The foregoing discussion has dealt with some of the factors which contribute to the 
persistence of organic carbon of living origin with the purpose of pointing out the 
marked influence of climate and soil factors. Obviously, the prediction of the fate of 
organic substances in a specific soil at a given location will require a knowledge of both 
climate and soil. Although our knowledge is much less extensive there is ample evidence 
to show that prediction of the fate of the chemicals classed as organic pesticides is 
dependent upon similar if not the same circumstances. 
Studies were reported by Foster, et al. (5) on the accumulation of insecticides in 
field soils at Beltsville, Md., State College, Miss., and New Brunswick, N. J. In this 
instance there were three soils, three climates, and nine insecticides involved. The 
tendency for accumulation was greatest at Beltsville and least at State College. The 
relative persistence of the nine insecticides also varied with location. It is not known 
to what extent these differences were caused by soil or climatic changes. One general- 
ization coming from these experiments was that many years will be required for the 
accumulation of sufficient evidence for accurate prediction of insecticidal persistence 
in soils. 
The high tendency for DDT to persist and accumulate in soils is well known. A 
number of years ago an experiment was conducted at Beltsville which demonstrates the 
differential influence of soils on decomposition rate of DDT. Radioactive p,p!-DDT 
was incorporated in 18 representative soils at a normal rate of application. After a 
6-month period of moist incubation the soils were extracted withacetone, and the amount 
of radioactivity recovered from each soil measured. The loss of radioactivity (C14) 
varied from 0 to 25 percent. An attempt was made to relate the degree of this loss to soil 
properties. No correlation was found with soil texture or organic carbon content and 
recovery, but a significant negative correlation was found with soil pH, although, there 
was considerable inconsistency between soils. There were five soils with a pH greater 
than seven, three showed a 25 percent loss of radioactivity. 
It seems evident that a satisfactory prediction of the influence of soil conditions on 
the instability of organic molecules will require aknowledge of the biological or chemical 
reactions which accomplish the actual molecular alteration. Another aspect is the reac- 
tions between soils and organic molecules which will inhibit these reactions. 
SOIL REACTIONS STABILIZING ORGANIC MOLECULES 
Many of the organic compounds added to. soils are not particularily resistant to 
decomposition yet they appear to have a degree of stability in the soil. The organic 
phosphorus compounds that accumulate in soils are a well-known example. Several 
hypotheses have been advanced to account for this lack of ready decomposition. A low 
level of microbiological activity in soil usually is a first consideration. The chemical 
approach deals with insolubility either through the formation of insoluble salts with 
polyvalent ions or the adsorption by clays and organic complexes. Molecular forces, 
hydrogen bonding, polar molecules, and basic amino groups are probably all involved. 
Soils high in clay have been shown to be the most retentive of organic matter. This 
observation has lead to the belief that some sort of an organic-inorganic interaction 
takes place in soils. In recent years the reactions between pure organic compounds and 
the clay montmorillonite have been subjected to detailed study. Gieseking (6) showed 
that complex organic cations such as methylene blue or brucine are strongly absorbed 
by the montmorillonite-beidellite-montronite clay minerals giving rise to increased 
c-lattice spacings. Increasing the amount of complex-cation added increased the spacing 
until a maximum was reached. These results were interpreted as showing that a portion 
of the complex cations were absorbed within the variable spacings of the minerals. 
Subsequently Hendricks (7) showed that the organic cations are held to the surface of 
silicate layers by both Coulomb forces between ions and van der Waals attraction of 
the molecules to the surface. 
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