This research involved two experiments and was conducted in the experiment station of Faculty of Agriculture, Islamic University of Riau, Indonesia. This experiment was arranged with a randomly simple design with two factors. First, the effects of water hyacinth (Eichhornia crassipes) compost and rice husk ash on Brassica juncea, and secondly the effects of water hyacinth compost and Rhizobium inoculant on soybean (Glycine max) were investigated. The data analysis was performed with Duncan’s multiple range test with significant differences at p <0.05. The net assimilation rate and mean relative growth rate were used as indicators for the dry weight accumulation of plants. The combination of water hyacinth compost and rice husk ash had an interacting effect on soybean and Brassica juncea. An increase of net assimilation rate was followed by an increase in mean relative growth and a significant increase in dry matter weight was also observed. A combination of water hyacinth compost and rice husk ash had a significant positive effect on plant growth in swamp land. Rice husk ash can potentially balance excess iron and aluminum levels in soil impedes the absorption of essential minerals in swamp land. Rice husk ash can potentially balance excess iron and aluminum levels in soil that impedes the absorption of essential minerals in swamp land. The combination of water hyacinth compost and rice husk ash in swamp land increased the production of seed dry weight of soybean and crop dry weight of Brassica juncea.
