Phytoextraction field experiments were conducted to determine the capacity of a variety of plant species to accumulate Cs-137. The soil contained from 0.39 to 8.7 Bq/g and had been contaminated at least thirty years ago. In one test, the field contained 88 randomized experimental cells (0.25 square meters each) with each cell receiving transplants of one of five species of plants: Amaranthus retroflexus/graecizans, Amaranthus aureus, Amaranthus cruteus x Amaranthus powelli, Brassica oleracea var. capitata, and Brassica juncea. Using concentrations of Cs-137 and biomass observed in the experimental cells, estimates were made of the amount of time required to attain cleanup goals. Species of Amaranthus generally provided the best concentration ratios (CR = [concentration in plants] / [concentration in soil]). For control plants, CR of Cs-137 was greatest for A. aureus and the Amaranthus hybrid, with average CR values of 1.0 ± 0.24 and 0.95 ± 0.14, respectively. The lowest value was for B. juncea at 0.36 ± 0.10. Under optimum biomass production and CR values, the clean-up target could be reached in 12 years. However, changes in soil chemistry induced by common soil amendments can seriously alter plant uptake patterns. For example, with a combination of composted manure and application of ammonium nitrate solution, the fraction of Cs-137 taken up from the soil was reduced by 57.4% (± 1.2) compared to controls. This was the result of release of competing ions, primarily Ca, released from the manure and was observed across all five plant species tested. The application of ammonium solution took place in the last two weeks before harvest. The reduction of plant Cs-137 content, by addition of the ammonium solution, as it interacted with the manure, indicates that substantial quantities Cs-137 can be released from the shoots of plants as a result of sudden changes in soil solution chemistry. Synchrotron x-ray fluorescence maps of element distributions in sectioned stems provides some insight to the mechanism.