the trunk-root flare will close within a growing season.

The most important thing to remember when making fertilizer decisions is to apply only what the plants will use. The care taken in learning about plant needs will pay off in healthy plants and a protected water supply. For most fertilization information, the North Carolina Agricultural Chemicals Manual should be consulted. The manual is available from the Department of Agricultural Communications at North Carolina State University (919-515-3173).

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