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.     

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.     

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.     

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.     

Growth and yield of maize alley cropped with Leucaena leucocephala and Faidherbia albida in Morogoro, Tanzania

This study examined the effect of alley cropping of Leucaena leucocephala and Faidherbia albida on wood biomass, maize grain yield and soil nitrogen status. The treatments were: trees planted alone at 1 × 5 m spacing; trees intercropped with maize and a sole maize crop. Mulch biomass averaged 6.18 and 0.97 t ha⁻¹ for L. leucocephala and F. albida, respectively. Corresponding wood production was 1.71 and 1.11 t ha⁻¹. Both total N and inorganic N (NO ₃ ⁻ –N plus ₄ ⁺ –N) were higher under F. albida and lowest under L. leucocephala. Similarly, foliar N concentration in maize was higher in plots intercropped with F. albida and least in L. leucocephala intercropping. Maize grain yield was little affected by the tree intercrop as competition for resources was reduced through periodic pruning and clean weeding. There was no gain in maize grain yield due to the presence of L. leucocephala and F. albida. These results suggest that alley cropping in Gario is justified for wood production but not for increasing maize grain yield. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

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.     

Competition for light between hedgerows and maize in an alley cropping system in Hawaii, USA

Successful agroforestry systems depend on minimizing tree-cropcompetition. In this study, field experiments and a simulation model were usedto distinguish between tree-crop competition for light and belowgroundcompetition in an alley cropping system. Maize (Zea maysL.) was harvested periodically in three treatments: between vertical barriers ofshade cloth, hedgerows of Flemingia macrophylla (Willd.)Merr., and sole maize. Radiation intercepted by the maize was calculated using asimulation model based on measured values for direct and diffuse light, hedgerowdimensions and leaf area, and solar trajectory. Radiation use efficiency wascalculated as biomass production per unit of intercepted radiation. Maizebiomass and yield in both the alley crop and the shade cloth treatment weregreatest in the center of the alleys. Grain yield between hedgerows was 3.5Mg ha⁻¹ (averaged across the alley), significantlyless than in the shade cloth (7.4 Mg ha⁻¹) or thesole maize (7.7 Mg ha⁻¹) treatments. Lightintercepted by the maize in the alley crop was about half that intercepted bythe maize in the sole crop. The shade cloth intercepted less light than thehedgerows because it did not have an appreciable width. Radiation use efficiencyin the three treatments was 0.75 g mol⁻¹ PAR anddid not differ significantly among treatments. Tree-crop competition wasoverwhelmingly for light. 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.     

Dry-season sesbania fallows and their influence on nitrogen availability and maize yields in Malawi

Nitrogen deficiency is widespread in southern Africa, but inorganic fertilizers are often unaffordable for smallholder farmers. Short-duration leguminous fallows are one possible means of soil fertility restoration. We monitored preseason topsoil (0 to 20 cm) ammonium and nitrate, fallow biomass production and grain yields for three years in a relay cropping trial with sesbania [Sesbania sesban (L.) Merr.] and maize (Zea mays L.). Sesbania seedlings were interplanted with maize during maize sowing at 0, 7400 or 14,800 trees ha^–1, in factorial combination with inorganic N fertilizer at 0 or 48 kg N ha^–1 (half the recommended rate). After maize harvest, fallows were allowed to grow during the seven-month dry season, and were cleared before sowing the next maize crop. Both sesbania fallows and inorganic N fertilizer resulted in significantly greater (P < 0.01 to 0.05) preseason topsoil nitrate-N than following unfertilized sole maize. In plots receiving no fertilizer N, preseason topsoil inorganic N correlated with maize yield over all three seasons (r ² = 0.62, P < 0.001). Sesbania fallows gave significantly higher maize yields than unfertilized sole maize in two of three years (P < 0.01 to 0.05). Sesbania biomass yields were extremely variable, were not significantly related to sesbania planting density, and were inconsistently related to soil N fractions and maize yields. Short-duration fallows may offer modest yield increases under conditions where longer duration fallows are not possible. This gain must be considered against the loss of pigeonpea (Cajanus cajan L. Millsp) harvest in the similarly structured maize-pigeonpea intercrop common in the region.This revised version was published online in November 2005 with corrections to the Cover Date.     

Resource competition across the crop-tree interface in a maize-silver maple temperate alley cropping stand in Missouri

In order to improve the management of temperate alley cropping, it is important to study the growth and physiological responses of plants arising from competition across the crop-tree interface. Maize (Zea mays L.) was established between rows of seven-year-old silver maple (Acer saccharinum L.) trees in north-central Missouri, USA with four imposed treatments: (1) an unmodified control with a standard rate of N fertilization (179.2 kg N (as NH₄NO₃) ha⁻¹), (2) trenching with root barrier installed, (3) supplemental fertilization treatment (standard N + 89.6 kg ha⁻¹ N), and (4) a combination of trenching with root barrier and supplemental fertilization. Whereas soil N status had little effect on maize physiology and yield at the interface, competition for soil water was substantial in both years. Without a root barrier, soil water content, predawn and midday water potential, and midday net photosynthesis of maize plants adjacent to the tree row were reduced compared with those of plants in the alley center, but no differences across the maize crop were evident in the presence of a barrier. Grain yield of border row maize plants lacking an adjacent barrier was depressed compared with that for maize plants with a root barrier present (8.42 vs. 6.59 Mg ha⁻¹ in 1997; 5.38 vs. 3.91 Mg ha⁻¹ in 1998). However, the barrier did not completely restore yield to that in the alley center, suggesting that reductions in light near the tree row also limited production. Top ear height showed a similar pattern of response to the presence of a root barrier. Silver maple trees responded to root barrier installation with reduced annual diameter growth and reduced water status on some sample days. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth and yield of eight agroforestry tree species in line plantings in Western Kenya and their effect on maize yields and soil properties

Between October 1988 and August 1992, field experiments were carried out in West Kenya to evaluate the suitability of Leucaena leucocephala, L. collinsii, Gliricidia sepium, Calliandra calothyrsus, Sesbania sesban, S. grandiflora, Senna siamea and S. spectabilis to provide a range of agroforestry products and services. The initial objective was to establish the growth rates and wood and leaf yields of these tree species, when planted in single rows. After the initial evaluation, it was evident that valuable additional information could be collected if the trees were converted to hedges and their effect on intercropped maize and soils was studied. At 21 months after planting, different species and provenances ranged in height between 3.5 and 6 m and varied considerably in phenotypic appearance. Wood production (1988–1990) varied from 3 to 33.8 t ha⁻¹ and leaf production varied from 0.62 to 10.1 t ha⁻¹. During intercropping (1990–1992), leaf production varied from 0 to 10.9 t ha⁻¹. Maize yields were higher in association with Leucaena and Gliricidia than with Calliandra, Sesbania and Senna. Cumulative maize grain and stover yields over four seasons were positively correlated with the total amount of tree leaves applied (r² range, 0.70–0.95). The effect of tree leaf mulch on crop yields decreased over time for all species. Leaves with high nutrient contents, which decompose fast (Leucaena, Gliricidia, Sesbania) are likely to have been more effective in sustaining crop yields than leaves with lower nutrient contents (Senna) or more complex decomposition patterns (Calliandra). Simple “leaf input-crop output” budgets to calculate the reserves for N, P and K in different systems explained crop yield differences in some cases. Compared to the fertility status of “zero-mulch” control plots, the status of soil C, N, P, K, Ca, Mg and S was to varying degrees improved under Leucaena, Gliricidia and Sesbania, much less under Calliandra but not under Senna. First season grain yields were related to the soil fertility status at the end of the tree fallow. The results of these experiments suggest that under subhumid tropical conditions with soils of relatively poor nutrient status, where light and water are not likely to be the major limiting factors to crop production, the application of sufficient quantities of high quality tree mulch may positively influence maize yields. When agroforestry tree species with contrasting decomposition and nutrient release patterns are evaluated jointly, it is more difficult to demonstrate a general relationship between quantities of mulch applied and improvements in crop yields and soil fertility levels. Therefore, further chemical, physiological and phenotypic characterization of free species with potential for fallow and intercropping systems is required.     

Effects of organic and mineral fertilizer inputs on maize yield and soil chemical properties in a maize cropping system in Meru South District, Kenya

Soil nutrient depletion as a result of continuous cultivation of soils without adequate addition of external inputs is a major challenge in the highlands of Kenya. An experiment was set up in Meru South District, Kenya in 2000 to investigate the effects of different soil-incorporated organic (manure, Tithonia diversifolia, Calliandra calothyrsus, Leucaena leucocephala) and mineral fertilizer inputs on maize yield, and soil chemical properties over seven seasons. On average, tithonia treatments (with or without half recommended rate of mineral fertilizer) gave the highest grain yield (5.5 and 5.4 Mg ha⁻¹ respectively) while the control treatment gave the lowest yield (1.5 Mg ha⁻¹). After 2 years of trial implementation, total soil carbon and nitrogen contents were improved with the application of organic residues, and manure in particular improved soil calcium content. Results of the economic analysis indicated that on average across the seven seasons, tithonia with half recommended rate of mineral fertilizer treatment recorded the highest net benefit (USD 787 ha⁻¹) while the control recorded the lowest (USD 272 ha⁻¹). However, returns to labor or benefit-cost ratios were in most cases not significantly improved when organic materials were used.     

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.     

Soil fertility changes and response of maize and beans to green manures of leucaena,sesbania and pigeonpea

Three multipurpose tree species (MPTS)-leucaena (Leucaena leucocephala), sesbania (Sesbania sesban var. nubica) and pigeonpea (Cajanus cajan) were pruned at a height of 60 cm above the ground every two months, and resulting plant biomass was incorporated into the soil as green manure. For comparison, maize (Zea mays) stover was also incorporated into some plots, while some other plots were left fallow. Varying quantities of plant biomass which were incorporated into the soil over a period of 12 months caused large changes in major soil plant nutrients, and it substantially improved soil fertility. To test for improved soil fertility, test crops of maize and beans (Phaseolus vulgaris) were grown on the plots after six biomass incorporations of 4806, 13603, 16659 and 7793 kg. ha^–1yr^–1 for pigeonpea, sesbania leucaena and maize, respectively. Responses of the test crops indicated that sesbania and leucaena green manures improved maize stover, cobs and grain yields; and bean haulms and grain yields by 77.6% when compared to fallow plots. Residual effects of green manures still resulted in significant (P < 0.05) yield differences in the test cropin the third testing season. Economic significance of green manures in increasing food crop yields to small scale farmers is discussed.     

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.     

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.     

Reduced soil nutrient leaching following the establishment of tree-based intercropping systems in eastern Canada

Tree-based intercropping (TBI) systems, combining agricultural alley crops with rows of hardwood trees, are largely absent in Canada. We tested the hypothesis that the roots of 5–8 years old hybrid poplars, growing in two TBI systems in southern Québec, would play a “safety-net” role of capturing nutrients leaching below the rooting zone of alley crops. TBI research plots at each site were trenched to a depth of 1 m on each side of an alley. Control plots were left with tree roots intact. In each treatment at each site, leachate at 70 cm soil depth was repeatedly sampled over two growing seasons using porous cup tension lysimeters, and analyzed for nutrient concentrations. Daily water percolation rates were estimated with the forest hydrology model ForHyM. Average nutrient concentrations for all days between consecutive sampling dates were multiplied by water percolation rates, yielding daily nutrient leaching loss estimates for each sampling step. We estimated that tree roots in the TBI system established on clay loam soil decreased subsoil NO₃ ⁻ leaching by 227 kg N ha⁻¹ and 30 kg N ha⁻¹ over two consecutive years, and decreased dissolved organic N (DON) leaching by 156 kg N ha⁻¹ year⁻¹ in the second year of the study. NH₄ ⁺ leaching losses at the same site were higher when roots were present, but were 1–2 orders of magnitude lower than NO₃ ⁻ or DON leaching. At the sandy textured site, the safety net role of poplar roots with respect to N leaching was not as effective, perhaps because N leaching rates exceeded root N uptake by a wider margin than at the clay loam site. At the sandy textured site, significant and substantial reductions of sodium leaching were observed where tree roots were present. At both sites, tree roots reduced DON concentrations and the ratio of DON to inorganic N, perhaps by promoting microbial acquisition of DON through rhizodeposition. This study demonstrated a potential safety-net role by poplar roots in 5–8 year-old TBI systems in cold temperate regions.     

Nitrate-N dynamics following improved fallows and maize root development in a Zimbabwean sandy clay loam

Improved or planted fallows using fast-growing leguminous trees are capable of accumulating large amounts of N through biological N₂-fixation and subsoil N capture. During the fallow phase, the cycling of nutrients is largely efficient. However, there are few estimates of the fate of added N during the cropping phase, after the 'safety net' of fallow-tree roots is removed. Nitrate-N at the end of the fallow phase, which is pre-season to the subsequent crop, was monitored in seven land use systems in successive 20-cm soil layers to 120 cm depth at Domboshawa, Zimbabwe in October 2000. Thereafter, nitrate-N dynamics was monitored during cropping phase until April 2001 at 2-week intervals in plots that had previously 2-year planted fallows of Acacia angustissima and Sesbania sesban, and in a continuous maize control. Pre-season nitrate concentrations below 60 cm soil depth were <3 kg N ha⁻¹ layer⁻¹ for S. sesban, A. angustissima, Cajanus cajan and natural woodland compared with the maize (Zea mays L.) control, which had >10 kg N ha⁻¹ layer⁻¹. There was a flush of nitrate in the S. sesbania and A. angustissima plots with the first rains. Topsoil nitrate had increased to >29 kg N ha⁻¹ by the time of establishing the maize crop. This increase in nitrate in the topsoil was not sustained as concentrations decreased rapidly due to leaching. Nitrate then accumulated below 40 cm, early in the season when maize root length density was still low (<0.1 cm cm⁻³) and inadequate to effectively intercept the nitrate. It is concluded that under light soil and high rainfall conditions, there is an inherent problem in managing nitrate originating from mineralization of organic materials as it accumulates at the beginning of the season, well ahead of peak demand by crops, and is susceptible to leaching before the crop root system develops. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of between-row (alley widths) and within-row spacings ofGliricidia sepium on alley-cropped maize in Sierra Leone

The effects of spacings between hedgerows (alley widths) and the spacings of trees within hedgerows ofGliricidia sepium on growth and grain yield of maize were investigated at Senehun in southern Sierra Leone. Four between-row spacings (2, 4, 6 and 8 m) were combined with three within-row spacings (0.25, 0.50 and 1.00 m) in a split block design. Maize, at densities of 20,000, 40,000 and 53,333 plants ha^–1, was established in the alleys and also as pure crops. N, P and K fertilizers were applied to all plots before pruning of the trees began. When pruning started, only the pure maize plots received fertilizer; prunings from the hedgerows were returned to the appropriate alleys in the other plots.Plots with the highest maize populations consistently gave the best yields before pruning started, but lower populations gave improved yields after pruning. Yields of maize increased with increasing alley widths before the start of pruning, after which the narrower alleys of 2 and 4 m outyielded the wider ones by almost double, probably because of the large amount of nutrients applied in prunings. Lack of light limited grain yields before the start of pruning, when there was some shading by the hedgerows. Alleys of 2–4 m wide, planted no closer than 0.50 m within rows, resulted in more than twice the yields of maize than in the 8-m alleys planted at 0.25 m within rows, once the hedgerows were well established and were being managed.