Carbon Retained -(gm.) 
is 
4 a —-® Sand + 10% Bentonite 
~ ~~ Sand + 30% Kaotin 
Sand only 
O 2 6 9 lO 
Time (months) 
BN-10717-xX 
Figure 2,--Effect of clay on retention of organic carbon, 
A well-known observation is that under similar climatic, age, degree of aeration, 
and cropping conditions the organic matter content of soils increases with their clay 
content. This suggests that the clay may in some way contribute to the persistence of 
organic matter in soils. Data from a controlled laboratory experiment by Allison, et al. 
(1) have been selected (Figure 2) to illustrate the influence of clay on the persistence 
of organic substances in soils. In this experiment 1 gm. lots of carbon as soybean plants 
were incubated with 100 gms. of sand or sand-clay mixtures, and the persistence of the 
carbon measured. The data clearly show that the introduction of clay into the system had 
a marked influence on the persistence of the organic carbon. Further, clay of the bentonite 
type (montmorillonite) appeared to be much more effective in this respect than kaolin. 
When considering the persistence of organic substance in soils, the tacit assumption 
is that the agent for decomposition is a biological process or its attendant enzymatic 
activity. However, for the purposes of this discussion the possibility that chemical or 
photochemical reactions in the absence of biological activity may alter organic molecules 
in contact with soils should not be precluded. The postulation of such reactions in soils 
is an old one, but there is only a limited amount of supporting evidence. For example, 
Dean and Dean (2) demonstrated the decomposition of citric acid in contact with soil to 
carbon dioxide and acetone. Presumably the soil supplied a necessary catalyst. 
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