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While phosphorus (P) and potassium (K) use efficiencies in alfalfa (Medicago sativa L.) are subject to genetic control, selection for these traits also affects the utilization of other nutrients. It is not clear whether other nutrients are as susceptible to genetic alterations directly or whether alteration of P and K uptake may simply alter the general synergisms and antagonisms among the elements in plants.Our objectives were to compare the P-and K-use efficiencies of five commercial alfalfa cultivars (CVs) growing on a low-P, low-K calcareous soil, and to evaluate the impact of P and K fertilization on the herbage concentrations of these and other elements. The CVs were selected for uniformity of disease resistance and other agronomic traits. Three levels each of P and K fertilizer were applied in factorial combination, with CV as the whole plot and fertilizer treatment as the split plot.Herbage samples, consisting of the top half of the shoot, were collected at the first cut in 1992 and 1993 and were analyzed by wet digestion and inductively coupled plasma emission spectrometry (ICP).There were appreciable amounts of aluminium (Al), iron (Fe), sulfur (S), and silicon (Si) in the alfalfa herbage but CV and soil treatment did not affect the concentrations of these elements. Plant breeding and selection for P-use efficiency 2447

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