Harry A. J. Hoitink, M. S. Krause, and Randy H. Zondag
Composts and mulches can be used successfully to improve tree vigor and health. Unfortunately, these organic amendments also can be applied in ways so that negative effects are the end result. Many factors contribute to success or failure in these soil organic management practices. This paper reviews the most critical factors that must be considered if mulches are to be used successfully.
Most fresh plant materials cause negative effects on plant growth and/or health for some time after application. For example, fresh straw used as mulch increases water retention in soils; immobilizes nitrogen, resulting in poor growth; and may also increase Phytophthora root rot. Fresh ground wood seems to have similar effects.
Composted yardwaste prepared from ground wood and grass clippings has the opposite effect. It improves plant growth, improves both drainage and water retention, and can provide biological control of Phytophthora root rot. The same results have been obtained with composted tree barks.
In gardening, the type of organic matter used must be considered also. Vetch plowed into soil as green manure increases Pythium damping-off of lettuce if the crop is planted within the first week after plowing. However, 10 days after plowing, when the green manure is fully colonized by soil microorganisms, the disease is suppressed and biocontrol prevails.
Why do fresh amendments or mulches have these temporary negative effects on plant health? Fresh plant tissues, incorporated into soil, release sugars, proteins, and other water soluble nutrients when they first begin to decay. This stimulates many plant pathogens as well as other soil microorganisms. Fresh organic matter undergoing high rates of decomposition also binds water, making it "slippery" when wet. The soil under mulch prepared from these materials remains too high in moisture content, and this can aggravate root rots.
As soon as the organic matter is partially decomposed and competition for nutrients begins among soil microorganisms, beneficial effects begin. Pathogens now are suppressed or killed, and beneficial microorganisms thrive, including mycorrhizal fungi. The structure of the soil is improved, which results in improved water retention under dry weather conditions and better drainage during periods of high precipitation. Soil fertility is affected also. While organic matter decomposes, nutrients are released and fulvic acids are formed. Fulvic acids are resistant to decomposition and polymerize to form humic substances in soils. Fulvic and some humic acids remain dissolved in water in soil early during the decomposition process. These acids chelate trace elements, such as iron, zinc, manganese, copper, etc., and improve the availability of these elements in soils. This is one reason why manures and sludges "green up" plants on some high pH soils.
Highly stabilized sources of organic matter, such as that in muck soils or peat, as well as humic substances in mineralized farm soils, do not provide these same beneficial effects. Pathogens typically cause heavy losses in such soils unless pesticides are used.
Composts prepared from yard wastes, manures, and sewage sludges tend to release significant quantities of nutrients for plant growth. They also may be high in salt content, which can present problems. These products need to be applied based on the fertility requirement for the crop. However, since the nutrients are released over several years, large amounts can be applied relative to the same amounts of nutrients in manufactured fertilizers. One half bushel of these products per tree gives positive effects on tree crops for the reasons given previously.
Composted manures and sludges contain large quantities of fine particles and tend to encourage germination of weed seeds. These types of materials should be incorporated into the soil during planting. They are not ideal when used as mulches. Coarser products should be used as mulches.
Hardwood bark and hammer milled pallets, etc., tend to consist of large particles, and these products immobilize nitrogen unless composted first. The best procedure is to nitrify this material with manure or composted sewage sludge (15% by volume), poultry manure (30 to 60 lbs/cubic yard), or urea (2 lbs/cubic yard). It should then be composted in windrows at temperatures of 120 to 140°F at 50 to 70% moisture content for six to eight weeks. This large-particle composted product is very effective, if applied as a mulch at a volume of one or more bushels per tree.
Composted yard wastes also enhance soil fertility and have provided some biological control of diseases caused by several soil-borne plant pathogens. The fine particles (less than one inch in diameter) screened out of composted yard waste make excellent soil amendments for top-soil preparation. The coarse fraction (greater than one inch in diameter) makes an excellent mulch that has long-lasting beneficial effects.
Fresh undecomposed materials and composts high in salinity must be applied in the fall or winter when pathogens and the crop are least active to allow for leaching and provide beneficial effects later. Composted sewage sludges and manures high in salinity, applied in the spring or summer when Phytophthora and Pythium are most active, often increase disease pressures rather than provide control. Application of these products in the late fall or mid-winter allows for leaching of salts and provides positive rather than negative effects.
Several producers of composts have learned to monitor and control the salinity and fertility effects of composts. It is possible today, therefore, to use these products beneficially at any time of the year and avoid negative side effects. The best approach is to blend high nutrient content materials with tree bark to provide long-lasting beneficial effects.
Most mulches need to be applied at a two-inch depth to provide weed control. Some landscapers apply mulches to a depth of four to six inches. Wood chips applied to a depth of four inches decrease the colonization of trees by mycorrhizal fungi as shown in a reforestation trial in Alberta. A two-inch-deep layer enhanced tree establishment and colonization by mycorrhizal fungi over the control in that work. Many reports have shown that mycorrhizal fungi are stimulated by organic amendments if the correct amount of material is applied, and this also promotes plant health and vigor.
Some mulches, as mentioned earlier, can be very high in nutrient content and salinity. Composted manures and yard wastes, for example, may contain up to 1% potash on a dry-weight basis. The nutrients in these materials must be considered or toxicity may result. Fertility guidelines must be followed for these materials.
The effect of a two-inch layer of composted hardwood bark lasts well into the third year. The lignin (dark material) and waxes in bark resist decomposition, and this is the reason for the long-term effect. Composted yard wastes break down much more rapidly because the principal material is cellulose, which decomposes readily. A two-inch-deep layer of such mulch lasted well into the third year on strawberries at The Ohio State University and lasted through three years in a mulch study on trees in nurseries.
Composted manures and green manures decompose even more rapidly and should be incorporated into the soil. The length of time that each product lasts depends on the chemistry of the original material and many other factors.
Mulches and composts, if used properly, provide beneficial effects through any of several mechanisms. It is best to apply composted products. Raw products should be applied in the late fall or winter. Do not apply more available mineral nutrients in the mulch than the amount required for the crop. Compost or manure analysis complete with soil analysis and crop need should form the basis for application rates. The frequency of application varies from crop to crop and product to product. It is most important to use these products when trees are first planted.