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Effect of Various Exchangeable Cation Ratios on Kinds of Fungi Developing during Decomposition of Organic Residues in Soil¹

ABSTRACT

Soils varying widely in exchangeable Ca, Mg, K, Na and H ratios were prepared from Yolo loam and Hanford sandy loam soils. The effect of these ratios on the kinds of soil fungi developing during the decomposition of organic residues in the soil was followed at various intervals over a 200-day incubation period.

In general, the nature of the organic material added to the soils exerted a greater effect on the kinds of fungi developing than did the exchangeable cation ratios. In the absence of organic residues, the fungus population remained essentially the same in all soils over the entire incubation period. Upon the addition of organic residues, Penicillium nigricans, which was present in high concentrations in the check series (no organic residues), became increasingly dominant in the most-acid soils. With few exceptions, other species became dominant in all other soils. The kinds of fungi becoming dominant depended on the nature of the organic residue added. The dominant species included Chaetomium sp., Cunninghamella bertholletiae, Fusarium spp., Monotospora brevis, Mucor spp., Sepedonium chrysospermum, Sporotrichum sp., Stachybotrys atra, and Stysanus stemonites. Some species developed quickly in large numbers and remained dominant for the remainder of the incubation period. Others developed in high concentration, then rapidly diminished in numbers or disappeared.

¹ Paper No. 786, University of California Citrus Exp. Sta., Riverside. Presented before Division III. Soil Science Society of America, Dallas, Tex., Nov. 18, 1953. keceived for publication Nov. 27, 1953.

² Associate Chemists in the Experiment Station. The authors wish to acknowledge the help of J. O. Ervin, who assisted with the soil preparation, and of Dr. Wm. Bridge Cooke, who kindly identified some of the fungus species.