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Onion Response to Lime on Acid Histosols as Affected by Ca/Mg Ratios¹

ABSTRACT

The objectives of this work were to determine critical ratios and concentrations of Ca and Mg in Histosols as related to onion (Allium cepa L.) growth and composition.

The molar Ca/Mg ratios extracted from soils were related to those found in onion and indicated that onion assimilated Mg more easily than Ca. Although the upper critical Ca/Mg ratio was not found, yields decreased rapidly as extracted soil and tissue Ca/Mg ratios became smaller than the lower critical ratio of 0.5. Onion yields, however, were not related to the soil extracted Ca/Mg ratio interval from 0.5 to at least 16, and a corresponding tissue range of 0.5 to 6.5.

Maximum yields were obtained when tissue concentrations of Ca+Mg ranged between 90 and 110 meq/100 g. These yields were obtained when soils released about 25–28 meq of Ca + Mg/100 g as extracted with a modified double acid procedure (0.05N HCl + 0.025N H₂SO₄). Concentrations higher than 16–18 meq Mg/100 g extracted from soils and 58–71 meq Mg/100 g in plants (depending on the concentration of Ca + Mg extracted from soils and absorbed by plants) exceed the upper critical limit and cause Mg-induced Ca deficiencies and reduced yields.

The lower critical concentration points for Mg, and conversely the higher critical concentration limit for Ca, was not identified. However, yields were not affected when soils released as little as 1.5 meq Mg/100 g and plants absorbed only 14 meq Mg/100 g if sufficient Ca was available to produce maximum yields. These data emphasize the importance of maintaining the Ca/Mg ratio in the proper range when liming acid histosols for vegetable production.

¹ Soil Research Service contribution no. 253, Quebec Ministry of Agriculture.

² Soil Scientist, Soil Research Service, 2700 rue Einstein, Sainte-Foy, Quebec, GIP-3W8, Canada; Soil Scientist, Agriculture Canada, Sainte-Foy; and Professor of Soil Science, Laval University, Sainte-Foy, respectively.

Received for publication September 18, 1978. Accepted for publication August 24, 1979.