Thermal Components of Soil Solarization as Related to Changes in Soil and Root Microflora and Increased Plant Growth Response

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Abstract
Solarized and shaded (moist soil covered by transparent polyethylene film but protected from solar heating by sheets of gypsum wallboard) soils were compared for population densities of selected soil microorganisms. Solarized soils usually contained the fewest microorganisms, untreated control soils had intermediate population densities (P < 0.05). Greater plant growth occurred in solarized soils, and sometimes in shaded soils, than in untreated control soils. Plant growth increases in the field often were correlated inversely (P < 0.01 or 0.05) with decreases in soil population densities of several groups of bacteria and fungi. The percentage of colonies of Gram-positive bacteria exhibiting in vitro antibiosis against Geotrichum candidum increased nearly 20-fold in solarized soil but not at all in shaded soil, as compared to untreated control soil. Six strains of fluorescent pseudomonad plant growth-promoting rhizobacteria colonized sugar beet and radish roots .apprx. 2 to 6 .times. more effectively in solarized soils of 4 textures than in the same soils that were not solarized (P < 0.05). Soil and root population densities of Pythium spp. were each reduced by 38% after postplant soil solarization of a 2-yr old almond orchard, but no reductions were seen after treatment of a 6-yr-old peach orchard. No visible differences in the extent of root infections by vesicular-arbuscular mycorrhizae (Glomus spp.) were apparent between roots from solarized and untreated at one orchard site.

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