F. O rganic Matter 



Additions of decomposable organic matter can reduce the uptake of strontium 

 (40, 80, 81). A major factor is probably the increased microbial population, although 

 adsorption to the organic matter itself is also important (56, 93, 108, 117). 



G. Fixation 



Fixation is a general name for processes occurring in soils that convert ions 

 from forms available to plants into those not available. Soil culture experiments, 

 such as those conducted with the Neubauer technique, have been used to evaluate the 

 amount of applied fertilizer that is available for plant uptake. Neutral normal salt 

 solutions are often used to extract the exchangeable cations, which are considered 

 to constitute the major source of the available quantities of many nutrients. The 

 difference between the applied amount and the available or extractable amount is 

 usually considered fixed. Thus, fixation is an arbitrary term which depends upon 

 the experimental conditions. 



Proposed mechanisms for fixation include precipitation as slightly soluble 

 materials, physical trapping between clay platelets and in other insoluble precipi- 

 tates, and diffusion into existing crystals (89, 97, 113, 114). 



Nonexchangeable amounts of strontium (82, 98, 103, 114) and cesium (82, 83, 

 113, 129) have been found in some soils. In other soils, no fixed strontium was 

 found (43, 47). The magnitude of strontium fixation ranged as high as 20 per cent 

 of fallout strontium-90 in North Carolina soil samples taken in 1958 (103). Labora- 

 tory studies showed that increasing the temperature from room temperature to 60° C 

 tripled the amount fixed in these soils. Increasing the equilibration time from one 

 to two weeks doubled the amount fixed at room temperature but had no effect at 60° C 

 (98). In other laboratory studies with these soils, fixed strontium appeared to be 

 extractable at 80° to 90° C. ^ 



H. Erosion 



Because most fission products are strongly adsorbed to clays, it is expected 

 that any redistribution of surface soil will cause a similar redistribution of fallout. 

 The strontium-90 concentration in runoff from field plots was 10-30 times the con- 

 centration in the soil and was almost entirely associated with the sediment (69). 

 The strontium-90 concentration on cultivated watersheds, where the amount of soil 

 erosion was known, was one third to two thirds of the concentration on watersheds 

 where there had been no erosion (25). Slope and cropping history appeared to be 

 related to the amount of loss. 



