Sodium as an element increasing nitrogen productivity - a case study on sugar beet

The objective of the study was to evaluate the effect of sodium-enriched nitrogen fertilizers against the background of pre-sowing sodium fertilization on sugar beet productivity, including technological quality of taproots. A field experiment, completed in 2001-2003, consisted of two main factors: (i) pre-sowing sodium application (0, 30 kg Na ha–1 in the form of NaCl), (ii) a set of nitrogen fertilizing variants, composed of two sub-levels: one consisting of four nitrogen rates (0, 90, 120, 150 kg N ha–1) and the other one comprising three chemical N fertilizer forms [(i) ammonium nitrate, 34%, AN, (ii) mixture of ammonium and sodium nitrates, 26%N + 6% Na (ASN1), (iii) mixture of ammonium and sodium nitrates, 21%N + 13% Na (ASN2)]. Depending on a nitrogen rate, the fertilizers were applied on two or three dates. The first N rate was applied only as ammonium nitrate. The in-season application of nitrogen and sodium as the 2nd and the 3rd rate of nitrogen allowed for discrimination of sodium rates, ranging from 0 to 44.2 kg Na ha–1. The effect of soil applied sodium was significant in the 2nd and 3rd year of study. The highest yields of taproots and sugar, despite changeable weather conditions, were harvested on the 120 kg N ha–1 treated plot. The response of sugar beet plants to in-season applied sodium was varied and depended on soil available sodium content and the course of weather during the growing season. The strongest response occurred in 2003, characterized by both the lowest amount of available soil sodium and shortage of water. The necessity of sodium application, as a nutritional factor increasing yields of taproots and sugar, was clearly demonstrable under low soil sodium content (< 5 mg kg–1 soil). Then, the optimum rate of in-season applied Na in the form of ASN1 ranged from 14.8 to 29.5 kg Na ha–1. The available sodium content, from 10 to 12 mg kg–1 soil, defined the upper limit of sodium fertilizer application. At that sodium fertility level, 7.4 kg Na ha–1 should not be exceeded. The highest unit N productivity, as attributed to the 90 kg N ha–1 treatment, responded positively to soil and in-season applied sodium. Therefore, it can be concluded that soil and/or in-season applied sodium can improve productivity of unit nitrogen, provided that a nitrogen rate will be reduced by up to 30 kg N ha–1 in comparison to its optimum rate.