Introduction
The preparation of compost
Optimum conditions for composting
Testing for compost maturity
Testing for compost quality
The use of compost in nurseries
Disease suppression
Quality of peat moss
For more information

Introduction

     Peat moss and pine bark in container mixes, manure and yard waste as field amendments, the use of composted material is widespread in the nursery industry.  However, the quality of products is variable and there are frequent reports of crop damage from poor media material.

     Since 1995, CropHealth Advising & Research has worked with the composting operation at Byland’s Nurseries, in Westbank.  Plants that are unsold or of lower quality are composted and 20,000 cubic yards of finished material is produced every year for use in containers and in the field.

     This presentation will review the preparation of compost, its use in nursery production and the research underway around North America on using composts for disease control.

The preparation of compost

     Composting is the biological decomposition of organic waste under controlled conditions.  The large molecules are broken down into simple molecules that can be utilized for plant growth.

     Plant residue can be composted in different ways, but one common method is to pile the material in windrows, about 10 feet high and 15 feet wide.  The process of composting usually occurs in three phases:

     - An initial hot phase of 1 or 2 days, during which the smaller material is rapidly degraded;

     - Followed by a few months of degradation by micro-organisms, a process generating hot temperatures;

     - A final curing phase when the temperature declines and the material is colonized by other microbes.

     At Byland’s Nurseries, we have found that controlling temperature and moisture is very important to obtain a mature compost within a reasonable time.  The temperature is maintained at 40 to 60^oC to stimulate microbial activity and to kill disease pathogens.  The pile is turned 3 to 5 five times over a 4 to 8 month period to aerate the center and control the temperature.



     The moisture should be maintained between 40 to 60% for optimum composting conditions.  With the hand-feel method, the material should feel moist, ball up easily but not release water.  A pile that is too dry or too wet will not compost properly, anaerobic microbes will flourish, and a foul odour will soon travel through the neighbourhood.

Optimum conditions for composting

     There is a consensus among scientists that controlling the carbon to nitrogen ratio (C:N) is the key to quick composting, odour control and quality of the finished product.  A this point, the best method to calculate the C:N ratio is through a regular laboratory soil analysis.

     Carbon is the main diet for the microorganisms responsible for composting, and they also scavenge available nitrogen in the process.  A high C:N, typical with dry leaves or sawdust, will result in a slow composting process.   A low C:N, from grass clippings and tree trimmings, will result in a loss of nitrogen and an odour problem.  The pile should have a balanced mix of rapidly decomposing materials and slowly decomposing materials to ensure adequate microbial activity.

Testing for compost maturity

     Once finished, it is important to test the compost to ensure it is a stable organic mass with reduced microbial activity.  A compost that is biologically active will hinder plant growth by tying-up nutrients or by releasing noxious gases.

     The simplest and most accepted procedure is the germination test.  Seed trays are prepared with the finished compost and with a standard potting soil.  Cress or radish seeds are used since they germinate rapidly and are affected by high salts.  About 30 seeds are planted in each material and germination and growth differences are compared between growing medias after 7 days.

     The Solvita maturity test, which measures microbial respiration, is also very accurate and has been used extensively at Byland’s.  Much like pH paper, this test provides a color rating of compost maturity with recommendations for container and field use.

Testing for compost quality

     A laboratory analysis will indicate the nutrient value of the compost and the fertilizers that should be added.  Typically, pine bark does not release a large quantity of nutrients and must be mixed with a complete fertilizer.  On the other hand, a compost of leaves is rich in potassium and sewage sludge is rich in nitrogen.  Therefore, these two composts must be supplemented with different fertilizers.

     Testing for salt is also important: the composting of any product will generally result in a moderate to high EC level (Electrical Conductivity).  High salts in a container mix can trigger root damage, water stress and poor plant performance.  Water leaching immediately after potting will lower the EC level.

     The compost material is then blended into the potting mix and tested for aeration porosity and water-holding.  It is difficult to achieve a consistent quality with composts and the blend may need to be slightly altered to get the desired result.  Aeration porosity should be at least 20% for most crops and 25% for crops sensitive to Phytophthora root rot.

The use of compost in nurseries

     Peat moss, for many years a standard in container mixes, is becoming scarcer and more expensive.  It is being replaced with less expensive composted materials such as bark, green waste or sewage sludge.  Pine bark is now widely used in floriculture production since it provides good aeration porosity for root growth.  Many growers use a 4:1 mixture of composted bark and peat as the organic component of their media.

     Some nurseries are using composted green waste instead of composted bark.  In California, for example, Monrovia Nursery reports that 40,000 cubic yards of clippings and prunings is composted every year and used in container media, while Flynn Rainbow Nurseries is composting 15,000 cubic yards with most of the finished product going into container media.

     When using composted green waste, each grower should design a potting mix that will provide the desired results.  Some plants require more aeration porosity or an acidic pH, and a compost product will vary from one region to another.  Some growers of bedding plants use a mix of 25% compost, 50% peat moss and 25% perlite, while some growers of woody ornamentals report using an equal volume of compost, sand and peat moss or pine bark.

     At Byland’s Nurseries, a field trial was set-up in 1995 to compare various potting mixes for ornamental shrubs.  We found that under our conditions, a mix that incorporates 25% to 50% composted yard waste resulted in excellent plant growth.  Many studies have since been published with similar results.  In 1996, R.C. Beeson, at the University of Florida, reported that Rhododendron liners grew best in a mix of 40% yard compost, 50% pine bark and 10% sand.

     Composts are also used in the field to supply organic matter, increase microbial activity and “revive” the soil.  Researchers in vegetable and grain crops are finding that compost application will not give a higher yield unless supplemented with nitrogen fertilization.  Compost is usually incorporated at 2 to 10 tons per acre or top-dressed at up to 50 tons per acre (1-inch thick layer when moist), the equivalent of 4 cubic yards per 1,000 ft².

     At Byland’s Nurseries, the application of composts in the field has been an excellent way to reload the organic matter content, so important for healthy root growth.  In one field, compost applied in 1996 raised the organic matter level from 2.8% to 5.3%, an effect that was still present in 1998.

Disease suppression

     This is a new area of research and, so far, the results are impressive:

     - At Ohio State University, composted pine bark reduced Phytophthora root rot in container production and suppressed Fusarium wilt in cyclamen production.

     - At the U.S. Department of Agriculture in Maryland, amending potting mixes with composted animal manure suppressed damping-off caused by Pythium and Rhizoctonia.

     - More recently, researchers in Spain have reduced root-knot nematodes in field tomato and pepper production with a top-dress application of composted chicken litter.

     The ability of compost to suppress diseases is linked to the beneficial bacteria and fungi that colonize the material during the curing phase.  These microorganisms will compete in different ways with the disease pathogens and offer protection to the plant.  The composting process must be done properly to obtain a disease-suppressive product: a poorly composted product will actually increase the risk of disease, while an over-mature product has little microbial activity.

     Various mechanisms appear to be responsible for disease-suppression:

     - In most composts, the beneficial microorganisms will compete for nutrients or produce antibiotics that suppress the growth of pathogens causing Pythium and Phytophthora root rot.

     - Less frequently, other microorganisms will colonize the material and parasitize the pathogens responsible for Rhizoctonia damping-off.

     -  Finally, recent research indicates that “systemic acquired resistance” may be at play, where plants grown in compost have a higher level of an enzyme associated with disease resistance.

     This field is promising and future research will provide guidelines on using composts for disease control.  Nursery growers should expect that in future years, “inoculated” composts will become commercially available to prevent specific diseases in specific crop situations.

Quality of peat moss

     Sphagnum peat is a primitive plant which grows in a bog.  The location within the bog from which the peat was extracted has a strong influence on the development of root diseases during production.

     - Dark fine peat, harvested from deeper layers in the bog, is low in microbial activity and often conducive to root diseases such as Pythium.

     - Alternatively, light fibrous peat, harvested from the top 1.2 meter of the bog, has the potential to reduce root rots.  Researchers have found the light peat comes with a microflora which competes for nutrients with the pathogen Pythium.

     The reduction in Pythium can be expected for 6 to 12 weeks, and up to 6 months in some cases.  This form of disease suppression is dependent on the ligno-cellulosic substances: once they are decomposed, the beneficial microorganisms decline in activity and the pathogens can recover.

For more information

   - Canadian Council of Ministers of the Environment, 1996, Guidelines for Compost Quality
Available for $6 from the Manitoba Statutory Publications, 200 Vaughn St., Winnipeg.
   - B.C. Ministry of Agriculture, Fisheries and Food, 1996,  B.C. Agriculture Composting Handbook.
Detailed and very informative.  Available for free from the Abbotsford office.
   - Northeast Regional Agricultural Engineering Service, 1992,  On-Farm Composting Handbook  Publication NRAES-54, edited by Robert Rynk
An excellent reference publication.  Available for $20(US) from 152 Riley-Robb Hall, Ithaca, NY, or access on the internet at http://www.cfe.cornell.edu/compost/Composting_homepage.html under ‘resources’.
   - H.A. Hoitink, D.Y. Yan, A.G. Stone, M.S. Krause, W. Zhang, W.A. Dick,  Natural Suppression  American Nurseryman, October 1997.
A good review of the topic in a style written for growers.
   - H.A. Hoitink, A.G. Stone, D.Y Han, 1997,  Supression of Plant Diseases by Composts  HortScience, 32(2): 184-186.
A technical paper on the microbial activity in composts, including sphagnum peat.
   - H.A. Hoitink, M.A. Rose, R.A. Zondag, 1997,  Properties of Materials Available for Formulation of High-Quality Container Media.
Special Circular 154.  Available from Ohio State University Extension.
   - The internet Web site http://www.cfe.cornell.edu/compost/Composting_homepage.html provides information and links to other sites.