Effects Of Increasing Levels Of Copper On The Mineral Nutrition And Growth Of Coffea Arabica L. In Kenya
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Copper fungicides are intensively applied on Coffea arabica L. in Kenya to control coffee berry disease (Colletotrichum coffeanum Noack) and leaf rust (Hemileia vastatrix Berk. & Br.) and to induce vigorous growth (tonic effect). It is thought that this continual use of Cu could increase Cu levels in the soil to an extent where the nutrition and growth of coffee would be affected. To study this possibility, experiments were carried out in the field in coffee plantations, some of which had been sprayed with Cu fungicides for over 5 years, whereas others had never been sprayed. Experiments were also conducted on young coffee seedlings in greenhouse to study both the effects of increasing Cu'concentrations on plant growth and also the extent to which such factors as pH and P fertilization could influence the uptake of Cu by the coffee plants. The field studies .howed soil Cu concentrations greater than 10 ppm in the regularly Cu-sprayed coffee plantations, whereas soil Cu values in the uns rayed plantations were lower than 3 ppm Cu. These levels of soil Cu, however, did not produce severe nutritional disorders in the coffee plants, although scattered signs of·interveinal chlorosis and yellowing were observed in the leaves of the Cu sprayed ptants; High m leaf-and-bean- Cu were also associated with increased Cu spray and in soil available Cu. The absence of pronounced Cu toxicity symptoms in the field at this stage, is thought to be due to the high level of soil-organic matter, and heavy fertilization with phosphate both of which contribute to Cu fixation in the soil. Also the strong accumulation of Cu in the top-soil may have contributed to this by rendering Cu unavailable for absorption by the deeper roots of mature coffee plants. Experiments in soil, in sand and in water culture with young coffee plants (in the greenhouse) showed that Cu treatments greater than 1.064 ppm in the water and sand cultures, and greater than 5 ppm in the soil culture, resulted in stunted plant growth, severe necrosis of roots and leaves and hindrance of nutrient translocation. Effects of high Cu in the medium were most marked in acid soils (pH 4.5-5.0), and less pronounced in moderately acid (pH 5.5-6.0) and neutral (pH 7.0-7.5) soils. Phosphorus treatments, from 140 to 240 ppm P, and Cu treatments at 1.064 ppm Cu in sand culture, separately promoted plant, growth compared to the control treatment. The increase in growth, at 1.064 ppm Cu and at 140 ppm P, over the control plants, grown in the complete nutrient solution, showed that the original levels of Cu (0.064 ppm) and P (40 ppm) in the solutions were inadequate. Nutrient-P levels of 140 - 240 ppm in combination with high Cu levels, (10.064 - 100.064 ppm Cu), caused severe reduction in plant growth. At high P levels (440 - 840 ppm P) in the presence of low Cu levels (less than 1.064 ppm tu), a decrease in leaf-Ca and-Mg, an' increase in leaf-K and severe chlorotic symptoms in the leaves were observed. The increase in 1eaf-K was undoubtedly due to the additonal K supplied (to the nutrient solution) by the P-carrier (KH2P04). At medium toxicity levels of Cu, increased P levels tended to reduce the Cu toxicity effects on the plants, but at severely toxic levels of Cu, the addition of heavy doses of P was ineffective. Significant Cu x P interactions occurred for the major nutrients in the leaves whereas there was a high accumulation of most of the trace elements and P in the roots at high Cu and P treatments. Anatomical observation on the leaves and roots of plants affected by high Cu absorption revealed severe cell destruction and abnormalities in the cellular structures. -Increasing the pH of acid soils to near neutrality and the application of P-fertilizers reduced the high Cu effects.