This thesis presents the results of field experiments conducted in the highly productive farming system of the Nepalese Terai. The main objective of the research was to investigate the value of Tithonia diversifolia (tithonia) as a green manure to increase productivity of the rice crop and improve soil fertility.
Field experiment results indicated that tithonia can be propagated either through seeds or stem cuttings from April to May under the Nepalese Terai environmental conditions. Seed propagation after May is not successful.
The growth rate of tithonia is slower than the commonly used green manure species sesbania (Sesbania cannabina). One to one and half tonnes of tithonia dry matter can be harvested from one hectare of land when tithonia is grown for 70-80 days. On average one tonne of tithonia dry matter will supply 31.5 kg N, 4.05 kg P and 10.4 kg K.
A litterbag technique was used to compare the decomposition and nutrient releasing characteristics of tithonia along with the common green manures asuro (Adhotoda vasica) and bakaino (Melia azedarchta) under three incorporation treatments. At an early stage tithonia decomposed faster than asuro and bakaino but later on there was no difference. Under surface mulching all three green manures decomposed more slowly, than when ploughed into paddy and irrigated. Bakaino released N faster whilst asuro released P faster. Tithonia was intermediate between asuro and bakaino in mass loss and nutrient releasing.
Field experiments were conducted for two cropping seasons on a sandy loam and slightly acidic soil under rice-wheat-fallow and rice-vegetables-maize/green manure or fallow cropping systems. Tithonia was tested for its potential value as a green manure to improve rice productivity, using biomass transfer. A comparable grain yield of rice was obtained from chemical fertilizer applied at a rate of 75:10:30 kg N: P: K ha-1 and from tithonia biomass when applied at a rate of 2.5 t ha-1 (dry matter basis). Productivity was further
improved when half the P was supplied from single super phosphate fertilizer and half fromtithonia biomass. The application of sesbania as a green manure did not result in a largerrice yield than with tithonia.
Tithonia can supply more than one essential plant nutrient to the soil and nutrients are released immediately after incorporation into the soils. The release of all three major nutrients was higher at early stage (10-15 days after incorporation) and declined gradually. Under flooded conditions NH4+ -N dominated the N pool but as the crop reached maturity,NO3--N dominated as the soil gradually dried. Soil phosphorus concentrations measured using both the Olsen-P and iron oxide filter paper method did not differ significantly
whether P was supplied from chemical fertilizer or tithonia. K was significantly higher in soil and in rice plants when tithonia biomass was applied compared to the treatments without tithonia and the concentrations of K was comparable with K in soil and plants treated with artificial fertiliser K.
For in situ green manuring tithonia can be fitted into the crop rotation after the wheat harvest and before the transplanting of main season rice. However, biomass production was dependent upon seasonal variation and the number of days the plant was grown in the field. Growing tithonia and sesbania together was not found to be feasible because of different planting times. The dynamics of soil N and P under in situ conditions and biomass transfer were similar. However, the field results conducted in the two seasons were inconsistent. Both tithonia and sesbania affected the main companion crop maize under a relay system but tithonia reduced the maize yield more heavily.
A participatory survey of farmers from two districts showed that farmers thought that green manure technology could potentially improve crop productivity of rice and enhance the fertility level of soils that are under intensive cultivation. Farmers liked rice bean, sunn-hemp and sesbania due to their fast growth, quick decomposition and positive effect on rice yield. However, there are some constraints associated with the promotion of green manure technology in the study area. The main constraints as indicated by the farmers are the unavailability of seeds for green manure crops, lack of irrigation and limited access to tractors to incorporate the green manure into the soil.
In conclusion tithonia may be a potential green manure for improving rice productivity under a biomass transfer system but for in situ green manuring there is a need for more field research before recommending it to farmers. The results have indicated that at least 2.5 t ha- 1 dry matter is required to get a satisfactory crop yield whereas the production of tithonia biomass under normal condition is only 1.5 t ha· , since under an intensive farming system the fallow period is quite short. There is an issue of sustainability under both biomass transfer and in situ systems because of nutrient mining in the former system and in latter system an external supply of plant nutrients is still required. However, tithonia could be a very promising plant for the areas of the lower hills where it could be grown in hedgerows in the sloping terraces and used for multiple purposes.