There was some long-range weed control resulting from a single fumigation. Follow- 

 ing the 1963 fall fumigation, soil samples were taken in selected plots, both fumigated 

 and unfumigated, to a depth of 6 inches. The surface inch of soil was discarded to 

 eliminate weed seed which had blown in since fumigation. Samples of the remainder of 

 the soil were then spread on containers in the greenhouse, and weeds counted as they 

 germinated. A total of only six weeds germinated on 37 samples of fumigated soils; in 

 contrast, 33 samples from unfumigated soils supported a total of 425 weeds. Fumigation 

 virtually eliminated viable residual weed seed to a depth of at least 6 inches. Many 

 of these weed seeds have a prolonged viability and might eventually be brought to the 

 surface by cultivation where they could germinate; thus, the treatments should provide 

 some long-range weed control benefits. 



A similar test of weed control was made in the fall of 1965 comparing soils 

 operationally fumigated with Trizone at 200 pounds per acre with unfumigated soils. 

 Soil samples were collected on October 10 after an early September fumigation. The 

 soil had not been disturbed in the period between fumigation and sampling. Twenty-four 

 randomly selected locations were chosen in both fumigated and unfumigated areas. At 

 each location a sample was obtained from each of three levels beneath the soil surface: 

 1 to 2 inches; 4 to 5 inches; and 8 to 9 inches. Weeds were then germinated from 

 measured subsamples in the greenhouse with the following results: 



Differences in weed population between fuiriigated and unfumigated soils were highly 

 significant (99% probability level). 



The deep-action effect of fumigation on residual weed seeds, as demonstrated in 

 these two tests, promises to provide a reduction in weed population for future crops. 



Soil de-pth 

 (Inches) 



Number of weeds per sample 

 Fumigated Unfumigated 



1 - 

 4 - 

 8 - 



- 2 



- 5 



- 9 



6.9 

 7.4 

 5.4 



44.9 

 46.0 

 29.6 



500 



Figure 1. --Weeding time as related 

 to the weed densities resulting 

 from various fumigant treatments 

 in the 1963 spring trials at 



100- 



Vapam 

 (40 gal. /acre) 



Coeur d'Alene nursery. 











5 



10 



15 



20 



25 



WEEDS (Number/square foot) 



6 



