Variation of Mineral Micronutrient Elements in Robusta Coffee (Coffea canephora Pierre ex A. Froehner) As Measured by Energy Dispersive X -Ray Fluorescence

Introduction: In many parts of the world, Coffea grows between latitude 25°N and 25°S and requires specific climatic conditions to produce high bean yield and quality. In Uganda, Robusta coffee is grown in a diversity of soils that range from red sandy clay or gravelly loam to soft laterite. Macro-elements such as K, Ca and Na though detectable through EDXRF are not as heavy as trace elements such as Fe, Zn, Cu, Mn, a factor that makes the micro-elements more amenable to EDXRF measurement. The EDXRF spectrometer instrument has a semi-conductor detector that measures the entire energy spectrum when placed closer to the sample to minimize loss of energy by the fluorescent X-ray.

Aim: Robusta coffee growing countries of the world such as Uganda, have experienced a decline in market prices since 2002 due to changed consumer preferences for other beverages. In Uganda, where Robusta coffee is the major income earner, one practical way of ensuring that coffee prices remain competitive is to enhance quality. The aim of this study was to analyze the extent of macro and microelement densities in the soils of Robusta coffee growing Ugandan farms, their relationships with density levels in the leaf and in the green bean and their ultimate contribution in the determination of cupping and marketing qualities. This information would enable coffee producers in Uganda to make decisions on which organic and inorganic fertilizer applications are needed to alleviate coffee quality which in turn is the key determinant of world prices.

Study Design: Soil, leaf and ripe cherry samples were derived from sixty seven tagged trees in twenty three on farm locations at Kawanda and Entebbe in fields that had no fertilizer application history.

Place and Duration of the Study: This work was conducted at the National Agricultural Coffee Research Institute (NaCORRI), Uganda and the Institute of Nuclear Science and Technology, University of Nairobi, Kenya, between January 2010 and December 2013.

Methodology: Nutrient densities of potassium (K), calcium (Ca), iron (Fe), titanium (Ti), zinc (Zn), copper (Cu), boron (B), lead (Pb) and manganese (Mn) of sixty seven soil, leaf and green bean samples collected from 23 districts were measured with Energy dispersive x-ray fluorescence (EDXRF). All the samples were oven-dried at 800C for 18-20 hours and ground to about 50 mm sieve-size. Three pellets of 2.5 cm in diameter were made and irradiated with a Cd-109 radioactive source. The X-rays emitted by the elements were detected by liquid nitrogen cooled Si (Li) detector and data analyzed with the Quantitative X-ray Analysis System (XQAS/AXIL) software.

Results: The results showed that the mean concentrations of Ca, Mn, Fe and Zn were highest in the soil but lowest in green beans. K concentrations were higher in green beans than in the leaf and were lowest in the soil. In most cases, the concentrations of microelements found in the soil determined the levels in the leaf and in the green bean. Microelement availability and concentrations were influenced by the interaction between the various elements, tree age, elevation, soil texture and location.

Conclusions: Mineral element K and microelement Cu were found in their highest concentration in the green bean. Mn, Zn and B were important elements in the determination of organoleptic cup attributes whereas soil, leaf and green bean nutrient concentrations were important in determining the medium size of the bean. EDXRF is an effective tool in the measurement of micronutrient concentrations of Robusta coffee growing soils, leaves and green beans. Cu was an essential microelement of physiological and metabolic processes in both leaves and in the green bean whereas Mn, Zn and Fe are critical microelements during photosynthesis. While all the trace elements measured, namely, Mn, Cu, Fe and Zn were crucial determining factors in bean and cup qualities of Robusta coffee, Zn and Mn were the more important microelements during fruit development. Soil Mn positively influenced flavor and aroma cup attributes whereas green bean amounts of B and Zn positively influenced trigonelline and caffeine concentrations.

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