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Harry A. J. Hoitink, The Ohio State University, Ohio Agricultural Research and Development Center, Plant Pathology; Daniel A. Herms, Pierluigi Bonello, |
Fresh vs. Composted Organic Matter
Fresh plant materials often have negative effects on plant growth and/or health for some time after application. For example, fresh straw mulch increases water retention in heavy soils, immobilizes nitrogen, and results in poor growth. It may also increase Phytophthora root rot. Fresh ground wood has similar negative effects. Another problem with fresh materials is that beneficial microorganisms which suppress plant diseases do not express their beneficial effects because of the excess of free nutrients. Therefore, pathogens grow and diseases develop.
In contrast, ground wood that has been stabilized by composting has beneficial effects. It improves plant growth and can also provide biological control of root rots. The same results have been obtained with composted tree barks and several other types of composts, but many factors must be considered to provide consistent effects. When composts are to be incorporated into soil, the stability of the organic matter must also be considered.
Why do fresh woody mulches have these negative effects on plant health? Fresh materials incorporated into soil release sugars and other readily available nutrients as they first begin to decay. This stimulates plant pathogens such as Rhizoctonia and Pythium spp. that cause damping-off of seedlings. The Armillaria fungus, which can kill mature trees, and nuisance fungi also are stimulated directly by application of fresh wood.
Fresh organic matter that is decomposing rapidly binds water, making it "slippery" when wet. Heavy soils that are low in organic matter, when mulched with these fresh materials, can remain too wet in the spring. This aggravates root rots caused by Pythium and Phytophthora spp. In dry climates and on soils covered with a layer of duff, as in forest litter, this does not occur.
Organic matter acquires beneficial characteristics as soon as it becomes partially decomposed and soil microorganisms begin competing for nutrients. Pathogens now are suppressed or killed and beneficial microorganisms thrive, including mycorrhizal fungi. The structure of the soil is also improved by partially decomposed organic matter. This results in better water retention under dry weather conditions and better drainage during periods of high precipitation, which in turn leads to root rot suppression.
Soil fertility is also improved by partially decomposed mulches or composts. As organic matter decomposes in or on the soil surface, nutrients are released and fulvic and humic acids are formed. Fulvic and some humic acids remain dissolved in soil water early in the decomposition process. These acids chelate trace elements such as iron, zinc, manganese, and copper, and improve their availability even at high soil pH. This is one reason why composted manures and sludges "green up" plants on some alkaline soils. These beneficial effects usually do not last more than one or two years.
After the organic matter in soil has fully decomposed, fulvic acids are no longer produced and iron deficiency due to high pH occurs again. Highly stabilized sources of organic matter, such as peat or organic matter in muck soils, or in soils low in organic matter, do not provide these same beneficial effects. Root rot pathogens typically cause losses in such soils and peat mixes unless pesticides are used.