Tag: Green manures
Alnus acuminata: an effective fertilizer tree for smallholder farmers in sub-humid regions of Rwanda
Fallow and sesbania effects on soil nitrogen dynamics in lowland rice-based cropping systems
Vast areas of rice-growing (Oryza sativa L.) lowlands in Asia are fallowed or cropped with non-rice crops for part of the year. Nitrate can accumulate during the fallow or non-rice crop, but this nitrate can be lost upon flooding for rice production. To determine fallow and green manure crop effects on soil nitrate and ammonium dynamics in lowland riceland, a 2-yr field study was conducted in the Philippines. Treatments before wet season rice were (i) Sesbania rostrata grown for either 45 or 60 d, (ii) weedy fallow, and (iii) weed-free fallow. Sesbania rostrata was sown with irrigation in late April-early May, rains started in early (1989) or mid-May (1990). Weeds and S. rostrata were incorporated after soil flooding on 23 June. Rains increased soil water-filled pore space to above 0.75 mL mL1 between mid-May and soil flooding. Weeds and S. rostrata assimilated soil nitrate, as evidenced by lower (P < 0.05) nitrate in those treatments than in the weed-free fallow. The decrease in soil nitrate in the weedfree fallow from 24 April to before soil flooding (15 kg N ha1) was apparently due to denitrification or leaching; additional nitrate (19 kg N ha1 in 1990) disappeared after soil flooding. Ammonium-N was rapidly released from incorporated weeds and S. rostrata. It reached a maximum by 36 d after incorporation, which correlated (r = 0.95) with N accumulation by rice at 45 d after transplanting. Results suggest that weeds and crops before rice can reduce soil N loss by assimilating nitrate-N and then cycling this N through incorporated plant residues back to the soil where it is rapidly mineralized and used by rice.
Organic materials as sources of phosphorus
Tithonia diversifolia as a green manure for soil fertility improvement in western Kenya: A review
Tithonia diversifolia, a shrub in the family Asteraceae, is widely distributed along farm boundaries in the humid and subhumid tropics of Africa. Green biomass of tithonia has been recognized as an effective source of nutrients for lowland rice (Oryza sativa) in Asia and more recently for maize (Zea mays) and vegetables in eastern and southern Africa. This paper reviews the potential of tithonia green biomass for soil fertility improvement based on recent research in western Kenya. Green leaf biomass of tithonia is high in nutrients, averaging about 3.5% N, 0.37% P and 4.1% K on a dry matter basis. Boundary hedges of sole tithonia can produce about 1 kg biomass (tender stems + leaves) m1 yr1 on a dry weight basis. Tithonia biomass decomposes rapidly after application to soil, and incorporated biomass can be an effective source of N, P and K for crops. In some cases, maize yields were even higher with incorporation of tithonia biomass than with commercial mineral fertilizer at equivalent rates of N, P and K. In addition to providing nutrients, tithonia incorporated at 5 t dry matter ha1 can reduce P sorption and increase soil microbial biomass. Because of high labor requirements for cutting and carrying the biomass to fields, the use of tithonia biomass as a nutrient source is more profitable with high-value crops such as vegetables than with relatively low-valued maize. The transfer of tithonia biomass to fields constitutes the redistribution of nutrients within the landscape rather than a net input of nutrients. External inputs of nutrients would eventually be required to sustain production of tithonia when biomass is continually cut and transferred to agricultural land.
Direct seeding of Sesbania sesban for green manure in agroforestry systems — a short communication
This study was based at the Agroforestry Research Centre, Maseno Kenya. The objective was to evaluate the effects of pre-treatment, seed rate and depth of sowing seed on direct seeding of Sesbania sesban. Direct seeding was shown to be feasible during the rainy season, provided a shallow seed bed was used. Direct seeding at 3 cm furrow depth led to significantly (P = 0.05) greater germination compared to 10 cm depth. There was no effect of pretreatment and seed rate on germination, early seedling growth and dry matter partitioning. Depth of sowing similarly had no effect on early growth and dry matter partitioning. Establishment by direct seeding is relatively non-labour intensive with a probable high adoption potential.
Agroforestry in the context of land clearing and development in the tropics
The background docuaent for this Workshop has highlighted the importance and urgency of developing scientific approaches to address the critical issues of land clearing. While reviewing and evaluating the current status of methods of land clearing and formulating technological packages for improvements, which are among the specific objectives of the Workshop, it is relevant as well as essential to consider the benefits and potentials of agroforestry and other integrated land, use systems. Land clearing in the tropics whether legitimate (with governmental knowledge and approval).or otherwise, is done either for bringing the land under some pre-determined use other than the existing one or for exploiting the timber and other natural resources, In government-approved land clearings for alternate land uses, it may be possible to adopt appropriate land management strategies to ensure adequate short-term productivity and longer-term sustainability, provided that the decision-makers and policy-planners are aware of the advantages of such modern approaches and are careful about implementing them. But in common practice, such a satisfactory situation does not exist mainly because of the lack of such modern knowledge
Short-term patterns of carbon and nitrogen mineralisation in a fallow field amended with green manures from agroforestry trees
The mineralisation of green manure from agroforestry trees was monitored with the objective to compare the temporal dynamics of mineralisation of litter from different species. Green manures from five agroforestry tree species were used on a fallow field during the long rainy season of 1997 (March–August) and from two species in the following short rainy season (September–January) in western Kenya. Different methods, i.e. measurements of isotopic ratios of C in respired CO2 and of soil organic matter (SOM) fractions, soil inorganic N and mass loss from litterbags, were used in the field to study decomposition and C and N mineralisation. Soil respiration, with the separation of added C from old soil C by using the isotopic ratio of 13C/12C in the respired CO2, correlated well with extractable NH4+ in the soil. Mineralisation was high and very rapid from residues of Sesbania sesban of high quality [e.g. low ratio of (polyphenol+lignin)/N] and low and slow from low quality residues of Grevillea robusta. Ten days after application, 37% and 8% of the added C had been respired from Sesbania and Grevillea, respectively. Apparently, as much as 70–90% of the added C was respired in 40 days from high quality green manure. Weight losses of around 80%, from high quality residues in litterbags, also indicate substantial C losses and that a build-up of SOM is unlikely. For immediate effects on soil fertility, application of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application.
Green manure production systems for Asian ricelands
Progress in adapting green manures (GMs) to Asia’s changing rice production
systems has been limited. Recent research has concentrated on the soil fertility
implications of GMs neglecting the socioeconomic and agronomic aspects that
ultimately determine the feasibility and cost effectiveness of GM systems. Recogni
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tion holistic evaluation and extrapolation of areas where GMs have a comparative
advantage over mineral fertilizer or other nonrice crops are needed. This paper
reviews the strategy for including GMs in rice farming systems and develops a
framework for analyzing the fit of soil
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improving legumes across a range of ecological
niches. Macroeconomic factors such as oil prices and national policy set the
general framework. Other important factors determining the specific adaptation of
GMs are the socioeconomic environment farm
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level economic and biophysical
conditions and availability of appropriate GM species and production technology.
The prognosis for soil
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improving legumes is seen to follow two key possibilities:
1) multiple
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use solutions where the GM also provides grain fodder or fuel; and
2) specificity of GMs to well
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defined niches within the cropping system. Agronomic
research in the hitherto neglected areas of species improvement seed production
crop establishment and pest management may expand the options for GMs in all
niches. The complexity and scale of the work require an international research effort
that provides leadership stimulates coordinated work and involves sharing priority
responsibilities among participating research institutions.