HERBICIDES IN SOILS’ 
T. J. Sheets and L. L. Danielson? 
INTRODUCTION 
Many herbicides are applied directly to the soil surface as selective preemergence 
sprays and as nonselective soil sterilants. Other chemicals are applied subsurface or 
are thoroughly mixed with the soil after surface application. Residues remaining on 
leaves after foliar applications are carried to the soil in rainwater or fall to the soil 
when injured leaves abscise and fall. Therefore at least part of all herbicidal sprays 
eventually reach the soil. 
Soils vary greatly in composition and reactivity. Many complex and ever-changing 
processes occur continuously in most soils. Soils are composed of mineral matter, 
organic matter, water, and air. The mineral fraction varies in amounts of sand, silt, and 
clay, and in types and amounts of clay minerals. The hydration and base saturation of the 
clay minerals also vary. The organic-matter fraction consists of decaying plant and 
animal residues and active soil flora and fauna. The organic and mineral colloids present 
in the soil contribute directly and indirectly to the extremely active nature of soil 
systems. 
This mixture of mineral and organic matter is permeated by pore spaces of various 
sizes. These spaces are filled with water and air in a more or less reciprocal relation. 
The soil water contains many soluble compounds and serves as an essential medium for 
many physical and chemical processes. The soil atmosphere is composed of oxygen, 
carbon dioxide, nitrogen, and several minor gases. The composition of the soil atmos- 
phere varies, particularly the oxygen and carbon dioxide contents. The complexity and 
variation of soil systems make the study of the fate of herbicides therein complicated 
and time consuming. 
METHODS OF ASSAYING HERBICIDE RESIDUES IN SOILS 
In most investigations on the persistence of herbicide in soils, researchers determine 
the presence of the active entity of the herbicide by growing sensitive plants. The influence 
of time on herbicidal residues has been measured by the growth of successive crops of 
test plants. This method, which has been used in both field and greenhouse experiments, 
is qualitative only. 
Quantitative bioassays have been developed for some herbicides. Holstun and Loomis 
(35) measured the elongation of young shoots of germinated millet seeds to determine 
the concentration of the sodium salt of 2,2-dichloropropionic acid [dalapon] in soils. 
Burschel and Freed (11) used heights and weights of seedling oats to determine the con- 
centrations of isopropyl N-phenylcarHarate Pipe! isopropyl N-(3-chlorophenyl) car- 
bamate [CIPC], and 3-amino-1,2,4-triazole [ amitrole] in soils. Rahn and Baynard (45) 
used weight of oat seedlings to assay quantitatively 3-(p-chlorophenyl)-1,1-dimethylurea 
Eaososonll in soils. Biological assays developed for solutions and vapors of herbicides 
could be adapted for use with soils (17, 46, 55). 
1 4 contribution from the Crops Research Division, Agricultural Research Service, U.S, Department of Agriculture, and the 
Mississippi Agricultural Experiment Station, 
2 Plant Physiologists, Crops Research Division, Agricultural Research Service, U.S, Department of Agriculture, Stoneville, 
Miss,, and Beltsville, Md., respectively. 
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