A beautiful article on soil 

We didn’t write this one, but an old friend did.

By Alex Stone, Dept. of Horticulture, Oregon State University

Soil health, is defined as: “the continued capacity of soil to: function as a vital living system within ecosystem and land-use boundaries; sustain biological productivity; promote the quality of air and water environments; and maintain plant, animal, and human health” (Pankhurst et al, 1997).  Soil health is vital to crop production and agro-ecosystem function

An often overlooked aspect of soil health is the ability of the soil to suppress plant diseases. One way to improve the soil’s potential to suppress plant diseases in field soils is through cover cropping and the addition of organic residues (e.g. manures, composts, or industrial organic wastes such as paper mill residues).  Organic residue-amended field soils have been shown to suppress a variety of soil-borne diseases.

The addition of organic residues to field soils can reduce disease by increasing the numbers and activities of beneficial organisms (e.g. organisms with potential for biological control of pathogens).  Beneficial soil microorganisms can directly inhibit the pathogen through competition for carbon and other nutrients (e.g. iron), competition for space, antibiotic production, and direct parasitism.

Composts have been shown to suppress root diseases caused by Pythium spp., Rhizoctonia spp., Phytophthora spp., and Fusarium spp. in a wide variety of plant species in containers (Hoitink et al, 1991). 

Organic matter-mediated suppression of these fungal diseases is potentially due to a variety of mechanisms: suppression of pathogen germination, destruction of pathogen resting structures and mycelia, competition for space and/or nutrients, and induction of systemic resistance in the host plant. There is also evidence that similar phenomena occur in organic matter-amended field soils

Manure additions and cover cropping suppressed Phytophthora root rot of avocado in commercial orchards in Australia (Malajczuk, 1983). Composted brewery waste applications have been shown to increase bean emergence, reduce snap bean root rot, and increase yield in New York field soils (Abawi and Widmer, 2000).  Grapevines from vineyards employing cover cropping and composting have been shown to exhibit significantly less root damage (due to Fusarium oxysporum and Cylindrocarpon spp.) than grapevines grown in vineyards in which these practices are not employed (Lotter et al, 1999).

Cover crops have been shown to reduce, increase, or have no effect on disease incidence depending on the host crop, cover crop, pathogen, and other factors.  A cover crop can act as a host for a pathogen, resulting in an increase in pathogen populations and disease incidence in subsequent host crops.  In other cases, cover crops can increase the populations of beneficial organisms and reduce disease incidence. Potato growers in eastern Washington are growing white mustard cover crops for suppression of Verticillium wilt (http://grant-adams.wsu.edu/agriculture/covercrops/green_manures/index.htm).

Snap bean root rot severity was shown to be reduced in container trials (with field soils) by the incorporation of ryegrass, oats, Trudan 8, grain rye, wheat, crown vetch, and rapeseed (Abawi and Widmer, 2000). 

  Plants may change the composition of the soil microbial community through selection of the microbes associated with their plant tissues – roots, leaves and stems.  Fusarium wilt of palm (caused by Fusarium oxysporum) has been shown to be suppressed by the growth of an understory leguminous cover crop.  This is thought to be due to an increase in the numbers of non-pathogenic Fusarium oxysporum and other Fusarium spp. in the soil, which compete with the pathogen for space and nutrients. 

Suppression was much stronger after 230 days of cover crop growth than it was after 49 days (Abadie et al, 1998). Sturz and Christie (1998) have shown that red clover harbors bacteria in its tissues that improve the growth of subsequent potato crops.  When red clover is grown in rotation with potato, the growth and yield of potato is enhanced.  Some of these red clover and potato–associated bacteria have also been shown to be active against potato tuber pathogens such as Fusarium sambucinum and F. oxysporum (Sturz et al, 1999).

Some crops, such as mustard family plants, can actually destroy pathogen propagules immediately after incorporation (Lewis and Papavizas, 1971; Muehlchen et al, 1990). These plants contain glucosinolates, which break down during soil incorporation into chemicals that have detrimental effects on the survival of fungal mycelia and resting structures such as sclerotia and chlamydospores.

A field trial was initiated in 1998 at the University of Wisconsin to determine the impact of organic amendment quality and quantity on the severity of common root rot of snap bean. Raw and composted paper mill sludge (PS) were applied because PS is a large volume industrial waste stream currently land-applied in WI on sandy soil processing vegetable acreage. Both raw and composted paper mill sludge strongly suppressed common root rot (Stone et al, 2003)

In recent work in Oregon, we have shown that fresh and composted dairy manure solids amended to field soils suppress root rot of sweet corn, and there is a strong relationship between disease suppression and microbial activity (Darby, 2003).  However, suppressiveness only lasts for one growing season, so manure must be applied each year (Darby, 2003). 

References:

• Abadie, C., V. Edel, and C. Alabouvette, 1998.  Soil suppressiveness to Fusarium wilt: influence of a cover-plant on density and diversity of Fusarium populations. Soil Biol. Biochem. 30:643-649.
• Abawie, G.S. and T.L. Widmer, 2000.  Impact of soil health management practices on soilborne pathogens, nematodes and root diseases of vegetable crops.  Appl. Soil Ecol. 15:37-47.
• Darby, H.M. 2003. Soil organic matter management and root health. Dissertation, Oregon State University, Corvallis.
• Doran, J.W., and M. Safley, 1997.  Defining and assessing soil health and sustainable productivity.  In: C.E. Pankhurst, B.M. Doube, and V.V.S.R. Gupta (eds.), Biological indicators of soil health.  CAB Intl.  New York, NY.
• Hoitink, H.A.J.
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, Y, Inbar, and M.J. Boehm, 1991.  Status of compost-amended potting mixes naturally suppressive to soilborne diseases of floricultural crops.  Plant Dis. 75:869-873.