The results of experiments conducted with higher plants in tightly sealed growth chambers provide definite evidence that the physical closure of a chamber has significant effects on many aspects of a plant's biology. One of these effects is seen in the change in rates of uptake, distribution, and re-release of nutrient elements by the plant (mass balance). Experimental data indicates that these rates are different from those recorded for plants grown in open field agriculture, or in open growth chambers. Since higher plants are a crucial component of a CELSS, it is important that the consequences of these rate differences be understood with regard to the growth and yield of the plants. This paper will focus on the description of a system for elemental analysis which can be used to monitor the mass balance of nutrient elements in CELSS experiments. Additionally, data on the uptake of nutrient elements by higher plants grown in a growth chamber will be presented.