Natural Fallow Research Articles

Diversity of plant-parasitic nematodes and their relationships with some soil physico-chemical characteristics in improved fallows in western Kenya

A survey was conducted in the food crop production area of western Kenya to characterise the nematode communities in different cropping systems and to monitor the effect of improved fallows on the abundance, diversity and community structure of plant-parasitic nematode assemblages. Soil samples were taken from short-term natural fallows (<3 years), old natural fallows (>20 years), maize/beans cultivated fields and improved fallows with Crotalaria grahamiana, Sesbania sesban and Tephrosia vogelii. The Shannon diversity index and evenness were used to assess nematode diversity in the different cropping systems. The relationships between plant-parasitic nematodes and the soil physico-chemical properties were investigated with principal component analysis and co-inertia analysis. Planting trees/shrubs as improved fallows in croplands dramatically increased the populations of nematodes in the soil. However, the diversity of plant-parasitic nematodes was reduced. The distribution of plant-parasitic nematodes was correlated with that of some soil physical and chemical properties. Scutellonema spp. were dominant in the improved fallows where the highest values of magnesium and potassium were recorded. The plant-parasitic Dorylaimida ( Xiphinema spp. and Paratrichodorus minor) were more abundant in the maize/beans plots and this abundance seemed related to high soil bulk density. Meloidogyne spp. and Rotylenchulus borealis had a strong positive correlation with soil organic matter and clay, and were abundant in the long-term natural fallows. The correlations between soil physico-chemical properties and nematodes indicate that, apart from the direct influence of the host plant, the soil characteristics play an important role in the abundance, distribution and structure of nematode communities. This validates the potential of nematodes as bio-indicator organisms of soil status.

Structural stability of an Alfisol under various fallow management practices in southwestern Nigeria

AbstractThe effects of legume‐based soil management on soil dispersion were studied on an Alfisol between 1994 and 1995 in an experiment which was established in 1989 in southwestern Nigeria. The fallow systems, which constituted the main plots in the split‐plot experiment, included natural fallow, Pueraria phaseoloides (Roxb.) Benth, and Leucaena leucocephala Lam de Wit. The subplots were 25, 33, 50 and 100 per cent cropping intensities. Apart from indices of soil dispersion such as water‐dispersible clay and dispersion ratio, the fractal theory was applied to describe the fragmentation of soil aggregates less than 4 mm under the systems and cropping intensities. Although water dispersible clay was less than 60 g kg−1 in the 0–15 cm soil depth because it was inherently low in clay content, the soil dispersion ratio was generally above 50 per cent. Also, the fractal dimensions, which ranged between 2.75 and 2.89, were similar among the fallow systems with cropping intensities for the surface soil. However, the interaction of slope position with season caused significant differences in fractal parameters, suggesting that the processes of soil degradation were different for the upper and lower slopes even with similar microaggregate distribution. The soil was inherently vulnerable to soil dispersion, although, the fallow management systems with less than 100 per cent cropping intensity would maintain soil structure at similar level as the forest. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Innovations within upland rice-based systems in northern Vietnam with Tephrosia candida as fallow species, hedgerow, or mulch: net returns and farmers’ response

Land degradation and crop yield decreases in tropical uplands in general and in southeast Asia have been recognised as the main problems arising from intensification of upland cultivation. With the purpose to search for solutions to these problems, agroforestry systems with a leguminous shrub Tephrosia candida (Roxb.) D.C. were experimentally tested on-farm in an upland rice ( Oryza sativa L.) system on sloping land in northern Vietnam during the period 1996–1999. The upland rice-based systems tested were: (1) fallow crop rotation, including natural fallow (NaFa) and Tephrosia fallow (TepFa); (2) continuous cropping, including monocropping (Mono), Tephrosia hedgerow intercropping (TepAl), and Tephrosia mulch transfer (TepMu). In this paper, findings concerning net returns per labour day and the response of farmers to the test systems are presented. The Tephrosia systems (TepFa, TepAl, and TepMu) and the existing systems (NaFa and Mono) were evaluated using both experimental measurements and participatory rural appraisal (PRA) techniques. In the different treatments over a 4-year rotation, 386–778 labour days ha −1 were used, while 2612–4924 kg ha −1 upland rice grain and 939–9601 kg ha −1 fuel wood were harvested. The upland rice grain harvested was enough to feed 8.7–16.4 persons ha −1. During the same period, the economic net present values were positive for the NaFa and TepMu treatments, but negative for the TepFa, Mono and TepAl treatments. The returns per unit labour were 3.7–6.7 kg rice or US$ 0.8–1.44 labour per day. Farmer criteria concerning management, labour input and soil conservation were well in accordance with the findings on labour input, crop yields and net returns. The Tephrosia systems seemed not to be rational alternatives for situations where the natural fallow systems are still viable. However, the Tephrosia fallow and the Tephrosia mulch transfer systems could increase crop yield per hectare at acceptable rates of return to labour.

The potential of mixed covers of Sesbania, Tephrosia and Crotalaria to minimise nematode problems on subsequent crops

Root-knot nematodes ( Meloidogyne javanica) associated with Sesbania sesban and Tephrosia vogelii and root-lesion nematodes ( Pratylenchus spp.) associated with Crotalaria grahamiana limit the effectiveness of these leguminous cover crops for replenishing soil fertility. As Crotalaria spp. are antagonistic to Meloidogyne, and S. sesban and T. vogelii suppress Pratylenchus, a field study was conducted on farms in western Kenya to test whether mixing S. sesban or T. vogelii with C. grahamiana can resolve the nematode problems associated with pure crops of the respective species. The pure and two-species mixed cover crops, grown for 10 or 12 months, were evaluated against natural and bare fallows. Maize ( Zea mays) and bean ( Phaseolus vulgaris) were then intercropped and the plots split to determine the effects of incorporating the biomass of natural fallows/cover crops against its removal on nematode infection of crops. T. vogelii and S. sesban were highly infected by Meloidogyne and their pure crops supported increased populations of this nematode. In mixed systems, C. grahamiana did not affect Meloidogyne infection of S. sesban and T. vogelii, but it significantly reduced the soil nematode populations compared with the respective pure crops. Root-lesion ( Pratylenchus zeae, P. thornei and P. pseudopratensis) and spiral nematodes ( Helicotylenchus pseudorobustus and Scutellonema brachyurus) were greatest in number following pure C. grahamiana. Mixing C. grahamiana with either S. sesban or T. vogelii did not restrict these nematode populations. Meloidogyne infection of bean following S. sesban+C. grahamiana and T. vogelii+C. grahamiana was restricted and the crop after these mixed systems did not lose yield compared with that after pure S. sesban and T. vogelii. All cover crops, except pure C. grahamiana and its mixture with S. sesban at Yala, increased maize yields compared with natural and bare fallows. In situ decomposition of the foliar biomass of cover crops and natural fallow did not affect nematode infection of both the annual crops. The Meloidogyne problem associated with S. sesban and T. vogelii covers on subsequent annual crops such as bean can be limited by mixing either of these shrubs with C. grahamiana.

Improvement of the physical fertility of a degraded Alfisol with planted and natural fallows under humid tropical conditions

Abstract. Topsoil (0–15 cm) bulk density, aggregate stability, soil dispersibility, water retention and infiltration were measured between 1989 and 1996 on an Alfisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and A. auriculiformis. Also, plots with natural fallow and maize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 year fallow periods. Under fallow, the soil bulk density decreased from1.56 to 1.11 t m73.The continuously cropped treatment (level tillage) had significantly higher bulk density than the fallowed subplots after 6 years. Mean soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plots and from 192 to 295 kPa for the continuously cropped (level tillage) subplot. Surface soilwater contentswere similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing.Soil structural improvement by planted fallows was similar to that by natural fallow, but the trees were more promising for long‐term fallow (&gt;6 years) than the herbaceous P. phaseoloides. However, the improvement in soil structure after 4 or 6 year fallow could not be maintained in subsequent cropping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultivated subplots compared with other fallow treatments. Thus, soil structure recovery under a fallow does not imply a sustained improvement when stress is applied to this soil. Post‐fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength will be necessary to enhance the improvements by fallow species.

Litter and biomass production from planted and natural fallows on a degraded soil in southwestern Nigeria

To rehabilitate a degraded Alfisol at Ibadan, southwestern Nigeria, Senna siamea (non-N-fixing legume tree), Leucaena leucocephala, and Acacia leptocarpa (N-fixing legume trees) were planted in 1989, and Acacia auriculiformis (N-fixing legume tree) in 1990. Pueraria phaseoloides (a cover crop) and natural fallow were included as treatments. Litterfall and climatic variables were measured in 1992/1993 and 1996/1997 while biomass production and nutrient concentrations were measured in 1993 and 1995. Total litter production from the natural and planted fallows was similar, with means ranging from 10.0 (L. leucocephala) to 13.6 t ha−1 y−1 (natural fallow) during the 1996/1997 collection. Leaves constituted 73% (L. leucocephala) to 96% (A. auriculiformis) of total litterfall. Acacia auriculiformis grew most quickly but S. siamea produced the highest aboveground biomass which was 127 t ha−1 accumulated over four years, and 156 t ha−1 accumulated over six years of establishment. The aboveground biomass of P. phaseoloides and natural fallow was only 6 to 9 t ha−1 at six years after planting. Nitrogen concentration in the leaves/twigs of was 2.5% for L. leucocephala, and 2% for other planted species and natural fallow. Pueraria phaseoloides had concentrations of P, K, Ca and Mg comparable to levels in the leaves/twigs of the tree species. Through PATH analysis, it was found that maximum temperature and minimum relative humidity had pronounced direct and indirect effects on litterfall. The effects of these climatic variables in triggering litterfall were enhanced by other variables, such as evaporation, wind, radiation, and minimum temperature. Improvement in chemical properties by fallows was observed in the degraded soil.

Decomposition Rates of Biomass Obtained from Six Month-Old Tephrosia vogelii, Tithonia diversifolia and Natural Fallow Vegetation at Maseno, Kenya

ABSTRACT Organic residues incorporated into soil release their nutrients through decomposition. This process may be slow or rapid depending on the biomass quality and environmental conditions. For a period of 8 months, an incubation study with litterbags was carried out in field conditions to determine the decomposition rate of Tephrosia vogelii Hook F., Tithonia diversifolia (Hemsley) A. Gray and natural vegetation fallows. Leaves, stems and roots of Tephrosia, Tithonia and natural fallow had high decomposition rates and less than 30% of the biomass remained undecomposed after 8 months of incubation. Tithonia leaves decayed within 1 month after incubation and Tithonia mixture released most of its N in mineral form during the first month. Tephrosia roots and leaves had a halflife (bo) of about 2 months, which is lower than that for Tithonia roots. Tephrosia stems and natural fallow roots were both low in N and decomposed slowly. Decomposition and nutrient release patterns were influenced by the quality of the material, in particular the N content, lignin and polyphenols.

Farming system-induced variability of some soil properties in a sub-humid zone of Ghana

This study assessed the effects of different farming systems, namely woodlot (WL), alley farming (AL), conventional tillage (CT) and natural fallow (NF) on the variability of organic carbon (OC) content and mean weight diameter (MWD) of a degraded Ferric Acrisol in the sub-humid zone of Ghana. The soils under woodlot accumulated the highest amount of organic carbon (18.6 g kg−1) with the least spatial variability apparently due to the greater additions of litter and minimum tillage. The conventionally tilled soil had the least OC content (13.1 g kg−1). Similar to the OC content, the woodlot soils also had the highest aggregate stability (MWD = 1.78 mm) and the least spatial variability. The stability of soil aggregates under the farming systems was greatly influenced by OC content; there was a good correlation between OC and MWD (r > 0.62**). Correlograms showed that OC and MWD are space dependent. The correlation length for OC under the different farming systems followed the order WL > NF > AL > CT, indicating that WL ensured a greater uniform distribution soil organic matter. The spatial distribution in MWD followed the same trend observed for OC. The MWD in the other farming systems was poorly related from point to point with shorter k-values, suggesting lack of uniformity due to low accumulation of OC. Generally, the woodlot system appeared to be a better, low-input restorer of soil productivity.

Farmers’ perceptions and management of soil organic matter — a case study from West Africa

A farm survey was carried out in 155 Ghanaian villages covering parts of the forest and savanna zones of West Africa to assess farmers' views on ‘soil organic matter’ (SOM) and its management. The results of a closed questionnaire accompanied by open discussions showed that most farmers are well aware of SOM and its importance for crop yields. In southern Ghana, farmers perceive SOM generally by its colour, while in northern Ghana, it is mostly assessed by the density and kind of vegetation. Farmers' perception of the properties of SOM was directed at its main functions as a primary provider of plant nutrients and its ability to conserve water. Other properties mentioned were the improvement of soil aeration and drainage, the loosening of soil structure as well as its impact on soil temperature. The major strategies farmers used in maintaining or augmenting SOM levels were: manure application, mulching with crop residues, slashing weeds without burning, composting, and shifting cultivation (natural fallow). Promoted technologies, such as green manuring, no tillage, or agroforestry were used only by a few of the farmers interviewed. The differences between farmers' views and strategies in the two zones as well as farmers' constraints in SOM management are discussed. It appears that the level of farmers' commitment to excellent soil management can vary with biophysical as well as socio-economic conditions.

Nitrogen management in 'adequate' input maize-based agriculture in the derived savanna benchmark zone of Benin Republic

Although the West-African moist savanna zone has a high potential for crop production, yields on farmers' fields are, on average, far below this potential, mainly due to the low use of external sources of nutrients. Since the mid-1990s, it has become clear that in order to upgrade crop production to levels needed to sustain the growing population without further degrading the soil resource base, inorganic fertilizers are required. Due to the physico-chemical nature of these soils and the relatively high cost of inorganic fertilizers, a general consensus exists in the research and development community that these inorganic inputs need to be complemented with organic matter. Here, we explore options to produce organic matter in-situ and evaluate the impact of combining inorganic and organic sources of N on maize yields, focusing on the densely populated derived savanna (DS) benchmark of Benin Republic. Although most of the farmers (93%) in this benchmark use inorganic fertilizer, applications rates are low (on average, 27 kg N ha−1). A significant response to N was observed for 96% of the studied farmers' fields. Grain and herbaceous legumes were observed to produce between 383 and 8700 kg dry matter ha−1 in the benchmark area. Inoculation with Rhizobia and inorganic P additions were shown to significantly improve biomass production on sites with low contents of Rhizobia and P. Although maize grain yield was observed to increase significantly following a legume compared with following a maize crop or natural fallow, these increases were insufficient in the case of a cowpea crop or were obtained at the cost of leaving the field `idle' for a whole year in the case of a herbaceous Mucuna fallow. Topping up a cowpea haulms equivalent of 45 kg N ha−1 with 45 kg urea–N ha−1 was shown to give maize yields similar to the yields obtained after applying 90 kg urea–N ha−1 on the poorest fields. Moreover, on these fields, a positive interaction between cowpea–N and urea–N sources of 200 kg grain ha−1 was observed. On the richest fields, the effects of applied organic matter and fertilizer were additive. Agroforestry systems are alternative cropping systems that produce organic matter in-situ. As tree roots go down below the rooting depth of food crops, sub-soil fertility was observed to influence tree biomass production. Yield increases in tree-crop intercrop systems – such as alley cropping – in the absence of inorganic inputs are often reduced by the occurrence of tree-crop competition. In cut-and-carry systems, where tree prunings are harvested from a field adjacent to the crop land, increases in maize grain yield caused by addition of those prunings were observed to be on the low side. Mixing these residues with urea, however, was shown to lead to added benefits of about 500 kg grains ha−1, relative to the treatments with sole inputs of organic matter or urea. Although residue quality was shown to affect maize N uptake in a pot trial, its impact under field conditions was minimal for the range of considered residue qualities. In an alley cropping trial, maize yield was shown to be sustained on a non-degraded site and enhanced on a degraded site, when a minimal amount of mineral fertilizer was added with the prunings, whereas fertilizer application alone failed to do so in both cases.

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.

Management of organic matter in the tropics: translating theory into practice

Inputs of organic materials play a central role in the productivity of many tropical farming systems by providing nutrients through decomposition and substrate for synthesis of soil organic matter (SOM). The organic inputs in many tropical farming systems such as crop residues, manures, and natural fallows are currently of low quality and insufficient quantity to maintain soil fertility hence there is need to find alternative or supplementary sources of nutrients. Knowledge gained over the past decade on the role of organic resource quality in influencing soil nutrient availability patterns (Synchrony Principle) and SOM maintenance (SOM Principle) provides a strong scientific basis on which to develop management tools. This scientific information must be linked with farmer knowledge and circumstances to provide a realistic approach to soil fertility and SOM management in the tropics. A decision tree has been developed for testing hypotheses about the resource quality parameters that affect nitrogen release patterns and rates. The decision tree is linked to an Organic Resource Database (ORD) with detailed information on the resource quality of agroforestry trees and leguminous cover crops providing a systematic means of selecting organic resources for soil fertility management. The decision tree has also been translated into a practical field guide for use with farmers in evaluating organic materials. The longer-term effects of organic inputs on SOM might also be addressed through the decision tree and database. It is generally believed that materials good for short-term soil fertility will not build or maintain SOM; if true then it is difficult to imagine practical means of maintaining SOM in the African context where short-term fertility issues will take precedence over longer-term maintenance of SOM.

Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Fallows with naturally regenerated or planted vegetation are important in many subsistence agricultural systems of tropical regions, but the underlying soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372-mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15–16 months), were Zea mays (maize), natural regrowth of vegetation (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultivated soil without vegetation (bare fallow). Inorganic N (nitrate+ammonium-N) to 2-m depth under bare fallow increased by 242 kg N ha −1 year −1 on the Oxisol and 54 kg N ha −1 year −1 on the Alfisol, indicating that N mineralization exceeded N losses. Subsoil inorganic N (0.5–2.0 m) remained relatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased during natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5–2.0 m by 75–125 kg N ha −1 year −1 down to a minimum N content between 40 and 80 kg N ha −1. After slashing sesbania and incorporating the above-ground biomass with 154–164 kg N ha −1, soil inorganic N increased within 2 months by 136 kg N ha −1 on the Oxisol and 148 kg N ha −1 on the Alfisol. Inorganic N decreased after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was small. A considerable part of the N in biomass of the natural fallow was recycled. Much of the total N accumulated by the sesbania fallow was removed with the wood and the amount of N recycled was similar on the Oxisol and Alfisol. We conclude that sesbania fallows can retrieve considerable subsoil inorganic N on deep soils with high subsoil N and effectively cycle this N through its rapidly decomposable biomass to subsequent crops.

Nitrogen and phosphorus uptake by maize as affected by particulate organic matter quality, soil characteristics, and land-use history for soils from the West African moist savanna zone

The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg–1) and lowest under natural fallow (6.3 mg kg–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg–1) and lowest in continuously cropped plots (107 mg N kg–1). After addition of all nutrients except N, a highly significant linear relationship (R 2=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R 2 increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability.

Crop Rotation to Improve Agricultural Production in Sub-Saharan Africa

A three years' trial was conducted in a farmers' field in northern Ghana to evaluate the effect of sole crops (cotton, cowpea, groundnut, soybean, and sunflower) planted once or twice on yield of the staple foods of the region, maize and sorghum. Sole cropping for only one year already resulted in significant yield increases for maize and partly for sorghum compared to the conventional cropping of mixed stands of maize–sorghum or maize–groundnut and natural fallow. Lowest yield of maize and sorghum was obtained where these cereals followed maize–sorghum (monoculture). Intercropping of maize with groundnut led to subsequent maize and sorghum yields which were similar to those obtained after maize–sorghum. After growing legumes and sunflower for one year the grain and straw yield of maize and sorghum was significantly higher in the two consecutive years than after cereal (maize–sorghum) monoculture. In this trial maize and sorghum were found to be not as tolerant to the disadvantages of monoculture or preceding cereals–legumes mixture. The results suggest that continuous intercropping with cereals under the given conditions has negative effects on soil fertility and can lead to an increase in soil-borne pests and troublesome weeds like Striga comparable to monocropped cereals.

Parasitic nematode populations in natural fallows and improved cover crops and their effects on subsequent crops in Kenya

A field study was conducted at two sites in western Kenya during 1995–1997, to evaluate the effects of 6- and 12-month sesbania ( Sesbania sesban) ‘cover crops’ in comparison with 6- and 12-month natural ‘weed fallows’, a 6-month crotalaria ( Crotalaria agatiflora) cover crop and continuous maize on the dynamics of parasitic nematode populations and their effects on subsequent maize and bean crops. Following 12-month sesbania, the plots were split for nematicide treatment during the cropping phase. Infestations of the root-knot ( Meloidogyne spp.), root-lesion ( Pratylenchus zeae), reniform ( Rotylenchulus variabilis) and spiral ( Helicotylenchus spp. and Scutellonema spp.) nematodes were monitored. Although a number of plant species in natural fallows were hosts to both M. incognita and M. javanica, even natural fallows 12 months long did not increase populations of these nematodes to cause any significant yield reduction in the subsequent nematode-susceptible bean. Crotalaria cover crop and continuous maize decreased M. incognita and M. javanica populations but increased P. zeae populations to levels that could limit their growth. Only the 12-month sesbania cover crop increased Meloidogyne populations greatly in soil and roots at the site of light-textured soil. Sesbania itself appeared to be fairly tolerant to the nematode over a 12-month growing period. Maize was not damaged by the root-knot nematodes after sesbania, but bean yield reduced by 52–87% following 12-month sesbania. A one-season interval with maize after sesbania cover was adequate to limit root-knot nematode damage on susceptible bean. Crotalaria can be considered as an alternative cover crop to sesbania where crops susceptible to root-knot nematode are an integral part of the cropping system after fallows. However, there is a need to screen Crotalaria species to identify species that produce large amounts of biomass without increasing Pratylenchus population. Neither cover crops nor natural fallows had any influence on reniform and spiral nematodes, and none of these nematodes affected subsequent crops.

Regeneration of Earthworm Populations in a Degraded Soil by Natural and Planted Fallows under Humid Tropical Conditions

Earthworm populations (predominantly Hyperiodrilus africanus and Eudrilus eugeniae) were sampled monthly for 1 yr during 1994 and 1995 in natural regrowth vegetation fallow (dominated by the natural fallow shrub Chromolaena odorata L.), planted fallow (the woody species Senna siamea Lam., Leucaena leucocephala Lam., and Acacia leptocarpa), and intercropped maize (Zea mays L.)–cassava (Manihot esculenta Crantz) established in 1989 in a degraded Alfisol (Oxic paleustalf) in southwestern Nigeria. Compared to leaves of Chromolaena (3.3% N), N concentrations were lower in those of Senna and Acacia, and higher in Leucaena Acacia and Leucaena had higher polyphenol relative to the natural fallow (2%). The lignin was lower in Leucaena than the natural fallow leaves (14%). For 65% of the sampling dates, earthworm numbers under all fallows were significantly higher than under continuous maize–cassava. The mean earthworm numbers (no. m−2) during the rainy season (April–October) decreased in the following order: Chromolaena (147), Senna (131), Leucaena (92), Acacia (80), and maize–cassava (14). Earthworm fresh weights in fallow plots were higher than in the maize–cassava plot, though this was significant for only 4 out of 11 sampling dates. Higher earthworm numbers and biomass in fallow plots were attributed to higher litterfall, lower soil temperature, and higher soil moisture. The mean earthworm numbers were directly correlated with the mean soil moistures (, P &lt; 0.05) in fallow plots and N/polyphenol ratios of fallow litterfall (, P &lt; 0.05). Increase in earthworm population by fallows led to an increase in leaf‐litter decomposition, soil organic matter, available P, and extractable cations and pH; and a decrease in soil bulk density and penetrometer resistance in the fallow plots.

The promising impact of ley introduction and herd expansion on soil organic matter content in southern Mali

In southern Mali, cultivated area and herd size increase together with population growth. Consequently, periods of natural fallow shorten and traditional farming and animal husbandry techniques lead to a decrease of soil organic matter (SOM) content. Between 20 and 45% of the land is cultivated while less than 60% is arable area. To increase efficiency of natural fallow, a reduction in livestock herds is often proposed. By means of a linear programming model, the feasibility of maintaining actual SOM content in two villages in different agro-ecological zones was investigated. By adjusting animal numbers and cropping pattern, the model maximized: (1) SOM content under the condition of positive farm labour income; or (2) farm labour income under the condition of a positive SOM content. The model results suggested that maintaining SOM content requires the use of cereal crop residues for animal feed and for manure through bedding in kraals, but also higher animal densities. The last was feasible only through: (1) herding cattle of several farms together to overcome labour constraints; and (2) introducing P-fertilized leys, for grazing in the dry season. Grazing of the leys together with crop residues allowed animal densities up to 44 tropical livestock units (TLUs) km −2, while less than 16% of the produced rangeland fodder was grazed. In such conditions, a positive SOM balance and higher income was obtained with a minimum of 16.7% of ley in the crop rotation. Limiting the area cropped with cotton stabilized income and contributed to a positive SOM balance. Zero-grazing during the warm season allowed SOM surpluses to be achieved with 12.5% of ley. However, as income decreased, seasonal zero-grazing could only be adopted for high-producing animals and small herds.

AN IMPROVED COVER CROP-FALLOW SYSTEM FOR SUSTAINABLE MANAGEMENT OF LOW ACTIVITY CLAY SOILS OF THE TROPICS

The potential of planted leguminous cover crop fallow as an alternative to the natural regrowth fallow for sustaining the productivity of low activity clay (LAC) soils in the tropics as the fallow period shortens was tested at Ibadan in the forest-savanna zone of southwestern Nigeria. The study, initiated in 1990, compared an improved system (cover crop-fallow) and a traditional system (natural fallow), with the aim of developing an improved cover crop-fallow system for sustainable management of LAC soils of the tropics. Pueraria phaseoloides, an herbaceous, N-fixing, perennial legume with a slow initial growth habit, was selected as a test cover crop. Pueraria was sown with a maize-cassava intercrop at the same season. In a 2-year cycle, 1 year of cropping was followed by 1 year fallow with pueraria for the cover crop-fallow system or with natural regrowth (mainly Chromolaena odorata) for the natural fallow system. Maize-cassava intercropping without a fallow period (continuous cropping) was included as a control. No fertilizer was applied throughout the experimental period. Pueraria produced 253 kg N ha -1 after the fallow period, compared with 109 kg N ha -1 for chromolaena; pueraria was also better in recycling P (11 kg P ha -1 ) than was chromolaena (9 kg P ha -1 ). Natural fallow for 1 year increased the maize yield from 75 to 350% from 1992 to 1996, and the cassava yield from 9 to 130% compared with continuous cropping. Cover crop-fallow resulted in a 22 to 72% higher maize yield than the natural fallow from 1992 to 1996. Although in 1992 and 1994 cassava tuber yield was lower with the cover crop-fallow than with the natural fallow, with better pueraria husbandry in 1996, cover crop-fallow increased the cassava tuber yield by 41%. With residue burning, the N balance was 27 kg N ha -1 yr -1 for the pueraria cover crop-fallow system and -15 kg N ha -1 yr -1 for the chromolaena natural fallow system. Cover crop-fallow maintained soil organic carbon status better than the natural fallow. Integration of a pueraria legume cover crop did not affect soil pH. Results indicate cover crop-fallow with pueraria could be a better alternative to traditional natural fallow under shortened fallow periods for raising or maintaining productivity of LAC soils of the humid tropics.

Influence of fallow management systems and frequency of cropping on weed growth and crop yield

The influence of weeds on maize intercropped with cassava was evaluated in three fallow management systems and four cropping frequencies from 1989 to 1996 in Ibadan, Nigeria, West Africa. Fallow management systems consisted of natural bush, Leucaena leucocephala hedgerows and Pueraria phaseoloides fallow. Frequencies of cropping were continuous cropping, 1 cropping year alternating with 1, 2 and 3 years of uncropped fallow. Shrubs and climbers contributed 60–70% and herbs (predominantly weeds) 30–40% to the total dry matter produced in all treatments. Weed dry matter and seedbank increased with time in all treatments but was higher in natural bush fallow by the fifth and sixth years and in continuously cropped plots. Weed seed density increased with time in the continuously cropped plots and in plots cropped after 1 or 2 years of fallow but decreased with time when the fallow period was up to 3 years. Maize grain yield decreased over time in both unweeded and weeded plots irrespective of treatment with the exception of the weeded P. phaseoloides fallow treatment. Cassava tuber yield decreased over time in unweeded plots in all treatments while tuber yield increased in weeded P. phaseoloides fallow treatment and remained relatively constant in L. leucocephala and natural bush fallow plots. The results from this study suggest that planted fallow involving legume species is more effective in weed management in the long term than natural fallow. Fallow period was beneficial to crop yield and yield improvement increased with the length of the fallow cycle, particularly for cassava.