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. Macroelements
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.