The diagnosis of zinc deficiency and effect of zinc on the growth and chemical composition of some tropical and sub‐tropical legumes

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Abstract
Eight tropical and sub‐tropical pasture and forage legumes were grown in pots of zinc deficient soil with varying additions of zinc chloride. Growth responses and the chemical composition of the plant tops and shoots were recorded, and from the latter, tentative critical concentrations of zinc were established and possible use of phosphorus‐zinc ratios were discussed. Species ranked in response to applied zinc in the order Desmodium intortum > Centrosema pubescens > Macroptilium atropurpureum > Neonotonia wightii > Vigna sinensis > Stylosanthes humilis > Lotononis bainesii > Medicago sativa. In the majority of species the relationship between dry matter yield of tops and zinc concentrations in plant tops showed a “Piper‐Steenbjerg Effect"1,2 which was associated with high concentrations of phosphorus in plant tissue, especially in plants displaying visual foliar symptoms of zinc deficiency. To overcome this problem in the diagnosis of zinc status of individual samples the authors recommend the use of multi‐element analysis of plant material, discarding samples or modifying the zinc diagnostic values for those samples with phosphorus concentrations in excess of 0.4% on dry weight basis. An alternative recommendation is to use diagnostic ratios of phosphorus to zinc. Critical zinc concentrations for the above species were 19, 20, 24, 22, 17, 34, 22 and 16 ppm respectively.