Harry A. J. Hoitink
Wei-Zheng Zhang
David Y. Han
Alexandra G. Stone
Matthew S. Krause
Warren A. Dick
Nursery operators and landscapers have recognized for centuries that composts can improve plant health. Many factors must be controlled, however, to obtain consistent effects.
The degree to which the raw material is heated during composting affects the potential for kill of pathogens and weed seeds. The degree to which the organic matter has been stabilized plays a role in disease suppression and plant growth. Furthermore, composts do not always become colonized naturally by beneficial microorganisms, and this can lead to failures.
Finally, the concentration of salts and the quantity of nitrogen released by composts plays a role. These factors are briefly reviewed here. General information is provided about composts widely available to the nursery industry and how best to use such products.
Most beneficial effects induced by composts are due to the activities of microorganisms in the rhizosphere, the area of soil immediately surrounding the roots. Some of these microorganisms produce plant growth hormones and stimulate plant growth directly. Others produce natural chelators called siderophores that, along with water-soluble humic substances in composts, keep iron at a high concentration in available form to plants in soil, even at pH 7.6. This probably explains why growers using composted biosolids can produce "acid-loving" plants such as azaleas at pH 7.4 in container media consisting of aged pine bark (60%), fibrous Sphagnum peat or composted long-grain rice hulls (20%), composted biosolids (10-15%), and silica sand, in regions where the irrigation water is high in carbonates. This is very difficult to do in peat mixes in areas with high carbonate water, because trace elements limit growth as the pH increases and their solubility decreases. The siderophores produced by beneficial microorganisms in compost-amended mixes reduce this problem. The soluble humic substances are abundant in manure and sludge composts.
Beneficial microorganisms that control diseases are known as biocontrol agents. Disease control obtained with this micro-flora is attributed to four mechanisms. The first is competition for seed, root, or leaf exudates (sugars, etc.) that leak out of seeds during germination or out of root tips as plants grow through the soil. Pathogens swimming to these sources of nutrients must compete with this beneficial micro-flora in the infection court. This reduces infections and therefore disease. Some biocontrol agents produce antibiotics that are effective against pathogens. Yet another group parasitizes pathogens. Micro-arthropods such as springtails and mites actually search out pathogen propagules in soils and devour them. The fourth mechanism involves the induction of systemic resistance in plants by microorganisms present in composts. A few beneficial microorganisms can induce all four mechanisms.
Ohio Agricultural Research and Development Center (OARDC) researchers have shown recently that some microorganisms colonizing roots in compost mixes actually activate biochemical pathways in plants, leading to resistance to root as well as foliar diseases. This mechanism explains the often-heard statement that plants on "healthy organic soils" are more able to resist disease. It has now been proven that compost can indeed support such effects. Details of this work are described here.
Most of the Sphagnum peat sold for use in container media is of a decomposition level that cannot support the growth and activity of beneficial microorganisms. OARDC research has determined that such peat mixes are conducive (no suppression) to all diseases tested. Figure 1 shows plants produced with one half of their roots in one mix and the other half in a second. The plant in the center had both sides in a Sphagnum peat mix (H4 on the von Post decomposition scale). The right side was in a mix infested with Pythium, a root rot and damping-off pathogen. Note that the roots of the center plant with both sides in the peat mix were small relative to the others. The rest rotted. The plant on the right had both sides in a composted pine bark mix with Pythium on the right side. Note the healthy root system. The plant on the left shows the systemic effect. The right side was in peat, also with Pythium, but the left was in the compost mix. When the compost was sterilized, it did not control the disease. Somehow, the microflora in the compost seemed to induce factors in the roots on the left that transferred to roots on the right in the peat mix which made the plant resistant to root rot.
Figure 2 shows an example of control of a fungal disease of cucumber (anthracnose) on the foliage of a plant produced in a composted pine bark mix. The plant on the left, where the disease was much more severe, was grown in an H4 peat mix. Some bacterial diseases in the foliage of plants can also be controlled in this way with several types of composts in the mix. This is an important finding because good control procedures for diseases such as fireblight, bacterial blight of lilac, Xanthomonas leaf spot on ivy, etc., are not available. It is important to stress, however, that fertility affects the severity of many bacterial diseases, and this may well prove very important to this type of disease control.
Several publications have shown recently that microorganisms on roots can induce this systemic response in plants. OARDC research has identified some of the microorganisms in composts that can induce this effect. Plants produced in any of several compost-amended mixes tested so far have higher concentrations of an enzyme related to host defense mechanisms. Plants grown in the peat mix that does not provide biological control do not have this elevated level of enzyme activity. In summary, this work shows that plants grown in substrates rich in biodegradable organic matter support microorganisms that induce systemic resistance in plants. These plants have elevated levels of enzyme activity relative to disease control and are better prepared to defend themselves against diseases.
It is important to realize that composts usually do not provide total disease control. When all conditions are favorable, composts offer the potential to reduce many diseases to below critical threshold levels. Pythium and Phytophthora root rots are among the most easily controlled diseases. Some foliar diseases such as Phytophthora die-backs may not be controlled at all, particularly when high fertility levels are maintained in the crop.
Some landscapers utilize fresh wood chips as mulch. The question is, can this lead to spread of diseases. The answer is yes, and such mulches also increase the activity of pathogens already on the site.
C. Ash, formerly at the University of Minnesota, has shown that fresh mulch, prepared from maple trees that died from Verticillium wilt, killed tomatoes mulched with this material. Verticillium was recovered from the dead tomato plants. This study demonstrated that pathogens in freshly ground infected trees can indeed cause problems in the landscape. Damping-off of bedding plants has been observed in Ohio landscapes mulched with fresh woody materials. Avocado trees mulched with fresh crop debris also suffer more from Phytophthora root rot. Rhizoctonia damping-off also occurs in bark mixes used during propagation even though Phytophthora root rot is controlled. How can these problems be avoided?
First of all, pathogens, insect egg masses, weed seeds, and so forth are killed when temperatures in compost piles exceed 130 deg F for just a few days. Turning of piles so that all parts are exposed to high temperature ensures that pathogen destruction occurs. Those that are not killed outright are weakened and are more susceptible to parasitism. Many technical articles support this statement.
It is important to stabilize organic matter in mulches. Organic matter must be stable enough so that plant pathogens cannot utilize it directly as a food base. Otherwise, the mulch actually increases the population of the pathogen. Rhizoctonia is an example of a plant pathogen that can actually grow on fresh mulches.
Another reason for partial composting of mulches is that some beneficial microorganisms grow strictly as saprophytes (on dead organic material) in fresh mulches. Once the mulch is partially decomposed, these beneficial microorganisms must now compete for nutrients at this stage of decomposition. Some of the beneficial microorganisms then produce several types of competitive products that lead to pathogen kill or inhibition. This does not occur in fresh wastes. Some Trichoderma isolates serve as examples of a group of beneficial microorganisms that behave in this manner in fresh woody materials (bark as well as sawdust and chips). In conclusion, application of fresh residues to crops or trees should be avoided when the crop is susceptible to disease. Fall or winter application avoids this problem.
What is the best method of composting fresh wood chip mulches to reach these beneficial effects? After just a few weeks of compost-ing, the organic matter in materials examined so far is already stabilized enough for most diseases to be controlled. The best way to accomplish this quickly with fresh ground brush is to enrich it with nitrogen. Add 1 lb. urea per cubic yard or grass clippings (10-20% by volume), composted sewage sludge (10-15% by volume), composted poultry manure (10-20 lbs. per cubic yard), or another source of nitrogen to decrease the carbon to nitrogen ratio to within the optimum range for composting. Be certain to add water to the pile to maintain a moisture content of 50-60% on a total weight basis. Ammonium volatilizes out of the pile as gaseous ammonia when it is too dry. The best procedure is to compost these materials for six weeks before use as a mulch. It should no longer give off ammonia odors then and should begin to smell like soil.
The procedures proposed here kill pathogens and adequately stabilize most materials for use as mulches. Depending on the material being composted, it may have to stabilize much longer before it is suitable for soil incorporation as compost.Very few beneficial microorganisms can survive in the high temperature part of compost piles. Most survive in the outer low temperature layer where they constantly re-establish their populations after turning of windrows if several factors are addressed. First, the moisture content must be above 35% in the organic matter fraction of composts for beneficial microorganisms to colonize the substrate. They actually grow as biofilms on the surface of organic matter, particularly if the moisture content is maintained above 45%. Dusty, dry composts and mulches become predominantly colonized by molds that cause a variety of problems. These problems can range from difficulties in wetting of the compost-amended soil or mix because fungal masses repel water, to inhibition of plant growth due to deleterious-to-growth microorganisms (minor fungal pathogens). Other fungi cause problems due to their large fruiting bodies, such as a number of Basidiomycetes that produce mushrooms.
Allowing composts to cure while maintaining a moisture content of 45-55% reduces the potential for these problems. Plant growth promoting bacteria and bacterial biocontrol agents naturally colonize such higher moisture content mulches and compost after peak heating because a thin layer (film) of water surrounds organic matter particles at this moisture content. Bacteria cannot readily colonize dry surfaces, whereas fungi thrive as long as the moisture content ranges from 15-34%.
When all factors are optimized, 20% of the compost batches tested still are somewhat deficient in natural biological control when the moisture content of the compost is kept above 40% on a total weight basis. To avoid this, composts must be inoculated with specific biocontrol agents. Commercial inoculants for compost consistently providing these beneficial effects are now being registered with the U.S. Environmental Protection Agency (EPA). Mixtures of cultures are better than single strains, and broad spectrum control of soilborne as well as some foliar diseases should soon become possible in practice.
The readily biodegradable fraction of the organic matter in composts sustains the activity of biocontrol agents. Humic substances do not support this activity; they are too resistant to decomposition to support such activity. Research has shown that the population of beneficial microorganisms steadily declines as decomposition proceeds. Each material has its own properties in this regard. These trends for Sphagnum peat and cow manure have been characterized utilizing direct infrared spectroscopy. The concentration and chemistry of ligno-cellulosic substances, not the humic acids, determine this effect. Once these materials are decomposed, the beneficial microorganisms decline in activity, the pathogen population recovers, and fungicides must be applied for sensitive crops to remain disease free.
For light Sphagnum peat (H2 to H3 on the von Post decomposition scale), the beneficial effect lasts 6-12 weeks in greenhouse crops. In outdoor containers in hot weather, the length of time may be reduced 50% because of the higher temperature. Pine bark can support this effect six months to one year. However, pine bark aged in large piles where pyrolysis or fires occur behaves more like charcoal and offers little disease control potential, even though it still has ideal physical properties for use in container media to control root rots. Hardwood bark incorporated at 15% by volume lasts two seasons in Ohio. Composted sewage sludges last through two years (at 10-15% by volume in a mix). Composted rice hulls and coconut coir (husks) also have an effect, but will undoubtedly be shown to be short-term in nature.
Some knowledge is available from field studies also. In general, the same material should last longer in the field because soil temperatures are lower than those in containers. The best results are obtained in the field if the compost is applied as a mulch on the soil surface. Only a small fraction (5-10 dry tons/acre) of the compost should be incorporated into the soil. The remainder (25-50 dry tons/acre) should be applied on the surface after planting. An exception to this general rule is where compost is applied ahead of a vegetable crop planted for an early harvest. Soil temperatures remain lower in mulched soils, and this can set back early vegetable crops. The quantity and form of mineral fertilizers applied need to be adjusted in succeeding years to avoid overloading with nutrients.
Control of diseases with composts in field soils can be illustrated by the following examples. Composted hardwood bark mulch applied to apple trees at planting controlled Phytophthora collar rot through two years in a 1978-1981 Ohio State University field study where inoculum of the pathogen was used. In Ohio orchards, this mulch effect (4 gal./tree at planting) seems to last much longer. A recent study by Dr. Richard Funt from the Ohio State University Department of Horticulture and Crop Science revealed that composted yard waste (50 tons/acre) on strawberry maintained plant stand beyond three years whereas plants in control plots declined from any of several pathogens. Five-year beneficial results have been observed by growers utilizing bark instead of straw mulches. In western Australia, a composted manure suppressed Phytophthora root rot of avocado for well into the second year. Similar results are being obtained in orchards in southern California today. Much more work needs to be done in this field, however, before general advice can be provided.
Some composts immobilize nitrogen (N), but most composts available today release nitrogen. Some are consistently high in salinity (dairy manures) and others vary in salinity. An increasing number of compost producers with experience in this field realize that the composition of raw materials, the composting process as well as curing, screening, and more must be kept constant to produce a consistent quality product. Furthermore, soil analysis laboratories increasingly are capable of predicting the fertility values of composts. Nutrient inputs must be balanced against crop needs and the residuals in the soil.
Composted pine bark and peat, because of their resistance to decomposition, do not release or immobilize significant quantities of nutrients. Therefore, essential micro-nutrients must be incorporated into the mix. Composted hardwood bark immobilizes nitrogen early but releases various nutrients, including trace elements, later.
Composted sewage sludges available in Ohio release 25% of the total nitrogen in the first few months after utilization. Therefore, the incorporation rate needs to be adjusted to the fertility needs of the crop. Generally, this means not using more than 10-20% of this compost by volume in a mix, depending upon the crop's fertility need. Overloading with this nitrogen-rich compost may increase fireblight, Phytophthora die-backs, and Fusarium wilts. All trace elements are supplied adequately by composted sewage sludges, particularly in high pH irrigation water regions. Trace elements do not need to be applied.
Composted leaves also supply trace elements, not much nitrogen, and significant quantities of potash. Composted yard wastes supply some nitrogen, if prepared with grass clippings, and may contain up to 1% available potash. All of these composts provide phosphorus, calcium, and magnesium. Most do not have to be amended with lime, and addition of sulfur to lower the pH to 5.5 may not be necessary even though most laboratories recommend it. It makes a lot of sense to blend low nitrogen with high nitrogen composts. Mixtures of composted yard wastes and composted sewage sludges increasingly are preferred in several applications.
Two studies (Ohio State University and Israel) have shown that concentrations of available iron, zinc, and manganese are very high at a soil pH above 7.0 in composted cow manure or sewage sludge-amended mixes. As mentioned previously, studies have determined that plants, including azaleas, grow very well and without trace element deficiencies at pH 7.0 or higher in these mixes because microorganisms producing siderophores (natural chelators) and soluble humic substances keep them in solution.
All composts can be high in salinity. As composts mature, mineralization proceeds, and the concentration of salts increases. Because compost piles often do not leach, salts accumulate with time. Thus it is always important to monitor the conductivity reading of a new batch. Incorporate based on salinity limits, if needed, or apply composts well ahead of planting to allow for leaching. Manure composts, which are now becoming more widely available, will have to be monitored most closely for salinity problems.
During the early 1980s, composted sewage sludge was applied to soybeans in Ohio in an attempt to control Phytophthora root rot. The disease increased in each of four years when the compost was applied directly ahead of planting. However, in plots where the compost was applied three months prior to planting (February) or in the previous fall, the disease was controlled and the yield increased. The damage done by the compost could be mimicked by an application of salt (NaCl) directly ahead of planting. It is well known that Phytophthora and Pythium root rots are aggravated by salinity. These factors must be considered carefully in biological control of plant diseases.
The foregoing is a brief synopsis of some of today's knowledge of compost utilization relative to maintenance of plant health. Some references are suggested for further reading.
Rynk et al., 1992. On-Farm Composting Handbook. NRAES-54. 152 Riley-Robb Hall, Extension Service, Ithaca, NY, 14853-5701. 187 p.
Hoitink, H. A. J., Stone, A. G. and Han, D. Y. 1997. Suppression of plant diseases by composts. HortScience 32: 184-187.