PROBLEMS OF SAMPLING AND ANALYSIS IN TRACE ELEMENT INVESTIGATIONS OF VEGETATION

Down through the ages lower forms of animals have eaten grasses or green vegetation and consumed it “as it comes.” But man, in his search for food, has commonly chosen fruits or tubers for palatability and nourishment and prepared them, in one way or another, for his consumption. One might say that the what, how, when, and where of scientifically sampling vegetation depends on the why. What is the purpose of the collection and how much time and money are available for the job? When these objectives are known, the many variables that may affect the sampling program can be considered and evaluated. Sampling vegetation for trace element analysis further compounds the number of variables. Regional and local factors of the environment are important as well as factors related directly to sample collection, storage preparation, and analysis. This paper considers these variables and describes a small comparative study that we made to assess the relative effects of several variables on the eventual reported values. The geographic landscape should be considered in advance of sampling in order to eliminate, regulate, or emphasize-as desired-any variation in soil or climatic conditions. Slope, exposure, and drainage may influence the composition of plants. Mitchell 1 has demonstrated an increase in Co, Ni, Mo, Zn, Ti, Cu, Mn, and Ba for red clover grown in poorly drained soils, and a decrease or no change in Fe, Cr, V, and Si. Analyses of a certain number of leaves or parts of leaves from a particular part of the current twig are commonly used in diagnosing the nutritional status of plant^.^-^ Guha and Mitchell have reported work by Biddulph showing that deficiency symptoms are due as often to failures in the distribution system as to variations in the initial absorption. Furthermore, percentage concentrations reported in dry weight can be misleading, for the ash content of the dry weight of leaves varies throughout the season. To minimize the possible variation in trace metal content with position on the plant (height above ground, exposure, or access of certain roots to mineralization), like vegetation should be sampled in a consistent and uniform manner. When a large quantity of sample is needed for analysis and the amount of mineral available in the soil is to be determined, the same plant organs on all sides of the plant should be sampled at a certain predetermined height above ground. Generally, grass or forage is sampled not closer than two inches above the ground, to avoid contamination from the soil; garden produce is sampled as randomly as possible on the plant; and leaves or twigs of specific age are sampled from the entire circumference of shrubs or trees at a selected height above the ground. Ideally, all collecting of samples for comparative purposes should be done during the same month. For the purpose of evaluating the