Landscape plants, like all living things, need nutrients to survive. Many of the elements that are essentialfor a healthy landscape are already in the soil, butfertilization is often needed to supplement these nutrients. When nutrients are added, care must be taken to apply only what the plants will use. Too much fertilizer can damage plants and can also impair water quality.

You do not, however, have to choose between having a healthy, attractive landscape and protecting water quality. Knowing how to determine proper nutrient applications for your landscape is the key. Soil properties, types of nutrients, plant needs, fertilizer types, application methods, and application timing need to be considered. This publication contains basic information to help you make decisions that will enhance your landscape and are environmentally sound.

Soil particles are grouped by size and designated as sand, silt, or clay. Sand is the largest particle size, and clay is the smallest. Soils contain different proportions of sand, silt, and clay that make up the soils' specific texture. In general, soil textures 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 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 rapidly depleted 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 pH, a measure of acidity, has a significant impact on the plant's ability to use nutrients. The scale of pH goes from 0 to 14. Seven is considered neutral; values below 7 are considered acidic, and values above 7 are considered alkaline. Most of North Carolina's landscape plants grow well in soil with a pH range from 6.0 to 7.0. Elthin 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 pH. The North Carolina Cooperative Extension Service publication SoilFacts: Soil Acidity and Proper Lime Use (AG439-17), available from your county Extension Center, provides good information on adjusting pH and the reasons for it.

The seven micronutrients (iron, manganese, zinc, boron, molybdenum, copper, and chlorine) are required in smaller amounts. Iron and manganese are the micro- nutrients most often deficient in landscape plants. An adjustment in soil pH usually corrects deficiencies of the micronutrients .

Each of these 16 elements has a specific role in plant growth. A deficiency or an excess of any one will impair plant growth until the problem is corrected. Table 1 lists some symptoms of nutrient deficiency in woody plants Watching plant growth, testing the soil, and analyzing the plant tissue are ways to assess nutrient needs.

Fertilizers may be divided into two broad categories, natural and synthetic. Natural fertilizers generally originate from unprocessed organism 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" can also 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 by microorganisms decomposing 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 efflciently 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 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 less for slow-release materials. Several categories of slow-release nitrogen fertilizers are available in garden centers. 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 plant starter, high-nitrogen fertilizers to minor element formulations. Chelated iron is used extensively for prevention and control of iron deficiency in azalea, rhododendron, and other popular ornamentals.

The following are guidelines for nitrogen application as it relates to the 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 suffficient to maintain plant vigor.

• • Plants growing in a restricted root zone should not receive large amounts of nitrogen, because restricted roots cannot support excessive top growth.

Nitrogen can also be added before planting. Inorganic forms, however, are not recommended at this time, because they leach easily, and at high levels 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 effficiently when they are actively growing 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.

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, preventing plant use. 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. The drill holes should be placed in concentric circles or in a grid system around the main stem beginning 3 to 4 feet from the main stem and extending beyond the drip line. Space the holes 2 feet apart and drill them 6 to 9 inches deep. The recommended rate of fertilizer for the area should be uniformly distributed among the holes and is 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 okay to place fertilizer on top of mulch in landscape beds.

Fertilizer Stakes or Spikes. 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 best for 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, yellowing of plants caused by 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 apply only what your plants will use. The care you take to learn about plant needs will pay off in healthy plants and a protected water supply. For most fertilization information, consult the North Carolina Agricultural Chemicals Manual. The manual is available from the Department of Agricultural Communications at North Carolina State University (919-515-3173). Call for cost and ordering information.

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