Arenosols cover extensive areas in coffee producing, humid tropical countries of Sub-Sahara Africa (Angola, DR Congo) and Southern America (Brazil). A laboratory experiment was undertaken to examine the potential of using coffee waste to improve the physico-chemical properties of an Arenosol from DR Congo. The amendment was applied at three rates to the topsoil and incubated in soil columns at field moisture capacity for 24 months. A control without any amendment was integral to the completely randomized experimental design involving three replicates. The soil columns were watered weekly with an amount of distilled water approximating to 87% of the average rainfall. Every 3 months, the soil chemical properties and the fraction of leached water and cations were measured. All application rates raised soil pH above 5.5 within 3 months. Exchangeable Ca, Mg and K showed respectively, 5 to 7-, 2 to 3-and 7 to 14-fold increases with increasing application rates. Organic C and total N significantly increased within 6 months to ca. 1.5 and 0.12% respectively, inducing a decrease in the C ⁄ N ratio from 17 to 13. The combined action of increasing soil pH and organic C contributed to a significant increase in cation exchange capacity. Increases in available P were significant, but temporary, with maximum values attained at 9 months. Coffee waste application significantly increased the fraction of retained soil water from 53% to 60%. It promoted the retention of basic cations, immobilized Mn, but increased the mobility of Fe. Coffee waste has the potential to be used as a liming material, an NPK fertilizer and has the benefits also of increasing water and nutrient retention.Coffee waste application to improve sandy soils 95

A 17‐year chronosequence of Acacia auriculiformis fallows on Arenosols of the Batéké Plateau (D.R. Congo) was surveyed and compared with virgin savannah soils to assess chemical soil fertility changes induced by these N‐fixing trees. Significant increases in organic carbon content, total nitrogen content, cation exchange capacity and sum of base cations were found after relatively short fallow periods of only 4 years and did not only affect the forest floor, but extended to at least 50 cm depth. The Acacia act as a major source of organic matter (OM), hence increasing organic carbon and nitrogen content and decreasing the C/N ratio. The increased OM content suggests that humification processes are the main cause of the significant decrease in pH. Total exchangeable cations initially increased slowly but doubled (topsoil 0–25 cm) and tripled (subsoil 25–50 cm) after 10 years. The point of zero net proton charge was systematically lower than soil pH and decreased with increasing OM content, thereby increasing the cation exchange capacity, although concurrent acidification retarded a significant beneficial impact at field pH on Acacia fallows of 10 years and older. Although the chemical soil fertility improves steadily with time, after 8 years of Acacia fallow the absolute amounts of available nutrients are still small and slash and burn practices are required to liberate the nutrients stored in the remaining biomass and litter before each new cropping period.