19 



Plowing surface -contaminated fields to a depth of seven inches resulted in 

 50 per cent of the activity in the zero- to four-inch level. Both rotary tillage to four 

 inches and two plowings resulted in>70 per cent of the activity in the top four 

 inches (110). The uptake of fission products placed at a 60-cni depth, compared to 

 a 30-cm depth, was as low as one thirtieth in the tops and one tenth in the seeds of 

 oats and peas (36). 



D. Moisture 



Variations in the soil moisture tension were found to affect cesium uptaJce and 

 the cesium-to-potassium ratio, but no effect was noted on calcium and strontium 

 (125). Several observations (128, 129) of a tenfold greater uptake of cesium by low- 

 land rice than by upland rice have been made. Evidence indicates that increased 

 amounts of ammonium ion under the reduced conditions caused the greater uptake. 



E. Prolonged Cropping 



Little variation in strontium availability is observed over a period of years 

 (14, 43), and the uptake of strontium per unit weight is fairly constant (86). The 

 effect of fixation has not been studied sufficiently in the field to determine whether 

 it is important under prolonged cropping. The uptake of cesium increased under 

 prolonged cropping (86), probably because the potassium became depleted by crop 

 removal. 



