Assessment of the potential of Siam weed (Chromolaena odorata) in enhancing soil fertility status in Serengeti District, Tanzania

Siam weed (Chromolaena odorata) is a plant that has infested a large area of agricultural land in Serengeti district and has adversely reduced crop land and crop yields. It is encroaching land and especially in the famous Serengeti National Park. The weed has been reported to contain high amount of plant nutrients in its tissue, hence its probable use as an organic soil amendments, with aspect to soil fertility improvement. A study was, therefore, conducted to address the Siam weed growth requirements, nutrient contents in its tissue and its suitability and use as an organic amendments as a way to manage the weed accordingly and appropriately. Soil and Siam weed plant sampling was done in the Serengeti district (Mara Region), an analysis was done in the Soil Science laboratory at SUA, Morogoro. Composite topsoil (0 – 20 cm) samples were collected and analysed for physico-chemical properties hence shows to have slightly acidity with pH (5.8 to 6.5), low to high CEC (11.6 - 29.4 cmol (+) kg −1 ) and very low to very high soil organic carbon (0.36 - 3.69%). Three soil profiles on a toposequence from the infested soil were excavated, described and sampled for soil laboratory analysis, hence shows medium acidic to medium alkaline (pH 5.84 to 7.93), low to high CEC (10.12 - 35.20 cmol (+) kg -1 ), low to medium SOC (0.11 - 1.97%) and high exchangeable bases 24.45 cmol (+) kg −1 based on the rating by Landon (1991). Using the morphological and laboratory data the soils were classified to the subgroup level of the USDA Soil Taxonomy as Ustic Torripsamments on sloping land, as Haplic plinthustults on midslope and as Plinthaquic paleudalfs on low land and to Tier-2 of WRB as Rendzic Lithic Leptosols (Eutric, Dystric, Tephric) on sloping lands, as Gleyic Plinthic Acrisols (Ferric, Humic) on midslope and as Haplic Stagnic Gleyic Luvisols (Chromic) on low or flat areas. Incubation study was done by incorporation of different parts of Siam weed plant with soil for 16 weeks. As a result of mineralization after incubation, the leaves released N (0.292%) and Fe (64.15 mg kg -1 ); bulbs released P (12.39 mg kg -1 ) and Mg (0.87Cmol (+) kg -1 ); stems released K0.22 cmol (+) kg -1 ) and Ca (5.13 cmol (+) kg -1 ) and roots released Cu (6.01 mg kg -1 ), Mn (77.18 mg kg -1 ) and Zn (1.65 mg kg -1 ). Nutrients released were increasing from the 0 to 12 th week and started to decrease at week 16 after incubation. Therefore, from incubation results, it is recommended that, farmers should grow their crops before the 12th week following incorporation of Siam weed biomass into soils for plants to absorb nutrients from the Siam weed.

AGRA.