Foliar Analysis as a Diagnostic Technique in Tropical Horticulture G. A. Cahoon Department of Horticulture Ohio Agricultural Research and Development Center The Ohio State University, Wooster, OH 44691 A brief history of the development of foliar analysis as a diagnostic technique for horticultural crops is reviewed and discussed. Terminology, together with a basic discussion of the components necessary to operate a successful foliar analysis program for growers, is presented. Some standard values, as they relate to 'normal' growth of selected tropical and sub- tropical horticultural crops, are listed. Keywords: foliar analysis, plant analysis, leaf analysis, petiole analysis', diagnostic techniques, plant nutrition, nutrient elements. Foliar analysis has been an active research procedure since the late 1930's. Some of the early comprehensive studies of impor- tance to us today were conducted on horticultural crops such as citrus, apples, grapes, peach, tomato, and potato and others. The goal of these studies, then, as well as now, was to develop a method whereby the nutritional status of the plant can be ac- curately determined. As we have gained knowledge and ex- perience, developed equipment and techniques, fertilizer pro- grams based upon foliar analysis have been devised which resulted in increased production and quality. Early researchers such as T. Wallace (Great Britain), P. Prevot (France), W. Reuther (USA), H.F. Clements (USA), A. Kenworthy (USA), M. Maume (France), C. Bould (Great Britain), G.T. Nightingale (USA) have given us much insight into this procedure and deserve credit as the pioneers in the field. The definition of foliar analysis, as used here, includes the terms tissue analysis, leaf analysis, petiole analysis, and plant analysis, somewhat inter- changeably. Although it is possible to use rapid tissue tests, the procedure is declining in popularity and is not developed or discussed further in this presentation. Modern Methods and Equipment During this early period, chemical analysis procedures were time-consuming, tedious and sources of considerable error. To- day, although the equipment is relatively expensive, much pro- gress has been made and it is possible to run accurate and rapid analyses of many elements on a wide range of tissues. However, accurate chemical analysis of a tissue is only one part of the diagnostic technique we call foliar analysis. Sampling procedures, including collection, preparation and interpretation of the data are still vital components and the ultimate success of the tech- nique generally depends on how accurately they are handled. Depending upon the tissue and time of sampling, leaf age and condition, drying and grinding procedures, etc., considerable variability can be introduced and are major sources of error. Regardless of the difficulties of past and present research, much of the progress and development of current information on nutrition of horticultural crops can be attributed to the use of foliar analysis. For example, before the general use of foliar analysis many fertilizer experiments were conducted to determine the effectiveness of a given type and rate of fertilizer on yield and quality of a crop. Yet because there was no 'common denom- inator,' or means of judging the relative nutrient status of the plant, much of the reference value was lost for anything but localized conditions. On the other hand, through the use of foliar analysis, the relationship between nutrient content of a specific 74 tissue, yield and quality has been utilized. This has been suffi- ciently reliable from year to year and from one soil type or cultural condition to another, that standards for many crops have been developed. Conducting field fertilizer experiments today without using foliar analysis is, in this author's opinion, almost inexcusable. For the commercial fruit producer, failure to take advantage of such an important tool is an invitation to failure. Thus, within the scope of this discussion, some of the fundamen- tal procedures will be discussed whereby both researchers and growers can avail themselves of this technique. Definition of Terms Nutritional studies of individual elements indicate that defi- ciencies or excesses can limit growth. However, it soon becomes obvious to the researcher that the concentration of one element within a plant cannot be changed very much without influencing another to a degree that growth may be affected. The term used for this is called nutritional interaction. Some of these interac- tions become very dominant on certain crops. For example, the relationship between potassium and magnesium on grapes is very obvious in any studies with either element. In recent years, studies on nutrient interactions have led to the development of procedures such as the DRIS system (Diagnosis and Reconmmen- dation Integrated System). A definition of terms for all the nutrient elements as used in this discussion can be defined as follows: Deficient: Plants are in the deficient range if they show visible leaf symptoms or distinctive decreases in yield and quality can result. Low: Plants are in the low range if the amount of an ele- ment present in the plant is inadequate to produce op- timum yield or quality. Sufficient: Plants are in this range if it is doubtful that further additions or reductions of the element will result in a desirable increase of growth, yield, or quality. Somewhere within this range is the absolute optimum, which together with the combination of the other elements present, will produce the most desirable production. High: Plants are in this range if the level of the element present in the plant is higher than necessary to produce op- timum yield, growth or quality. This amount may indicate unfavorable quality relationships or an imbalance of other nutrients. Excess: A plant contains an excess amount of an element if visible leaf symptoms are present or definite reductions in yield, vigor, or quality result. PROCEEDINGS of the CARIBBEAN FOOD CROPS SOCIETY-VOL. XX