Tithonia and senna green manures and inorganic fertilizers as phosphorus sources for maize in Western Kenya

Efforts to overcome declining soil fertility on small holder farms in western Kenya must be consistent with the reality of low utilization of inorganic fertilizers. Likewise organic inputs alone cannot supply adequate nutrients. The use of two organic resources, Tithonia diversifolia (tithonia) and Senna spectabilis (senna) leaves, and their combination with inorganic P for improving soil fertility and maize yields was investigated on a P limiting soil in Western Kenya. Treatments included: 1) control, no inputs; 2) 5 t ha⁻¹ (dry matter) tithonia leaves; 3) 5 t ha⁻¹ senna leaves; 4) 5 t ha⁻¹ tithonia leaves + 25 kg P ha⁻¹ as triple superphosphate (TSP); 5) 5 t ha⁻¹ senna leaves + 25 kg P ha⁻¹ (as TSP); and 6) 25 kg P ha⁻¹ of TSP. Maize was used as a test crop. Decomposition and P and N release of tithonia and senna leaves were determined in a litterbag study. Tithonia + TSP applications tripled maize yields compared to the control, senna + TSP and tithonia sole application doubled yields, while senna sole applications did not increase yields substantially. A large residual yield was produced in the tithonia treatments in a subsequent crop. These yield results were consistent with the higher quality and faster release of N and P from the tithonia leaves compared to senna. The tithonia biomass transfer system can improve yields in the short term but has limitations because of the large amount of biomass and the associated labor requirements. This revised version was published online in June 2006 with corrections to the Cover Date.     

Short fallows of Tithonia diversifolia and Crotalaria grahamiana for soil fertility improvement in western Kenya

Managed short-duration fallows may have the potential to replace longer fallows in regions where population density no longer permits slow natural fallow successions. The purpose of fallows is not only to improve subsequent crop performance but also to restore soil fertility and organic matter content for the long term. We therefore evaluated the soil organic matter and nutrient flows and fractions in a short fallow experiment managed in the western Kenya highlands, and also compared the experimental area with a 9–12-yr-oldadjacent natural bush fallow. The factorial agroforestry field experiment with four land-use and two P fertilizer treatments on a Kandiudalfic Eutrudox showed that 31-wk managed fallows with Tithonia diversifolia(Hemsley) A. Gray and Crotalaria grahamiana Wight &Arn. improved soil fertility and organic matter content above those of a natural weed fallow and continuous maize (Zea mays L.). Post-fallow maize yields were also improved, although cumulative three-season increases in yield were small (0–1.2 Mg ha⁻¹) when the yield foregone during the fallow season was accounted for. Improvements in yield and soil quality could be traced to quantity or quality of biomass recycled by the managed fallows. The non-woody recycled biomass produced by the continuous maize, weed fallow, and tithonia treatments was near 2Mg ha⁻¹, whereas crotalaria produced three times more recyclable biomass and associated N and P. Increases in topsoil N due to the fallows may have been attributable in part to deep acquisition and recycling of N by the fallows. Particulate macro-organic matter produced by the fallows contained sufficient N(30–50 kg ha⁻¹) to contribute substantially to maize production. Organic Paccumulation (29 kg ha⁻¹) similarly may play a significant role in crop nutrition upon subsequent mineralization. The effect of the P fertilizer application on soil properties and maize yield was constant for all land-use systems (i.e., no land-use system × P fertilizer interactions occurred). There was an indication that tithonia may have stimulated infestation of Striga hermonthica (Del.) Benth., and care must be taken to evaluate the full effects of managed fallows over several seasons. This revised version was published online in June 2006 with corrections to the Cover Date.     

Leucaena + maize alley cropping in Malawi. Part 1: Effects of N, P, and leaf application on maize yields and soil properties

Yields under alley cropping might be improved if the most limiting nutrients not adequately supplied or cycled by the leaves could be added as an inorganic fertilizer supplement. Three historic leaf management strategies had been in effect for 3 years ina Leucaena leucocephala alley cropping trial on the Lilongwe Plain of central Malawi : 1) leaves returned; 2) leaves removed; and 3) leaves removed, with 100 kg inorganic N ha⁻¹ added. An initial soil analysis showed P status to be suboptimal under all strategies. A confounded 3⁴ factorial experiment was conducted with the following treatments: leaf management strategy (as above), N fertilizer rate (0, 30, and 60 kg N ha⁻¹), P fertilizer rate (0, 18, and 35 kg P ha⁻¹), and maize population (14,800, 29,600, and 44,400 plants ha⁻¹). Both N and P were yield limiting, and interacted positively to improve yields. The addition of 30 kg N and 18 kg P ha⁻¹ improved yields similarly under all leaf management strategies by an average of 2440 kg ha⁻¹. Increasing the rates to 60 kg N and 35 kg P ha⁻¹ improved yields an additional 1990 kg ha⁻¹ in the ‘leaves returned’ and leaves removed + N’ strategies, but did not improve yields under the ‘leaves removed’ strategy. Lower yields were related to lack of P response at the highest P rate in this treatment, which may have induced Zn deficiency. Plots receiving leaves had higher organic C, total N, pH, exchangeable Ca, Mg, K, and S, and lower C/N ratios in the 0–15 cm soil layer than did plots where leaves had been removed. Leaf removal with N addition was similar to leaf removal alone for all soil factors measured except for organic C and total N, which were higher where N had been added. The results show that N and P were the primary yield-limiting nutrients. Historic N application maintained the soil's ability to respond to N and P on par with leaf additions.     

Combining Tithonia diversifolia and fertilizers for maize production in a phosphorus deficient soil in Kenya

The ability of Tithonia diversifolia, fertilizers and their combination to improve maize production in a P-deficient Ferralsol was studied in western Kenya. Tithonia and fertilizers were applied separately or combined in different proportions to give equal rates of165 kg N ha⁻¹, 15.5 kg P ha⁻¹ and 155 kg K ha⁻¹ in two consecutive maize growing seasons, followed by two residual maize crops. Maize grain yields and P recovered in the above-ground biomass were higher in the pure Tithonia than the pure fertilizer treatments. Maize yields increased with increasing proportions ofTithonia in the mixed treatments. When less than 36% of theP applied in the mixture was supplied by Tithonia, there was no additional yield benefit in the combined treatments compared to the pure fertilizer treatments. However an added value ranging from 18 to 24% in yield was observed at greater Tithonia proportions. Economic returns were greater when Tithonia was applied alone than when fertilizers were used, with a larger profit whenTithonia was collected from existing niches than when produced on site. Collecting Tithonia from current niches resulted also in larger net returns for all the combined treatments compared to the fertilizer treatments. The results of this study indicate that a high quality organic residue such as Tithonia can increase maize production to a greater extent than fertilizers. In low input systems, the combination of Tithonia and fertilizers is a valuable alternative when resources are scarce and an added benefit can be obtained by maximizing the proportion of Tithonia in the mixture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus uptake and maize response to organic and inorganic fertilizer inputs in Rubona,Southern Province of Rwanda

Green manure of multipurpose trees is known to be a good source of nutrients to crop. However, most agroforestry species do not have adequate phosphorus (P) in their leaves. Supplementing green manure with moderate dose of P is a beneficial strategy to improve food security in Rwanda. This study examines the effects of Calliandra calothyrsus Meissner, Tithonia diversifolia Hensley A.Gray and Tephrosia vogelii Hook.f. green manure applied independently or in combination with triple super phosphate (TSP) and lime on maize yield and P uptake in the Oxic Tropudalf of Rubona, Rwanda. The treatments were the control, lime at 2.5 t ha⁻¹, TSP at 25 and 50 kg P ha⁻¹, leaf of C. calothyrsus, T. diversifolia, and T. vogelii each at 25 and 50 kg P ha⁻¹, respectively. Leaf shrubs biomass, TSP and lime were applied for four consecutive seasons (2001–2004). The results showed that the combination of green manure with TSP at a rate of 50 kg P ha⁻¹ significantly increased maize yield from 24 to 508% when compared to the control and T. divesifolia combined with TSP was leading (508%). Equally, the same treatments as indicated above showed higher P uptake (15.6–18. 6 kg P ha⁻¹) than the control (5 kg P ha⁻¹) and 65% of maize yields variation was explained by total P uptake. The plant residues quality such as C:N ratio, total plant N, and P significantly influenced the variability of maize grain yields.     

Tree and crop productivity in gliricidia/maize/pigeonpea cropping systems in southern Malawi

This study examined the hypothesis that incorporation of Gliricidia sepium (Jacq.) Walp.) (gliricidia), a fast-growing, nitrogen-fixing tree, into agroforestry systems in southern Malawi may be used to increase the input of organic fertilizer and reduce the need for expensive inorganic fertilizers. The productivity of maize (Zea mays L.), pigeonpea (Cajanus cajan L.) and gliricidia grown as sole stands or in mixed cropping systems was examined at Makoka Research Station (latitude 15° 30′ S, longitude 35° 15′ E) and a nearby farm site at Nazombe between 1996 and 2000. Treatments included gliricidia intercropped with maize, with or without pigeonpea, and sole stands of gliricidia, maize and pigeonpea. Trees in the agroforestry systems were pruned before and during the cropping season to provide green leaf manure. Maize yields and biomass production by each component were determined and fractional light interception was measured during the reproductive stage of maize. Substantial quantities of green leaf manure (2.4 to 9.0 Mg ha⁻¹ year⁻¹) were produced from the second or third year after tree establishment. Green leaf manure and fuelwood production were greatest when gliricidia was grown as unpruned sole woodlots (c. 8.0 and 22 Mg ha⁻¹ year⁻¹ respectively). Improvements in maize yield in the tree-based systems also became significant in the third year, when c. 3.0 Mg ha⁻¹ of grain was obtained. Tree-based cropping systems were most productive and exhibited greater fractional light interception (c. 0.6 to 0.7) than cropping systems without trees (0.1 to 0.4). No beneficial influence of pigeonpea on maize performance was apparent either in the presence or absence of gliricidia at either site in most seasons. However, as unpruned gliricidia provided the greatest interception of incident solar radiation (>0.9), coppicing may be required to reduce shading when gliricidia is grown together with maize. As pigeonpea production was unaffected by the presence of gliricidia, agroforestry systems containing gliricidia might be used to replace traditional maize + pigeonpea systems in southern Malawi. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen Dynamics in Cropping Systems in Southern Malawi Containing Gliricidia sepium, Pigeonpea and Maize

This study tested the hypothesis that incorporation of green leaf manure (GLM) from leguminous trees into agroforestry systems may provide a substitute for inorganic N fertilisers to enhance crop growth and yield. Temporal and spatial changes in soil nitrogen availability and use were monitored for various cropping systems in southern Malawi. These included Gliricidia sepium (Jacq.) Walp. trees intercropped with maize (Zea mays L.), with and without pigeonpea (Cajanus cajan L.), sole maize, sole pigeonpea, sole gliricidia and a maize + pigeonpea intercrop. Soil mineral N was determined before and during the 1997/1998, 1998/1999 and 1999/2000 cropping seasons. Total soil mineral N content (NO₃⁻ + NH₄⁺) was greatest in the agroforestry systems (p<0.01). Pre-season soil mineral N content in the 0–20 cm horizon was greater in treatments containing trees (≤85 kg N ha⁻¹) than in those without (<60 kg ha⁻¹; p<0.01); however, soil mineral N content declined rapidly during the cropping season. Uptake of N was substantially greater in the agroforestry systems (200–270 kg N ha⁻¹) than in the maize + pigeonpea and sole maize treatments (40–95 kg N ha⁻¹; p<001). Accumulation of N by maize was greater in the agroforestry systems than in sole maize and maize + pigeonpea (p<0.01); grain accounted for 55% of N uptake by maize in the agroforestry systems, compared to 41–47% in sole maize and maize + pigeonpea. The agroforestry systems enhanced soil fertility because mineralisation of the applied GLM increased pre-season soil mineral N content. However, this could not be fully utilised as soil N declined rapidly at a time when maize was too small to act as a major sink for N. Methods for reducing losses of mineral N released from GLM are therefore required to enhance N availability during the later stages of the season when crop requirements are greatest. Soil mineral N levels and maize yields were similar in the gliricidia + maize and gliricidia + maize + pigeonpea treatments, implying that addition of pigeonpea to the tree-based system provided no additional improvement in soil fertility.     

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) m^–1 yr^–1 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 ha^–1 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.This revised version was published online in November 2005 with corrections to the Cover Date.     

Organic amendment and inorganic fertilization affect soil properties and quality of Larix olgensis bareroot stock

Bareroot Changbai larch (Larix olgensis Henry.) seedlings were reared with inorganic fertilizer (nitrogen (N):phosphorus (P) = 1:1, W/W) applied at a rate of 100 (F100) or 200 kg N ha⁻¹ (F200) with (+) or without (−) chicken manure as a soil amendment (O) in north-eastern China. An unfertilized control treatment was included. Inorganic and organic fertilizer treatments tended to increase soil ammonium, nitrate, available P, total P, organic carbon content and electrical conductivity, and biomass and N concentration in seedlings. Organic amendment improved first order lateral root number, tap root length, fine root morphology (length, surface area, volume) in seedlings, while the F100 treatment increased N accumulation in needles and stems compared to the F200 treatment, on average. Most fertilizer treatments tended to increase P content in combined stems and roots, but F200 − O and F100 + O treatments diluted P in needles. Organic amendment combined with inorganic fertilizer at a rate of 100 kg N and P ha⁻¹ is recommended to improve seedling growth and N reserves in woody tissues.     

Leucaena + maize alley cropping in Malawi. Part 2: Residual P and leaf management effects on maize nutrition and soil properties

Agroforestry systems involving leaf removal for animal fodder may result in rapid depletion of soil fertility. The purpose of this research was to determine if the effects of leaf removal on soil fertility parameters and maize yield in a Leucaena leucocephala alley cropping system could be reversed. Three leaf management strategies in a Leucaena alley cropping trial that had been in effect from 1987 to 1991 were investigated: 1) leaves returned, 2) leaves removed, and 3) leaves removed, with 100 kg inorganic N ha⁻¹ added. In the 1990/91 season, a 3⁴ confounded factorial design was utilized to investigate the effects of leaf management strategy, N rate (0, 30 and 60 kg N ha⁻¹); maize plant population (14,800, 29,600, and 44,400 plants ha⁻¹); and P rate (0, 18, and 35 kg P ha⁻¹). In the 1991/92 and 1992/93 seasons, leaves were applied equally to all plots, and no P was applied. The N rate and plant population treatments were continued, and the same confounded factorial design was implemented to investigate residual leaf management strategy, residual P rate, n rate, and plant population. The yield gap between the plots where leaves had been returned vs. removed narrowed each season due to uniform leaf application. Application of N improved yields during both seasons. Residual effects of the initial P application decreased to only 10% of the total yield in 1992/93. Plant population affected yields only during the season of very good rainfall. Leaf additions resulted in a relative increase in soil pH, total N,and exchangeable Ca, Mg, and K and a decrease C/N ratio in plots that had not previously received leaves. Leaves supplied more K and Zn to the upper 15 cm of soil than were being extracted by the maize crop, but uniform leaf additions eliminated differences in K and Zn uptake. Plant Zn uptake decreased with increasing P rate and plant population, and increased with increasing N rate and a history of leaf return. The results show that applying leaves equilibrated yields within two seasons, and resulted in a relative improvement of several soil properties. The residual effect from P applications was not adequate to maximize yields.     

Water balance and maize yield following improved sesbania fallow in eastern Zambia

Sesbania [Sesbania sesban (L.) Merr.] fallows are being promoted as a means for replenishing soil fertility in N-depleted soils of small-scale, resource-poor farmers in southern Africa. Knowledge of soil water distribution in the soil profile and water balance under proposed systems is important for knowing the long-term implications of the systems at plot, field and watershed levels. Soil water balance was quantified for maize (Zea mays L.) following 2-year sesbania fallow and in continuous maize with and without fertilizer during 1998–1999 and 1999–2000 at Chipata in eastern Zambia. Sesbania fallow increased grain yield and dry matter production of subsequent maize per unit amount of water used. Average maize grain yields following sesbania fallow, and in continuous maize with and without fertilizer were 3, 6 and 1 Mg ha⁻¹ with corresponding water use efficiencies of 4.3, 8.8 and 1.7 kg mm⁻¹ ha⁻¹, respectively. Sesbania fallow increased the soil-water storage in the soil profile and drainage below the maximum crop root zone compared with the conventionally tilled non-fertilized maize. However, sesbania fallow did not significantly affect the seasonal crop water use, mainly because rainfall during both the years of the study was above the normal seasonal water requirements of maize (400 to 600 mm). Besides improving grain yields of maize in rotation, sesbania fallows have the potential to recharge the subsoil water through increased subsurface drainage and increase nitrate leaching below the crop root zone in excess rainfall seasons. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alley cropping of maize with calliandra and leucaena in the subhumid highlands of Kenya: Part 2. Biomass decomposition, N mineralization, and N uptake by maize

A major challenge in developing agroforestry approaches that utilize tree-leaf biomass for provision of N to crops is to ensure synchrony between the N released from decomposing prunings and N demand by crops. A study was conducted in the subhumid highlands of Kenya to assess the rate of decomposition and mineralization of soil-incorporated Calliandra calothyrsus Meissner (calliandra) and Leucaena leucocephala (Lam.) de Wit (leucaena) tree biomass and maize roots (Zea mays L.) both in an alley cropping and a sole cropping system. The amount of mineralized N peaked four weeks after planting (WAP) maize in all the treatments during both seasons of 1995. Cumulative mineralized N at week 20 ranged from 114 to 364 kg N ha⁻¹ season⁻¹, the absolute control treatment giving the lowest and the prunings-incorporated treatments giving the highest amounts in the two seasons. Total N uptake by maize, ranging from 42 to 157 kg ha⁻¹ season⁻¹, was lowest in the 'alley-cropped, prunings-removed' treatments, and highest in the 'non alley-cropped-prunings-incorporated' treatments. The apparent N recovery rate by maize was highest in the fertilizer applied treatments in the two seasons. Decomposition rate constants (kD) ranged from 0.07 to 0.21 week⁻¹, and the rates among the different plant residues were as follows: leucaena < calliandra < maize roots. Nitrogen release rate constants (kN), ranging from 0.04 to 0.25 week⁻¹, followed a similar pattern as the rate of decomposition with leucaena releasing the highest amount of N followed by calliandra and lastly by maize roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Pasture production and tree growth in a young pine plantation fertilized with inorganic fertilizers and milk sewage in northwestern Spain

The use of organic waste materials such as milk sewage as an organic fertilizer could have the dual advantages of organic-waste disposal and reduced dependence on inorganic fertilizers. The effects of fertilization with (1) conventional mineral fertilization, (2) milk sewage sludge at 40 kg N ha⁻¹ target rate and (3) no fertilization on pasture production and tree growth were examined in an experiment consisting of two pasture mixtures under a one-year-old Pinus radiata plantation with a density of 2500 trees ha⁻¹. The two pasture mixtures were: (1) Dactylis glomerata L. var. saborto (25 kg ha⁻¹) + Trifolium repens L. group Ladino (4 kg ha⁻¹) + Trifolium pratense L. var. Marino (1 kg ha⁻¹); (2) Lolium perenne L. var. Tove (25 kg ha⁻¹) + Trifolium repens L. group Ladino (4 kg ha⁻¹) + Trifolium pratense L. var. Marino (1 kg ha⁻¹). The experiment began in the spring of 1995 using a randomized block design with three replicates in Castro Riberas de Lea (Lugo, Galicia, north-western Spain). Plot size was 12 × 8 m², with a 1 m buffer strip between plots. Two-year data showed that fertilization with either material had a positive effect on pasture production, with no significant difference between the two fertilization treatments. Tree growth in the milk sewage sludge plot was significantly higher than in the control plots. Inorganic fertilization increased pasture production, but affected tree growth negatively. The results show that milk sewage sludge could be used as a fertilizer in silvo-pastoral systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of improved fallow with <Emphasis Type="Italic">Sesbania sesban</Emphasis> on maize productivity and <Emphasis Type="Italic">Striga hermonthica</Emphasis> infestation in Western Kenya

Striga hermonthica is a major constraint to smallholder subsistence agriculture production in the sub-Saharan African region. Low soil fertility and overall environmental degradation has contributed to the build-up of the parasitic weed infestation. Improved cropping systems have to be introduced to address the interrelated problems of S. hermonthica and soil fertility decline. Thus, the effects of improved fallow with leguminous shrub Sesbania sesban on maize yields and levels of S. hermonthica infestation on farm land in the bimodal highlands of western Kenya were investigated. The experimental treatments were arranged in a phased entry, and randomized complete block scheme were six months Sesbania fallow, 18 months Sesbania fallow, six months natural fallow consisting of regrowth of natural vegetation without cultivation, 18 months natural fallow, continuous maize cropping without fertilizer application, and continuous maize cropping with P and N fertilization. Results show that Sesbania fallows significantly (p<0.05) increase maize yield relative to continuous unfertilized maize. S. hermonthica plant populations decrease in continuous maize between the first season (mean = 428 000 ± 63 000 ha⁻¹) and second season (mean=51 000 ± 15 000 ha⁻¹), presumably in response to good weed management. S. hermonthica seed populations in the soil decrease throughout the duration of the experiment in the continuous maize treatments. Short-duration Sesbania fallows can provide modest yield improvements relative to continuous unfertilized maize, but short-duration weedy fallows are ineffective. Continuous maize cultivation with good weed control may provide more effective S. hermonthica control than fallowing.     

Adoption potential of selected organic resources for improving soil fertility in the central highlands of Kenya

Soil fertility decline is the major cause of declining crop yields in the central highlands of Kenya and elsewhere within the African continent. This paper reports a study conducted to assess adoption potential of two leguminous trees, two herbaceous legumes, cattle manure, and Tithonia diversifolia either solely applied or combined with inorganic fertilizer, for replenishing soil fertility in the central highlands of Kenya. The study examined biophysical performance, profitability, feasibility and acceptability, and farmers experiences in managing and testing the inputs. The study was based on a series of studies incorporating both sociological and experimental approaches for two and a half years. Results of on farm trials showed that manure + fertilizer and tithonia + fertilizer treatments increased yields by more than 100% above the control. These treatments were the most profitable having highest net benefits and benefit cost ratios. They were also the most commonly preferred by farmers who used them on larger plots compared to the other inputs. In conclusion, cattle manure and tithonia were found to be the organic materials with the highest adoption potential for soil fertility improvement in this area. Calliandra calothyrsus and Leucaena trichandra, on the other hand, have potential for use as animal fodder. The herbaceous legumes had the least adoption potential due to poor performance recorded on the farms that possibly led to low preference by the farmers. However, issues of sustainable seed production could have played a role. This study recommends some policy issues for enhancing adoption and research issues focusing on exploring strategies for increasing biomass production and use efficiency on farms. Andre Bationo was formerly working for Tropical Soil Biology and Fertility Institute of CIAT (TSBF-CIAT) as the African network Coordinator.     

Coppicing improved fallows are profitable for maize production in striga infested soils of western Kenya

Striga hermonthica (striga) weed is a major threat to crop production in sub-Saharan Africa, and short duration improved fallow species have recently been found to reduce the effects of this weed because of their ability to replenish soil nitrogen. The objective of this study was to compare the efficacy and profitability of coppicing improved fallow species (Gliricidia sepium [gliricidia], Leucaena trichandra [leucaena] and Calliandra calothyrsus [calliandra]) and non-coppicing species (Sesbania sesban [sesbania], Mucuna pruriens [mucuna], and Tephrosia vogelii [tephrosia]), in controlling striga. Natural fallow and a sole maize crop were included as control treatments. The fallow treatments were split into two and either fertilized with N or unfertilized. The results showed that coppicing fallows produced higher biomass than non-coppicing fallows. For example, Callindra (coppicing fallow species) produced 19.5 and 41.4 Mg ha⁻¹ of leafy and woody biomass, respectively after four cumulative harvests as compared with Sesbania (non-coppicing species), which produced only 2.3 and 5.9 Mg ha⁻¹ leaf and woody biomass, respectively. Improved fallows reduced striga population in proportion to the amount of leafy biomass incorporated into the soil (r = 0.87). N application increased cumulative maize yield by between 15–28% in improved fallow systems and by as much as 51–83% in the control treatments. Added total costs of the coppicing fallows did not differ significantly from those of the non-coppicing fallows and control treatments. However, the added net benefits of the coppicing fallows were significantly higher (US$ 527 for +N and 428 for −N subplots; P < 0.01) than those of the non-coppicing fallows (US$ 374 for +N and 278 for −N), and the least for the control treatments. The most profitable fallow system was Tephrosia with net added benefits of US$ 453.5 ha⁻¹ season⁻¹ without N, and US$ 586.7 ha⁻¹ season⁻¹ with added N.     

Soil nitrogen mineralization at various levels of canopy cover in red pine (Pinus resinosa) plantations

The objective of this study was to determine the rate of nitrogen (N) mineralization in response to various levels of canopy cover in red pine (Pinus resinosa Ait.) stands. Experimental plots consisted of various levels of canopy cover,i.e., clearcut, 25% (50% during first sampling year), 75%, and uncut in red pine plantations in northern Lower Michigan, USA. Net N mineralization and nitrification in the top 15 cm of mineral soil were examined during the first two growing seasons (1991–1992) following the canopy cover manipulations, using anin situ buried bag technique. Mean net N mineralization over the course of both growing seasons (May–October) ranged from 26.9 kg ha⁻¹ per growing season in the clearcut treatment to 13.4 kg ha⁻¹ per growing season in the uncut stand. Net N mineralization and nitrification increased significantly in the clearcut treatment compared to the uncut treatment during the second growing season only. However, net N mineralization and nitrification did not differ significantly between the partial canopy cover treatments and the uncut stand. Increased N mineralization and nitrification in the clearcut during the second growing season may be associated with increased soil temperature and changes of organic matter quality with time since canopy removal. This study was supported in part by the USDA Forest Service and Michigan Technological University.     

Profiles of carbon stocks in forest, reforestation and agricultural land, Northern Thailand

A study was conducted to assess carbon stocks in various forms and land-use types and reliably estimate the impact of land use on C stocks in the Nam Yao sub-watershed (19°05'10"N, 100°37'02"E), Thailand. The carbon stocks of aboveground, soil organic and fine root within primary forest, reforestation and agricultural land were estimated through field data collection. Results revealed that the amount of total carbon stock of forests (357.62 ± 28.51 Mg·ha-1, simplified expression of Mg (carbon)·ha-1) was significantly greater (P＜ 0.05) than the reforestation (195.25 ±14.38 Mg·ha-1) and the agricultural land (103.10±18.24 Mg·ha-1). Soil organic carbon in the forests (196.24 ±22.81 Mg·ha-1) was also significantly greater (P＜ 0.05) than the reforestation (146.83± 7.22 Mg·ha-1) and the agricultural land (95.09 ± 14.18 Mg·ha-1). The differences in carbon stocks across land-use types are the primary consequence of variations in the vegetation biomass and the soil organic matter. Fine root carbon was a small fraction of carbon stocks in all land-use types. Most of the soil organic carbon and fine root carbon content was found in the upper 40-cm layer and decreased with soil depth. The aboveground carbon(soil organic carbon: fine root carbon ratios (ABGC: SOC: FRC), was 5:8:1, 2:8:1, and 3:50:1 for the forest, reforestation and agricultural land, respectively. These results indicate that a relatively large proportion of the C loss is due to forest conversion to agricultural land. However, the C can be effectively recaptured through reforestation where high levels of C are stored in biomass as carbon sinks, facilitating carbon dioxide mitigation.     

Potentials of Millettia thonningii and Pterocarpus santalinoides for alley cropping in humid lowlands of West Africa

This study was conducted to assess the suitability of two fallow species that are indigenous to West Africa, M. thonningii (Schum and Thonn) and P. santalinoides (L'Her), for alley cropping with maize and their effect on soil chemical properties. It was carried out during the rain-fed cropping season at Ibadan, Nigeria and Mbalmayo, Cameroon in 1993 and 1994. Total dry matter of P. santalinoides prunings was higher at the two sites than that of M. thonningii by about 35% to 37%. Maize grain yield in plots supplied with prunings was significantly higher (P > 0.05) than in control (no prunings or fertilizer application) at Ibadan. Grain yield in plots supplied with prunings plus 40 kg ha⁻¹ urea fertilizer gave significantly higher yields than plots supplied with 80 kg N ha⁻¹ urea fertilizer only. At Mbalmayo, there was no significant difference between grain yield in plots supplied with 80 kg N ha⁻¹ and plots supplied with prunings plus 40 kg N ha^-1 urea fertilizer though the latter had higher yields. Grain yield was also higher in the middle rows than in rows adjacent to the hedgerows and these were not significantly different. Weed dry matter was reduced by 27% to 43% when Pterocarpus prunings were applied and 13% to 31% with application of Millettia prunings. Weed flora in both locations changed from grasses to broad leaved. Soil chemical changes at soil depth 0 to 10 cm showed significant increases (pH, C, N, P and Ca) after two cropping seasons in plots supplied with prunings or prunings plus fertilizer than the initial values. At Mbalmayo, K was lower after cropping in treatments than the initial values while at Ibadan, K and Mg were lower except in plots supplied with Pterocarpus prunings only. P. santalinoides and M. thonningii have significant potential for agroforestry in this sub-region. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth and yield of maize and timber trees in smallholder agroforestry systems in Claveria,northern Mindanao,Philippines

On-farm experiments were conducted in the Philippines to study over a 4-year period the growth of two timber trees, gmelina (Gmelina arborea R. Br.) and bagras (Eucalyptus deglupta Blume), and their impact on the grain yield of intercropped maize. The experiment consisted of maize monocropping plots (control) and maize intercropped between trees planted in block (2 × 2.5 m), and hedgerow arrangement (1 × 10 m). Three maize crops were planted in the block plots before canopy closure, and seven maize crops were planted in the hedgerow and monocropping plots. Maize grain yield in the hedgerow and in the block arrangement with gmelina were respectively 37% (16.58 tons ha⁻¹) and 68% (8.3 tons ha⁻¹) lower than in monocropping (26.21 tons ha⁻¹). In the plots with bagras, maize grain yield in hedgerow and in block arrangement were respectively 19% (24.8 tons ha⁻¹) and 66% (10.4 tons ha⁻¹) lower than in monocropping (30.6 tons ha⁻¹). For both tree species, the diameter at breast height (dbh) was greater in hedgerow than in block arrangement, with the difference being more pronounced with age. It was estimated that gmelina planted in hedgerows would produce 6–8 m³ ha⁻¹ of merchantable volume more than if planted in block. The study verifies the hypothesis that intercropping between widely-spaced trees rows (planted at 10 m or more) is more profitable and feasible to smallholders than either maize monocropping or woodlots, and concludes with recommendations on how to further improve the productivity of tree-intercropping systems.