Seasonal dynamics of nutrients in leaves and xylem sap of coffee plants as related to different soil compartments

The northwestern province of Costa Rica is a marginal coffee growing area. At the onset of the rainy season low redox potentials probably induce the mobilization of soil Mn resulting in enhanced plant uptake of Mn. To test this hypothesis we monitored from April to the end of June 1995 the mobile Mn in the soil and nutrient and Mn concentrations in leaves and xylem sap of coffee plants. Every 2 weeks we took aggregate and bulk soil samples. The aggregates were mechanically separated into interior and exterior, air-dried and all soil samples were extracted with 1 M NH₄NO₃. We also extracted the field moist soil with distilled water. In addition, the 3rd and the youngest pair of coffee leaves and xylem sap were sampled and analyzed. According to the results of leaf analyses the nutrient supply of the coffee plants in general seemed to be balanced. However, Mn concentrations of 223 mg kg^-1 in the 3rd leaf pair at 18 April were above the optimum and the youngest leaves indicated Fe deficiency, but senescent leaves accumulated Fe and overcame the deficiency. Manganese concentrations in the xylem sap showed a pronounced maximum 2 weeks prior to a similar maximum of mobile Mn in the aggregate exterior. But in general the temporal variation of nutrient concentrations (especially Ca and Mg) in the plants are well correlated with the easily extractable nutrient concentrations in bulk soil. Probably due to its specific absorption and high rates of redistribution within the plant, K in the soil extracts did not correlate with plant concentrations. Element concentrations of youngest leaves could not be correlated with soil concentrations and are not considered to be an adequate tool for monitoring current nutrient uptake. Since plant element concentrations did not correlate with the aggregate interior, plants probably cannot use that nutrient source efficiently.