Speaker
Description
Proliferation of the aquatic weeds water hyacinth and hippo grass in freshwater bodies of tropical and subtropical areas causes serious ecological and socio-economic problems. The weeds have been reported to block waterways, clog hydroelectric power generator turbines and to deplete oxygen for aquatic life. Various efforts to control these weeds have been ineffective. In areas with soils of low fertility, using the aquatic weed biomass as soil amendments for crop production might be an effective way of controlling the weeds. We investigated the potential of using biomass from these two weeds as sources of nitrogen (N) for sorghum on a low fertility tropical soil in a greenhouse study. The objectives were: (i) to determine the N mineralization rates constants (k) of the biomass (ii) to establish the relationship between k and N uptake by sorghum, and (iii) between k and sorghum dry matter (DM) yield. An incubation study was conducted to determine N mineralization from soils amended with weed biomass and a pot study to establish N uptake by sorghum and sorghum DM yield in 44 days. Treatments used were:(i) untreated weed biomass (ii) bokashi from weed biomass (iii) compost from weed biomass (iv) chemical fertilizer (10 % N: 20% P2O5 :10 K20) and (v) soil alone. The weed biomass and chemical fertilizer applied per pot contained 1.13 g N, equivalent to a field application of 90 kg N/ha to a sorghum crop with 80,000 plants per ha. The soil moisture during the study was maintained at 60% of the soil’s total pore volume. Nitrogen mineralization from the organic amendments followed the first-order kinetics equation N min〖(t)〗=Nₒ(1-e^(-kt)); where k (day-1) is the N mineralization rate constant; Nmin(t) is the cumulative mineralized N at time t, and N0 is the potential mineralizable N at t0. For water hyacinth, compost had the highest k of 0.0113 day-1 followed by untreated biomass with 0.0096 day-1, and bokashi with 0.0083 day-1. For hippo grass, compost had the highest k of 0.0119. day-1, followed by bokashi with 0.0092 day-1 and untreated biomass with 0.0054 day-1. Grouped k values were: 0.0116 day-1 for compost, 0.0088 day-1 for bokashi, and 0.0075 day-1 for untreated biomass, showing that compost had the highest N mineralization rate, then bokashi, and lastly untreated biomass. Nitrogen mineralized from water hyacinth biomass, reflected the order of k values of the biomass, with 297 mg N/kg, 333 mg N/kg and 351 mg/kg for untreated weed, bokashi and compost respectively. A similar trend was observed for hippo grass biomass, with 199 mg N/kg, 291 mg N/kg and 285 mg N/kg for untreated biomass, bokashi and compost respectively. The grouped mean sorghum DM yields were: 15g/pot for compost, 12g/pot for bokashi and 7g/pot for untreated biomass. The N uptake by sorghum followed the same pattern as the DM yield. A significant (p = 0.034) strong positive linear correlation (r=0.854) was found between sorghum DM yield and the biomass k. A similar significant (p=0.023) strong positive linear correlation (r=0.874) was found between sorghum N uptake and the biomass k. The mean agronomic effectiveness of aquatic weed biomass in producing vegetative sorghum DM compared to chemical fertilizer was 163 % for compost, 97 % for bokashi, and 1.4 % for untreated biomass. We concluded that the two aquatic weeds can supply adequate N for sorghum on low fertility tropical soils when made into compost or bokashi.
| Keywords | Aquatic weed, compost, bokashi, nitrogen mineralization rate, nitrogen uptake |
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