Leaf analysis as a means of assessing the nutrient status of deciduous fruit trees and vines in the Western Cape Province
Thesis (DScAgric)--University of Stellenbosch, 1958.
OBJECTIVE. High economic production has ever been the aim and aspiration of the agriculturist and no less that of the fruit farmer. In striving towards this aim the latter has for a long time been at a disadvantage with regard to control of his nutritional programme. Even on naturally fertile soil, the question continually arises as to what the correct fertilizer treatment should be to maintain high productivity and how such a decision can be arrived at. A satisfactory answer to these questions could have been obtained from fertilizer trials if it was not such a difficult matter, in view of the extensive and long-term nature of such trials with fruit trees, to establish a sufficient number for each fruit species on different soil types and under different climatic conditions. Efforts to find a new approach to the problem have turned attention to the plant itself and its chemical make-up as affording the best index of its nutritional requirements. Intensive work in this direction has resulted in the evolution of a new tool in agriculture, the technique of diagnostic leaf analysis or 1Toliar diagnosis" as originally proposed by Lagatu and Maume in France and Thomas in u.s.A. A review of the literature is presented indicating the prodigous amount of research which has been applied to studies of the relationship between plant response and nutrient supply in terms of plant composition. Agriculturists have been quick to recognize the potentialities of leaf analysis as a practical guide in nutritional problems and advisory services based on foliar analysis have already been established for certain crops overseas. The experimental basis for formulating such a scheme for deciduous fruit in the Western Cape Province is provided by the factual evidence presented in this thesis.THE TECHNIQUE. The technique of diagnostic leaf analysis comprises sampling of leaves, preparation of sample for analysis and the analysis itself followed by interpretation of the analytical results by comparison with previously determined nutritional standards. Numerous factors were found to influence the final composition of the leaf sample as determined by analysis, such that strict adherence to a standardized procedure through all phases of sampling and preparation of leaf samples for analysis is required to eliminate or reduce errors likely to cause misleading interpretations. Experimental data are presented suggesting how the leaf sample should be selected on a tree and how it should be handled, cleaned, dried, ground and stored to reduce sampling and other errors. The final procedure as adopted eliminates most of the potential sources of experimental error but two unavoidable sources of e~ror remain to be accounted for, that due to tree variation and seasonal effect. The variation in leaf composition from tree to tree was found to be very considerable, so that aampling from a large enough group of trees (6 to 10) to reduce the error involved is essential in order to obtain leaf data which correctly reflects the nutrient status of the portion of the orchard concerned. Secondly, on the grounds of marked consistency found in different fruit species as to seasonal and year to year variation in mineral nutrient concentration, correction factors have been formulated and are suggested as a means of overcoming these sources of error. THEORETICAL BASIS. A diagnosis of the nutrient status in terms of the analytical results as finally determined is obtained by comparison of the data with previously established leaf composition standards of reference and by correct interpretation of the deviations from these standards. The theoretical basis for setting up these index values is discussed. The criterion used is based on the concept of Optimum Values which aaequately integrates the known relationships between plant response and nutrient supply in terms of internal nutrient concentration. A modification of this concept is proposed to the effect that for maximum growth and yield there exists an optimum range of nutrient concentrations with upper and lower limits for each of the functional elements, and that within this range the interrelationship between the individual nutrient elements is also optimal. Since no local fertilize~ trials with deciduous fruit trees are available and only one for grapes, data from highly productive plants in commercial orchards and vineyards were used to determine the upper and lower limits of the "optimum range", on the following premise. If leaf analysis data are available from a sufficient number of high performance orchards in different localities representing a wide range of nutrient supply and environment, the highest and lowest values obtained may be considered to represent a close approximation of the limits of the range required for optimum performance. It is contended that index values obtained in this way must be of practical value in assessing the nutrient status of fruit trees. It is further postulated that the lower limits for the micro-nutrients and even for magnesium may be justifiably adjusted according to the concentration levels associated with symptom expression. INDEX VALUES. The necessary data for determining standards of leaf composition were obtained from leaf analysis surveys of orchards and vineyards and from a grape fertilizer experiment in the Western Cape Province. Visual symptoms of prevailing nutritional disorders are described (supplemented by photographic illustrations) and their relation to leaf composition indicated. Tentative index values have been determined on the basis indicated for each fruit species, apple, pear, peach, apricot, plum, prune and grapes. These nutritional levels comprise upper and lower limits for the nutrients N, P, K, Ca, Mg 1 Mn 1 Fe and Cu, as well as the upper limits for B and Na. DIAGNOSTIC INTERPRETATIONS. Assessment of the nutrient status in terms of these index values suggests that many orchards and vineyards in the Western Cape Province, particularly prune, apricot and grapes, are suffering from malnutrition in some form and are likely to show a marked response to nutritional treatment as suggested by foliar diagnosis. The use of diagnostic leaf analysis constitutes an important advance in dealing with orohard problems in that an immediate decision is possible regarding nutrient status and related aspects such as selection of suitable sites for fertilizer trials and adjustment of the fertilizer programme.
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