A field experiment was conducted on a recently reclaimed calcareous soil in the Exiphrates' basin, Syria, to study the effect of soil applications and foliar sprays of Fe, Imo, Zn and Cu sulphates on cotton (var. Alepo 40) yield and fibre characteristics. The soils are highly calcareous (15-25% CaCO3), very low in organic matter (<l%),, and rich in total iron and manganese oxides, but DTPA extracted very little "available" iron. There appeared to be adequate Ma and Cu but DTPA extractability of Zn declined to near critical levels after 90 days of cotton growth. Submergence (anaerobism) alone was ineffective in increasing iron solubility when no organic source was applied to the soil. Easily decomposed organic material (starch or freshly collected green manure) could bring much Ire 2+ into solution. This solubility increased greatly when fresh hydroxides were also included. Green manure (clover) enhanced Pe, Mn and Zu availability in the field up to 75 days after incorporation. In pot experiments, green manure was effective in improving Ma, Zn and particularly Fe nutrition of cotton. Iron supplied by the organic complexing activity of green manure or its products was clearly quite different from inorganic iron from ferrous sulphate additions. The green manure prevented iron precipitation within the root even in the presence of phosphate, and it increased iron translocation to the upper plant portions. Natural organic cbelate sources of iron are thus much more mobile and active within the plant. Iron and zinc sulphate application to soil caused 5 and 10% increases in the cotton yield, while their application to plants as sprays resulted in 12 and 23% yield increases respectively. Iron also improved significantly lint smoothness. The yield of cotton in the field was increased by ferrous sulphate sprays and soil applications largely because of the predominance of Mn solubility over Fe in the root medium rather than because iron "availability" was low. There was clear Ma/Fe and also P/Fe antagonism in the plants.