Bioassay for Residual Soil Fumigants
General Principles Information Note 5 (GPIN-005)
Charles W Averre, Extension Plant Pathologist



Soils are fumigated to control plant diseases and problems caused by soil-borne fungi, bacteria, nematodes, insects, and weeds. The length of time required for fumigants to escape from the soil before plants can safely be planted varies greatly. Typically seven or fewer days are needed under summer conditions, however in the winter and early spring it may take months. The release period is short with: 1) highly volatile fumigants, e.g. methyl bromide; 2) low rates of fumigant; 3) light soil; 4) high soil temperatures; 5) low soil moisture; 6) shallow application depth; and 7) repeated cultivations after fumigation. Seeded crops are less susceptible to residual soil fumigant injury than transplanted crops. In general, fumigants escape slowly from cold, wet, heavy soils.

A simple test to assay for harmful, residual soil fumigants before planting or seeding a crop:

1. With a trowel dig into the treated soil to, or just below, the depth of application. Remove 2 to 4 small (1-2 oz) soil samples, mix briefly, and immediately place a portion in an air tight jar so that fumes will not escape. Use mason jars, wheat germ jars or similar jars with gas tight lids.
2. Sprinkle lettuce seed on the moistened surface of the soil and recap immediately. Prepare a similar jar with untreated soil (an untreated check) for comparison.
3. Place the jars at 65 to 85 degrees F in indirect sunlight (direct sunlight may kill the seed by overheating). Lettuce seed will not germinate in the dark.
4. Inspect the jars for germination in one to three days.
5. The soil is safe to plant if seed germinate in the treated soil.

Caution

Be sure to sample the field properly in several areas.
Be sure that the lids are air tight, (no grit under the seal).
Be sure that the jars are placed in the light, (not direct sun).

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. Last printed 02/91 (Revised)

Published by North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted Nov. 2000 by Plant Disease and Insect Clinic