Assessing the residual benefit of soil-applied zinc on grain zinc nutritional quality of maize grown under contrasting soil types in Malawi.

Abstract A proper understanding of the residual value of zinc (Zn) is necessary for sustainable biofortification of food crops. This study aimed to establish the extent to which application of Zn at the national rate, plus two experimentally elevated rates, in one year provided any benefit to plant yield and nutritional quality in the following growing season. Residual effects of soil-applied Zn on grain Zn concentration and uptake were estimated by an experiment in which maize was grown in successive seasons at two agricultural research stations in Malawi, with Zn applied to the soil in the first season but not the second. At each site two common soil types were used: Lixisols and Vertisols. The study used three Zn fertilizer rates of 1, 30 and 90 kg Zn ha -1 applied to the soil in the previous cropping season, arranged in a randomized complete block design (RCBD) with 10 replications at each experimental site. At harvest, maize grain yield and Zn concentration in grain and stover were measured; Zn uptake by maize grain and stover were determined and Zn harvest index was calculated. Effects on grain yield and Zn uptake by the crop were assessed in relation to residual Zn fertilizer and soil type. Maize grain yield on plots in the second season where 30 kg Zn ha -1 had been applied exceeded that on second season plots where 1 kg Zn ha -1 had been applied by 25%. The grain Zn concentration and Zn uptake in the second season after fertilizer application were larger by 13% and 30% respectively on the plots which had received 30 kg Zn ha -1 than those which had received 1 kg Zn ha -1 . There was no evidence that applying Zn at 90 kg Zn ha -1 resulted in larger crop yield, grain Zn concentration, or Zn uptake the second year after application than was seen in plots the second year after application of 30 kg Zn ha -1 . The magnitude of the benefits attributed to residual effects of soil-applied Zn did not depend on soil type. Conclusively, the residual effects of 30 kg ha -1 of soil-applied Zn in the preceding season benefited the subsequent maize compared to the national recommendation of 1 kg Zn ha -1 . The benefits of larger applications of Zn than the current national recommendations should be considered across at least two seasons and for different crops.