Soil zinc fertilisation does not increase maize yields but improves nutritional quality

AIMS Fertilisation of crops with zinc (Zn) is considered important to enhance agricultural productivity and combat human deficiencies in sub-Saharan Africa. However, it is unclear on which soils Zn fertilisation can lead to higher yields and increased grain Zn concentrations. This study aimed to find soil properties that predict where soil Zn is limiting maize yields and grain Zn concentrations, and where these respond positively to Zn fertilisation. METHODS Zinc omission trials were set up at multiple farm locations in Kenya (n=5), Zambia (n=4) and Zimbabwe (n=10). Grain yields and tissue Zn concentrations were analysed from plots with a full fertiliser treatment as compared to plots where Zn was omitted. RESULTS Zinc uptake (R2 = 0.35) and grain Zn concentrations (R2=0.26) on the plots without Zn fertiliser could be related to a limited extend to soil Zn measured in extractions that measure labile Zn. A positive maize yield response to soil Zn fertilisation was found at only two out of nineteen locations, despite soil Zn levels below previously derived critical concentrations at most locations. Soil properties nor plant concentrations were able to explain maize yield response to Zn fertilisation. However, a positive response in Zn uptake and grain Zn concentrations to Zn fertilisation was found at the majority of sites. CONCLUSIONS We conclude that soil Zn fertilisation can increase maize grain Zn concentrations, especially in soils with low pH and organic carbon content. Predicting a yield response to Zn fertilisation based on soil properties remains a challenge.