CONTROL OF THE NITROGEN STATUS OF LETTUCE BY NITRATE ANALYSIS OF PLANT SAP

Authors:	D. Alt, A.-M. Füll
Abstract: In a field trial with lettuce the influence of increasing amounts of nitrogen on yield and on NO₃ concentration in the sap of midribs was investigated. The aim of this investigation was to test whether monitoring of N nutrition by sap analysis is possible. With increasing N supply yield increases were observed; 100 kg N/ha was sufficient. N fertilization had a strong influence on NO₃ concentration in the sap of midribs. In zero-N plants very soon no nitrate was found any longer. In plants receiving N, NO₃ concentration first increased and later decreased again. NO₃ concentrations in sap rose and remained high longer with increasing amount of N applied. From the time course of NO₃ concentration, a critical value of 2300 mg NO₃/1 sap was derived. Below this value N fertilization is recommended. By means of the NO₃ test incipient N deficiency may be detected earlier than by means of visible symptoms. It could therefore be concluded that the NO₃ quick-test is in principle suitable for monitoring N nutrition.
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