Nursery Crop Science
Using Compost in Landscape Beds and Nursery Substrates
Prepared by: T.E. Bilderback and M.A. Powell
Extension Horticulture Specialists
Published by: North Carolina Cooperative Extension Service
Publication Number: AG 473-14 Last Electronic Revision: March 2007
Compost has been used by gardeners in backyard landscaping and gardening for many years. The benefits are substantial, especially when compost is used on the compacted, nutrient-deficient soils common in many urban areas. Commercial growers and professional landscape contractors, on the other hand, have usually relied on peat moss or pine bark as a soil amendment or a medium in which to propagate or pot plants. The main deterrent to the commercial use of compost has been the lack of a reliable supply of a consistent, fully stabilized product. The nutrient content of compost and its air and water relationship can vary widely. If current technical composting guidelines are followed and the process is closely monitored, however, it is possible to produce uniform compost that the horticulture industry will find acceptable. That industry represents a tremendous market for properly composted materials in North Carolina.
Incentives for Composting
Recent federal and state laws required that the amount of material deposited in North Carolina landfills be reduced by 25 percent by June 30, 1993. To help meet that requirement, North Carolina passed a law that prohibits depositing organic yard wastes such as leaves, grass clippings, or tree trimmings in the state's landfills. (Prior to passage of the law an estimated 20 percent of all landfill deposits consisted of organic yard wastes). To comply with this law, many municipalities have been forced to develop composting and recycling facilities. These facilities represent a significant cost to the town or county, some or all of which must be recovered by the marketing of the composted products. To ensure a market for these products, alternative uses must be developed for composted yard wastes, municipal solid waste, agricultural wastes, animal wastes, food processing wastes, and any other industrial material that can no longer be sent to a landfill but that can be composted. If the compost is reasonably priced and of uniformly high quality, it may find widespread use in the horticulture industry.
Mulch or Soil Amendment?
Some yard wastes, such as wood chips, are very difficult to compost fully and are therefore not suitable for incorporation into the soil. However, these materials can be used as mulches to modify soil temperature and reduce moisture loss in natural areas and informal landscapes. Shredded leaves can also be used in this manner. These materials should not be incorporated into the soil like stabilized composts. Incorporating unstabilized organic matter often creates problems such as depletion of soil nitrogen, leading to nitrogen deficiencies in plants.
What is "Good" Compost?
Most of the compost currently being used by the landscape and nursery industries is classified as unrestricted grade. This material includes yard, agricultural, and silvicultural wastes plus untreated wood products that have been treated by pathogen-free requirement procedures (PFRP)-that is, maintained at 131°F (55°C) for 48 to 96 hours. This treatment ensures that the compost is free of disease-causing organisms. Municipal solid waste (MSW) compost or any compost that contains sewage sludge must also meet the requirements for unrestricted grade. These requirements are stated in Section .1407 of the North Carolina Solid Waste Compost Rules, "Classification/ Distribution of MSW Compost Products." All MSW composts must be subjected to the pathogen-free requirement procedures. In addition, MSW compost is classified by its physical and chemical properties and degree of stabilization. The compost must be marketed as fresh if the organic matter content has been reduced only 20 to 40 percent during composting. It is considered semimature if it has undergone a 40 to 60 percent reduction in organic matter and mature if the organic matter reduction is greater than 60 percent. For all three classifications, the maximum particle size is 1.0 inch, and the maximum proportion of inert solids is 6 percent of the dry weight. Another requirement for unrestricted grade MSW compost is that its electrical conductivity, often referred to as its soluble salts level, must measure less then 10 millimhos (mmhos) per centimeter (mmho/cm) dry weight unless the product is marketed as a fertilizer with a guaranteed fertilizer analysis. A mho is a measure of conductivity. It is the reciprocal of an ohm, which is a measure of electrical resistance. A millimho (mmho) is one thousandth of a mho. Conductivity is measured by an instrument called a solubridge or conductivity meter. More information can be obtained from the Solid Waste Section of the North Carolina Department of Environment, Health, and Natural Resources.
Nutrient Content of Compost
As one might expect, the nutrient content of compost is quite variable. It is affected greatly by the raw materials that are composted. Typical yard wastes, such as leaves or tree bark, may contain less then 1 percent nitrogen and phosphorus (dry weight), whereas animal wastes may contain nearly 2 percent nitrogen and even higher percentages of phosphorus and potassium. Most compost also contains as much as 2 percent calcium. The amount of time that the compost is allowed to "cure" after the initial composting process also affects the final nutrient content. It is a good idea to have compost analyzed before using it. This precaution ensures that landscape plants will not be injured and that there will be no negative environmental effects related to over-application of nutrients.
Screening and Testing for Stability
Compost should be screened before use. Passing the compost through l/2-inch mesh removes hazardous materials such as glass or metal objects that are sometimes collected with the organic matter. It is also imperative that the compost be stable before it is applied in a commercial landscape or nursery. If the producer has had the compost tested for stability, the results of those tests will indicate the odor potential. If the compost has not been tested, three simple tests can be applied in the field to gauge its stability:
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Put a handful of the material in a plastic bag and seal it for 24 hours. If it does not have an offensive smell when opened, it is probably stable.
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None of the materials in the compost should be identifiable. The compost should appear and smell like rich, organic earth.
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If a small pile (3 cubic yards) of compost does not heat more than 20°F above the temperature of the surrounding air in 24 hours, it is probably stable.
Before using compost, be sure to study a copy of any soil or waste chemical nutrient analyses, pesticide and heavy metal analyses, and stability tests that the producer of the compost performed. Items to look for in the results of soil and waste nutrient analyses are pH, soluble salts level, and percentages of all elements, especially nitrogen, phosphorus, potassium, calcium, and magnesium. Most stabilized composts have a pH of about 7.0. If the compost contains animal manure, the soluble salts level can be relatively high (4.0 mmhos/cm or higher when measuring a solution of two parts water and one part compost). This reading would indicate that the compost should be leached (rinsed) before use until the conductivity measures no more than 2.0 mmhos to prevent damage to ornamental plants.
Information on the amounts of metals such as zinc, iron, lead, nickel, and cadmium in the compost should also be studied. Metals content should be well below the threshold values shown in Table 1.
Table 1.
Parameter |
Maximum Allowable Content |
|---|---|
| Mercury (Hg) | 10.0 |
| Cadmium (Cd) | 10.0 |
| Nickel (Ni) | 200.0 |
| Copper (Cu) | 800.0 |
| Lead (Pb) | 250.0 |
| Chromium (Cr) | 800.0 |
| Zinc (Zn) | 1,000.0 |
| Total polychlorinated biphenyls (PCBs) | 2.0 |
Application Rates for Landscape Use
As a soil amendment, compost is thought to enhance the physical, chemical, and biological properties of soils. In clay soils, organic materials such as compost and pine bark increase drainage and air space. Compost tends to raise pH and increase biological activity in the soil. In sandy soils, compost increases the ability to hold moisture and nutrients. Landscapers find that all of these characteristics help ornamental plants become established and develop a good root system. Table 2 gives suggested application rates. For landscape use as a soil-incorporated amendment, animal waste compost should be applied in a layer 1/2 to 3/4 inch thick on the soil surface, then incorporated. This amount is approximately equal to 880 to 1,300 pounds per thousand square feet. Yard waste compost can be applied at a higher rate because it contains fewer nutrients. A 2- to 3-inch layer can be spread over the soil surface, then incorporated into the root zone. This amount equals approximately 2 to 3 tons per thousand square feet. The weight of the finished compost is influenced by the source of the material and the moisture content. After the material has been incorporated, it is always advisable to sample the soil again to determine future nutritional needs.
Use in Nursery Potting Substrates
Organic materials that have been properly composted are very acceptable as components of nursery potting mixes. Several nursery crops have shown favorable responses to compost. For container use, be cautious and use approximately 10 percent compost by volume in pine bark mixes. The compost should be considered a substitute for peat moss and sand. It is good to subject the medium to a soil test before adding fertilizer. Addition of minor elements will probably be recommended. For container production, the use of slow-release fertilizers is also recommended. Dolomitic limestone should be omitted or reduced to no more than 3 pounds per cubic yard of potting mix. The soluble salts level and pH should be monitored during the growing season. Before using compost on any nursery crop, it is wise to establish a small test area to determine the material's suitability for the particular ornamental species.
Table 2.
| Type of Use | Suggested Rate |
|---|---|
| Container media | 10% (by volume) of medium |
| Landscape beds and nursery field production | |
| Yard waste compost | 2 to 3 inches incorporated (2 to 3 tons/1,000 sq ft) |
| Animal waste compost | 1/2 to 3/4 inch incorporated (880 to 1,300 pounds/1,000 sq ft) |
Because North Carolina has a large poultry industry, large amounts of turkey litter is composted, and the compost is readily available. North Carolina State University tests have found that turkey litter compost has high electrical conductivity (soluble salts level) and a pH range from 6.0 to 6.2. These values are slightly higher than those of commercial potting mixes. Mixes containing approximately 10 percent (by volume) of turkey litter compost initially contain higher amounts of nutrients and particularly higher phosphate levels than currently available potting mixes. The greatest advantage of using animal waste composts may be that phosphate levels remain within the suggested range longer during the growing season. This factor may be important in flower bud development in late summer and fall for nursery crops such as hybrid rhododendron.
Summary
As water quality and solid waste disposal regulations become stricter, the amount of organic material being composted will increase. The landscape and nursery industries in North Carolina are prime candidates for using these products, but only if the materials produced meet high quality standards and can be used in an environmentally responsible manner. The incorporation of composted organic materials into soil and nursery media will provide environmental and economic benefits.
Additional Information
For more information on composting, obtain the following North Carolina Cooperative Extension publications from your county Extension Center.
Soil Facts: Using Municipal Solid Waste Compost. North Carolina Cooperative Extension Service publication AG-439-19.
Composting for Home Gardens. North Carolina Cooperative Extension Service publication AG-467.
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
AG-473-14
