Recycling coffee and tea wastes to increase plant available Fe in alkaline soils

Coffee beans and tea leaves contain large amounts of potentially metal-chelating substances which could remain in the wastes after extraction by hot water. The following two experiments have been carried out: a) an incubation experiment with the objective of verify whether coffee grounds and green tea wastes could be used as an Fe chelating agent to increase Fe availability to plants in the soil; b) a pot experiment to verify the effect of those composts on the Fe content of the edible part of vegetables. Japanese leaf radish (Raphanus raphanus sp), whose the leaves are the edible part, was chosen as test plant. Calcareous subsoil (shell fossil soil) with original pH 9.3 and a B horizon of Andisol (Typic melanudand) with pH adjusted to 7.7 were used. For the incubation experiment, the treatments included of the direct addition of Fe at rates of 0 (control), 10, 20 and 40 μg g−1 dry soil as ferrous sulfate (FS); coffee waste compost (CWC) and tea waste compost (TWC). Both composts contained approximately 40 g Fe kg−1 dry mass. Thus, the total amounts of CWC and TWC added were of 0, 0.25, 0.5 and 1.0 mg g−1 soil. Considering a soil density of 1 g cm−3 and 10 cm of plow layer, the total amounts of compost applied were of 0, 0.25, 0.5 and 1.0 ton ha−1. Soil samples were collected after 30 and 60 days of incubation and then analyzed for plant available Fe. For the pot experiment, the doses of 0 (control) and 1 mg g−1 soil of CWC or TWC were used to grow radish. Plants were harvested after 60 days. For samples incubated for 30 days, the CWC and TWC treatments led to the largest increase in the ammonium bicarbonate diethylene triamine pentaacetic acid (AB-DTPA) extractable Fe levels of both soils (P < 0.05). After 60 days of incubation the amounts of AB-DTPA-extractable Fe in soil samples treated with both composts were always higher than in those treated with FS alone. For both soils, the application of 40 μg Fe g−1 dry soil as CWC or TWC enhanced significantly (P < 0.05) the total Fe content of radish shoots compared to the control. We concluded that it has been possible to increase the plant-available Fe in neutral to alkaline soils using coffee grounds and tea leaf wastes composted with FS. However, more research on the effectiveness in field conditions are necessary.