The status of selected micronutrients in Zambian soils and the response of wheat to their application
The chemical properties of twelve Zambian soils, sampled from different agro-ecological zones, were studied especially for total and available micronutrient contents. Total contents ranged from 25.1 to 73.2, 5.2 to 116.5, 151 to 1297 μg g^-1 and 1.5 to 12.7% for Zn, Cu, Mn and Fe respectively. Available (DTPA-extractable) Zn and Cu, (hot-water-soluble) B and (ammonium oxalate-extractable) Mo contents were mostly low and for B and Mo often below the minimum critical limit for plant growth. This was especially the case for the highly leached soils of the high rainfall areas. Total Cu content of the soils was highly correlated with the extractable amounts. A selection of extractants was compared for B, Cu, Fe, Mn and Zn. The amounts extracted by the different extractants for each element were highly correlated with each other in most cases, especially for the 8 Cu extractants. Hydrochloric acid (0.1M)-extractable Cu had the lowest correlation coefficients with other extractable Cu contents; ranging from 0.609 to 0.758. Ammonium bicarbonate-DTPA-extractable Cu had the highest correlation with total soil contents. Magnesium chloride (1.0M) gave the lowest correlations among the extractants for Zn. The extractants evaluated for Fe and Mn correlated well with each other. In a pot experiment on one soil (Misamfu) and one wheat variety (Whydah), zinc application increased dry matter yield and plant concentration and uptake of both Zn and Cu. Copper application increased the concentration and uptake of Cu and Zn. Liming increased total dry matter and the uptake of both Zn and Cu. It decreased the residual DTPA-extractable Zn and Cu in soils and the shoot concentration of these nutrients. In another pot experiment, B application to two varieties (Aldan and Whydah) at the rate of 1.0 μg g-1 increased grain yield for Aldan, but suppressed dry matter yield for both. The growth of wheat seedlings on agar was investigated to screen varieties for aluminium tolerance. Despite the problem of root establishment, there were differences between varieties in plant height and CO2 respiration rate.