Ann. Tabac, 1991, Sect. 2-23, p. 105-26., ISSN.0399-0354
Effect of calcium-magnesium corrective supply on the characteristics of soil and Burley tobacco BB 16
SEITA, Institut du Tabac, Bergerac, France
Since 1972, supplies of magnesium-limestone (CaO 30-35%, MgO 20-25%) are regularly _ i.e. each two years _ applied on an acid alluvial sandy soil, at the rate of 1 t.ha-1 (C1), 2 t.ha-1 (C2), and 4 t.ha-1 (C4), in order to study, by comparison to a non-limed control the effects of these supplies on the characteristics of the soils, the sub-soils and the tobacco, cultivated in a two year rotation with winter wheat. This report sums up the situation about the changes in soils and sub-soils since the study began until 1988, and in Burley tobacco cv. BB 16 from 1980 to 1988. After 16 years of liming, i.e. a total amount of 10-11 t.ha-1 for C1, 20-22 t.ha-1 for C2, and 36-38 t.ha-1 for C4, pH (KC1) in soils increased by 1 - 1,7 and 2,2 units, respectively. The sub-soils were modified in the same way and the changes in pH (KCl) were similar to those in the soils, indicating a fast downward translocation of the cations. The corrective effect of magnesium-limestone on soil acidity was greater during the first years of liming than in later years : when the doses are accumulated, the effectiveness of liming decreases with time. During the same period, exchangeable Ca-contents in the soils of C1, C2 and C4 were multiplied by 1,3 - 1,7 and 1,9 respectively. The same changes were noticed in the sub-soils. Exchangeable Mg-contents in soils raised markedly and were multiplied by 2,3 - 3,5 and 3,8 respectively. Mg-contents in the sub-soils were modified in a very similar way. When the largest amounts of Mg-lime were applied (C4), a significant part of the Ca, and possibly of the Mg as well, moved beyond the depth of 60 cm. Exchangeable K-contents in the soils decreased with time in all plots, and the doses of liming had no influence. Among the extractible oligoelements of the soil, only manganese and copper seemed to be influenced by liming. The doses of magnesium-limestone applied seemed to have no effect on the organic matter-contents in the soils and the sub-soils. The effect of liming on yields and quality-index of the Burley tobacco cv. BB 16 was noticeable in only a few years, with a general downward trend when the largest doses were applied. Changes in burning rates were inconsistent. On the other hand, the chemical characteristics of cured leaves were sometimes markedly modified : Ca-content in tobacco slightly decreased, K-content decreased systematically and notably, reaching a lower threshold in the most limed plots, Mg-content increased significantly, being multiplied by two fold in C4, ash-content decreased slightly. C1, total N and NO3-content were not modified by liming, whereas total alkaloids-content was generally the lowest in tobaccos from the non-limed plots. Oligoelement-contents in the cured leaves were mostly closely related to the amounts of magnesium-limestone applied : Mn-content and Zn-content rapidly decreased with supplies, B and Cu-content exhibited the same trend, but in a lesser way. Only Mo-content increased with the soil pH, whereas iron-content was not modified by the limestone supplies.