Intensification of subsistence agricultural on the Nembi Plateau, Papua New Guinea 2. Organic fertilizer trials.

Abstract

Four sources of organic fertilizer were compared as part of a research programme to find means of intensifying subsistence agriculture on the Nembi Plateau. The organic fertilizers tested were: compost formed within sweet potato (Tpomoea batatas (L.) Lam.) mounds using the technique that is traditional in the Papua New Guinea highlands but not widely used in the study area; the aquatic nitrogen fixing fern Azolla pinnata R. Br.; pig manure; and coffee pulp. The following were conducted: (a) three sweet potato rate-of-compost application trials, (b) a sweet potato organic fertilizer trial that compared Azolla pinnata, pig manure and coffee pulp, (c) a trial evaluating the effect o/ Azolla pinnata and soil cultivation on taro (Colocasia esculenta (L.) Schott) in a drained doline. Large responses were obtained to compost in each trial. An optimum economic application rate of S3 t/ha of fresh organic matter was indicated. A lower application rate of 20 t/ha is recommended for the Plateau. Azolla pinnata did not increase taro yields and resulted in a nonsignificant increase in sweet potato yield. Pig manure gave a significant yield response when applied to sweet potato and had a positive residual effect on sweet potato yield in a demonstration plot. Coffee pulp gave the largest increase in sweet potato yields. Examination of the major nutrients supplied in the sweet potato trial suggests that the response was due mostly to potash contained in the various organic fertilizers. Limitations on the usage of various organic fertilizers arc discussed. The use of compost and coffee pulp as fertilizers should be promoted in sweet potato gardens on the Plateau. Pig manure will have an important role in improved farming systems, but further research is needed on this and also on the usefulness of Azolla pinnata.