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.
Tag: fallow systems
Food-crop-based production systems as sustainable alternatives for Imperata grasslands?
Purely annual crop-based production systems have limited scope to be sustainable under upland conditions prone to infestation by Imperata cylindrica if animal or mechanical tillage is not available. Farmers who must rely on manual cultivation of grassland soils can achieve some success in suppressing Imperata for a number of years using intensive relay and intercropping systems that maintain a dense soil cover throughout the year, especially where leguminous cover crops are included in the crop cycle. However, labour investment increases and returns to labour tend to decrease in successive years as weed pressure intensifies and soil quality declines. Continuous crop production has been sustained in many Imperata-infested areas where farmers have access to animal or tractor draft power. Imperata control is not a major problem in such situations. Draft power drastically reduces the labour requirements in weed control. Sustained crop production is then dependent more solely upon soil fertility management. Mixed farming systems that include cattle may also benefit from manure application to the cropped area, and the use of non-cropped fallow areas for grazing. In extensive systems where Imperata infestation is tolerated, cassava or sugarcane are often the crops with the longest period of viable production as the land degrades. On sloping Imperata lands, conservation farming practices are necessary to sustain annual cropping. Pruned tree hedgerows have often been recommended for these situations. On soils that are not strongly acidic they may consistently improve yields. But labour is the scarcest resource on small farms and tree-pruning is usually too labour-intensive to be practical. Buffer strip systems that provide excellent soil conservation but minimize labour have proven much more popular with farmers. Prominent among these are natural vegetative strips, or strips of introduced fodder grasses. The value of Imperata to restore soil fertility is low, particularly compared with woody secondary growth or Compositae species such as Chromolaena odorata or Tithonia diversifolia. Therefore, fallow-rotation systems where farmers can intervene to shift the fallow vegetation toward such naturally-occurring species, or can manage introduced cover crop species such as Mucuna utilis cv. cochinchinensis, enable substantial gains in yields and sustainability. Tree fallows are used successfully to achieve sustained cropping by some upland communities. A variation of this is rotational hedgerow intercropping, where a period of cropping is followed by one or more years of tree growth to generate nutrient-rich biomass, rehabilitate the soil, and suppress Imperata. These options, which suit farmers in quite resource-poor situations, should receive more attention.
“Fertilizer trees” and Malawi’s new food security initiative
The first priority of the United Nation’s Millennium Development Goals (MDG) is to eradicate extreme poverty and hunger by 2015. Malawi is one of Africa’s chronic hunger hotspots. Over the past ten years, several agricultural reforms and food security Initiatives have been undertaken by the government, Including: liberalized market reform, food reserves during years of hunger, the targeted input programme (TIP), promotion of organic manure, small scale Irrigation schemes, and currently, the Targeted Fertilizer Subsidy Programme (TFSP) is underway.
Farmer assessment and economic evaluation of shrub fallows in the Humid Lowlands of Cameroon
Food crop production in highly populated areas around major cities of the humid lowlands of Cameroon is highly dependent on a fallow system (two–four years duration) mainly of Chromolaena odorata. Where such fallows have been in use for some time, problems of soil fertility with declining crop yields and higher incidence of weeds were reported. Although improved fallows have been widely adopted in sub-humid zones, there is no evidence of successful adoption of agroforestry-based technologies for soil fertility improvement in the humid forest areas. In response, ICRAF has developed a short fallow system with Cajanus cajan for soil fertility improvement in the humid lowlands of West Africa. Farmers’ response to these cajanus fallows is positive. Benefits reported are higher crop yields after cajanus fallows compared to natural fallows, clearing of cajanus is easier and the shrubs shade out the weeds. Women particularly appreciate the technology for its low labour demand and for the fact that these shrubs can be planted on land with less secure tenure. Economic analysis of cajanus fallows compared to natural fallow over six years shows that cajanus fallows are profitable under most tested scenarios, both in terms of returns to land and to labour. It seems that improved fallows with Cajanus cajan are a good response to shortening natural fallows for households in the humid lowlands of Cameroon with land constraints. However, wider dissemination of the technology requires a targeted extension approach and adequate seed supply strategies, which should be based on joint efforts between farmers, extension services and research.
Agroforestry for Small Landholders of Eastern and Southern Africa
Soil and water resources worldwide are under stress from the accelerated demands of increasing population. Reduction in soil fertility is driven by increased human population that has reduced land availability and caused a breakdown of traditional fallow systems that smallholder farmers relied on for soil fertility replenishment. Agroforestry is among a suite of sustainable agricultural practices that can rebuild soil fertility and soil organic matter (SOM), and break the cycle of poverty. Many agroforestry species are used for biological fixation of atmospheric nitrogen (N) into available N, root uptake, and recycling of nutrients. Nitrogen that accumulates in the leaves of planted tree fallows and intercrops is released when the biomass decomposes after being incorporated into the soil. Because farming practices of African smallholders tend toward multipurpose mosaics rather than uniform field management, the number of useful combinations of crops with agroforestry is constantly increasing. New tools such as “Useful Tree Species for Africa” facilitate the choice of trees within farming systems. Short-term agroforestry species have increased cereal yields from 10% to 200%, while yield differences under long-term parkland species such as Faidherbia albida have ranged from slight decreases to doubling of yields. Parkland systems have long been used by farmers but are now being recognized by the development community. The multiple sources of the yield benefits under parkland management are currently being documented by researchers. While rebuilding soil fertility, agroforestry also increases biomass buildup and carbon (C) sequestration in farming systems. This increase, however, is highly variable throughout eastern and southern Africa, and the residence time of soil organic carbon (SOC) is controversial. All agroforestry systems for which data are available accumulate biomass faster than the natural systems they emulate. The range of C sequestration by smallholder agroforestry in the tropics has been bracketed between 1.5 and 3.5 Mg C ha-1 year-1. Addition of agroforestry species to farming systems has the potential to either enhance or reduce soil C and greenhouse gas (GHG) emissions. Thus, the study of GHG emissions with agroforestry practices is critical in describing the trade-offs between smallholder and ecosystem benefits from agroforestry
Influence of improved fallow systems and phosphorus application on arbuscular mycorrhizal fungi symbiosis in maize grown in western Kenya
A field study was carried out on a six-year-old on-farm field trial during long-rains season (April–August) 2003 to investigate the effect of improved fallow systems and phosphorus application on arbuscular mycorrhiza fungi (AMF) symbiosis in maize. The trial comprised of maize rotated with a fast growing leguminous Crotalaria grahamiana fallow and a non-leguminous Tithonia diversifolia fallow for 3 years followed by continuous maize. The experiment was randomized complete block design with three cropping (continuous maize, Crotalaria fallow and Tithonia fallow) systems and two phosphorus levels (0 and 50 kg P/ha). AMF colonization in maize roots, maize yield and macro-nutrients uptake were recorded. Phosphorus applications improved (P < 0.05) early (<8 weeks old maize) AMF colonization, nutrient uptake and maize yield in improved fallow systems. Greater differences due to phosphorus application were noted in maize in Tithonia fallow than in Crotalaria fallow. Following phosphorus application, a positive relationship existed between early AMF colonization and maize yield (r = 0.38), and phosphorus and nitrogen uptake (r = 0.40 and r = 0.43, respectively), demonstrating the importance of phosphorus fertilization in enhancing low-input technologies (improved fallows systems) in phosphorus deficient and acidic soils of western Kenya.
Agroforestry approaches or rehabilitating degraded lands after tropical deforestation
There are three main types of degraded lands that result from slash and burn agriculture: degraded fallows, degraded pastures and Imperata cylindrica grasslands. Together they may cover as much as 250 million hectares, and have the advantage for recuperation of being close to road, market and urban infrastructure. Land degradation can be due to one or more causes: physical (e.g. soil compaction, erosion), chemical (e.g. increases in soil acidity, decline in available nutrients) and biological degradation (e.g. loss of microsymbionts, weed encroachment). The use of bulldozers to clear land accelerates degradation and generally requires more labour, energy and/or purchased inputs for reclamation. The current research knowledge base has focused upon alternatives to slash and burn that start with fertile, cleared forests rather than degraded systems. In the past, agroforestry approached for reclaiming the three types of degraded systems. Int he past, agroforestry approaches for reclaiming the three types of degraded lands were employed but most of the experiences are not based on sound research. The establishment of Acacia mangium on Imperata grasslands is one success story. In the case of degraded fallows and pastures, land regeneration strategies have been identified (e.g. economically and biologically improved fallows; silvopastoral systems) but the essential research is still to be done. Fundamental concepts of restoration ecology with emphasis on nutrient cycling, plant succession and weed dynamics should set the state for understanding how to turn degraded areas into productive lands. Coupled with appropriate policies and other socio-economic considerations, such work will provide viable alternatives to land abandonment after tropical deforestation.
The economics of short rotation improved fallows: evidence from areas of high population density in Western Kenya
This study assesses the economics of improved fallow in the densely-populated areas of western Kenya, where soils are nutrient-depleted. A formal survey of 71 farmers showed that half periodically fallow 10–50% of their land for at least one season, mainly for soil fertility restoration. This is a higher proportion than expected for areas of high population density. An improved-fallow prototype, established by direct seeding Sesbania sesban, an indigenous tree, into the maize crop preceding the fallow, was subjected to an economic analysis, based on twenty farmer-managed trials where the technology was tested. One would intuitively expect that the yield of maize grown after a fallow would need to be at least double that obtained before the fallow in order to compensate for the lost production during the fallow period. Instead, the break-even yield increase following the one-season fallow, compared to continuous cropping, was only 21% of the long rains yield of 600 kg/ha for the base scenario. It was relatively, low, because the foregone maize yield during the fallow was compensated by savings in crop labour. Improved fallow is a promising technique for reclaiming depleted land, especially for households with access to off-farm income or having low labour-to-land ratios. The farm trials facilitated a realistic economic analysis and farmers’ input into the design of the technology to help focus research on improving the practice
The science and practice of short-term improved fallows: symposium synthesis and recommendations
This paper summarizes the reports submitted by working groups who met during the International Symposium on the Science and Practice of Short-term Improved Fallows held in Lilongwe, Malawi from 11–15 March 1997. The four working groups focused their discussions on the themes around which the symposium was structured, namely (1) traditional fallows and indigenous knowledge, (2) case studies on research and development of improved fallows, (3) how and where fallows work, and (4) dissemination mechanisms and policy requirements for adoption. Through their deliberations, each group provided a synthesis of key lessons learned and recommendations for future research and action.
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.