CHEMISTRY OF PESTICIDES IN SOILS 
L. A. Dean! 
The ultimate fate of pesticides added to soils depends upon physical, chemical, or 
biological processes operating singly or in sequence. Literature reviews reveal that 
pesticidal behavior cannot be predicted from studies with a single soil or from a single 
pesticide on several soils. This discussion deals withcertain principles of soil chemistry 
as they are related to behavior of organic substances added to soils. The purpose is to 
examine the types of information required for a satisfactory prediction of pesticidal 
behavior in soils. 
As early as 1907 (13), U. S. Department of Agriculture scientists were interested 
in the occurrence of organic toxic substances in soil. In fact, the theory was then ad- 
vanced that such substances contributed to the unproductivity of certain soils. A poor 
soil was found to contain the phytotoxic substance dihydroxy-stearic acid. 
Soils consist of mineral and organic matter, water and air combined in three 
phases: Solid, liquid, and gaseous. Soil chemistry furnishes the basis of our knowledge 
of the composition and the interrelations of these components. For the purpose of this 
discussion perhaps the most important characteristic of soils is the differences among 
them. Every farm consists of several kinds of soils or soil types, and throughout the 
world there must be hundreds of thousands of soils. Individual areas of a soil type are 
three-dimensional bodies having length, breadth, and depth. Differences between neigh- 
boring soils may be large or small. They reflect changes in such factors as topography, 
drainage, or parent material. Regional soil differences are generally greater than those 
of local associations of soil types. Most frequently these differences reflect the dis- 
tribution of climate and living organisms. For convenience soil types are grouped in 
respect to common features thus bringing out relationships between soils and their 
management in relation to agriculture. The soils of the arid Western States can be con- 
trasted with those of the humid Southeast. The former are neutral or alkaline in reaction, 
very low in organic matter and clay minerals of the montmorillonite expanding-lattice 
types predominate. The latter are acid, contain appreciable quantities of organic matter, 
and in the clay fraction kaolin and the hydrous oxides of iron and aluminum predominate. 
These are examples of the contrasts in properties that may exist between two soil zones. 
THE PERSISTENCE OF ORGANIC SUBSTANCES IN SOIL 
One common feature of soils is the accumulationor persistence of varying quantities 
of organic residues. For the most part these residues originate from plant and animal 
tissues returned to the soil, however, in recent years an ever-increasing variety of 
organic materials are being added to soils by man. Many factors have been studied in 
relation to the persistence of organic residues in soil including climate, oxygen supply, 
and chemical and physical properties of soils. 
The climatic factors of temperature and rainfall have a strong influence on the rate 
of decomposition of organic matter. As an example of this influence of climate, an 
experiment conducted at 11 locations in the Sierra Nevada mountains is cited (8). The 
annual decomposition of alfalfa under these different climates is shown in Figure l. 
At first sight the decomposition curve seems erratic, but actually, it shows that the 
effects of rainfall and temperature are closely recorded. The first three points show the 
influence of sharply increasing rainfall. Points 3 to 7 show a decreasing decomposition 
rate with decreasing temperature but constant rainfall. The remaining points reflect 
a temperature-rainfall interaction. 
1Research Investigations Leader, Soil and Water Conservation Research Division, Agricultural Research Service, U. S, Depart- 
ment of Agriculture, Clemson, S, C, 
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