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Agronomy Journal 93:1191-1199 (2001)
© 2001 American Society of Agronomy

SOIL FERTILITY

Maize Yield as Affected by Organic Inputs and Urea in the West African Moist Savanna

B. Vanlauwe*,a, K. Aihoub, S. Amanc, E. N. O. Iwuaford, B. K. Tossahe, J. Dielsf, N. Sangingaf, O. Lyassef, R. Merckxg and J. Deckersh

a Tropical Soil Biol. and Fertil. Progr., P.O. Box 30592, Nairobi, Kenya
b Inst. Natl. des Recherches Agricoles du Bénin, B.P. 884, Cotonou, Benin Republic
c Cent. Natl. des Recherches Agronomiques, Abidjan, Côte d'Ivoire
d Inst. of Agric. Res., Zaria, Nigeria
e Inst. Togolais de Recherche Agronomique, B.P. 1026, Lomé, Togo
f RCMD, IITA, Ibadan, Nigeria, c/o L.W. Lambourn and Co., 26 Dingwall Rd., Croydon CR9 3EE, UK
g Lab. of Soil Fertil. and Soil Biol., Faculty of Agric. and Appl. Biol. Sci., K.U. Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium
h Inst. for Land and Water Manage., Faculty of Agric. and Appl. Biol. Sci., Vital Decosterstraat 102, 3000 Leuven, Belgium

* Corresponding author (b.vanlauwe{at}cgiar.org)

Received for publication December 12, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Nutrient depletion is a major constraint to crop production for moist savanna soils, and inputs of nutrients are required to overcome this constraint. The impact of sole and combined applications of organic inputs (OIs) [fresh tree prunings, Pueraria phaseoloides (Roxb.) Benth. residues, and manure] and urea [(NH2)2CO] on maize (Zea mays L.) performance was investigated at four sites in West Africa. Interactions between OIs and urea resulting in added benefits from their mixed rather than sole application were quantified, and likely causes were evaluated. Maize in the mixed treatments, receiving 45 kg ha-1 urea N and 45 kg ha-1 N as OIs, produced 1.6 and 3.7 Mg ha-1 grain in Sékou and Glidji, respectively. Based on the yields from sole application of either OIs or urea, added benefits from the mixture were 0.49 Mg ha-1 grain (P < 0.001) in Sékou and 0.58 Mg ha-1 (P < 0.15) in Glidji. These benefits were generated during grain filling, which was characterized by drought, and they were likely caused by improved soil water conditions with mixed applications compared with sole applications. Nitrogen recovery from urea was higher in the combined treatments (44% in Sékou and 32% in Glidji) relative to the sole urea treatments (22% in Sékou and 15% in Glidji). Positive interactions between OIs and urea occurred at two of four sites and were likely caused by improved soil water conditions after applying OIs. Organic inputs can alleviate constraints to crop growth other than N depletion and, as such, improve the use efficiency of N fertilizer.

Abbreviations: DAP, days after planting • NR, N recovery • OI, organic input • TSP, triple super phosphate • WAP, weeks after planting


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
ALTHOUGH the West African moist savanna zone has a high potential for crop production, yield levels obtained under farmer conditions are usually low. Some of the major causes for these low yields are the low use of external sources of nutrients and the near absence of fallows long enough to restore the soil nutrient base. Vanlauwe et al. (1999) demonstrated a significant maize response to fertilizer N and P for 98 and 78%, respectively, of the soils randomly selected within two research villages in the derived savanna and Northern Guinea savanna of the Ecoregional Program for the Humid and Sub-Humid Tropics of Sub-Saharan Africa (EPHTA, 1996).

In earlier work on soil fertility in the tropics, it was demonstrated that applying N fertilizers tended to deteriorate the soil physico-chemical conditions of the often physically and chemically fragile moist savanna soils in the long term, especially in the case of ammonium sulfate [(NH4)2SO4] (Pieri, 1992). This observation, together with the increasing attention paid to environmental degradation partly caused by agrochemicals, led to the development of farming systems where inputs of inorganic fertilizers were minimized or avoided. It also led to an increased research effort on the biological components of soil fertility dynamics in the tropics (Swift, 1985). Alley cropping is a good example of such a low-input farming system (Kang et al., 1990). However, it became quickly apparent that OIs alone, applied at reasonable rates, could sustain crop yield only at limited levels (Kang et al., 1990). Moreover, Vanlauwe et al. (1998a)(1998b) demonstrated that recovery of N from high quality Leucaena leucocephala (Lam.) de Wit residues by a maize crop was low. A significant part of the L. leucocephala N was found in the soil organic matter pool, and thus was not immediately available for crop growth.

It has recently been generally accepted that to increase crop production in West Africa, both inorganic inputs and OIs are needed (Buresh et al., 1997; FAO, 1999). Organic inputs are needed to maintain the physical and physico-chemical health of the often shallow, sandy to sandy loam topsoils in the West African moist savanna zone while fertilizers are needed to supply a sufficient amount of nutrients to the crop. Several hypotheses have been formulated concerning possible positive interactions between OIs and fertilizer when applied simultaneously (Giller et al., 1998), resulting in added benefits in terms of improved crop yield, soil fertility status, or both (Palm et al., 1997). Because some hypothesized positive interactions require sufficient contact between the organic and inorganic inputs, the way of applying the OIs (surface vs. incorporated) may influence interactions between organic and inorganic inputs. Traditionally, organic materials are surface-applied in the derived savanna while organic materials are incorporated in the old furrows before reridging the fields at the start of the growing season in the Northern Guinea savanna.

Although a large body of information exists in literature on the impact of inorganic N, organic N, and the combination of both on crop yield (Pieri, 1992; Palm et al., 1997), most trials in the West African moist savanna deal with manure or crop residues as a source of OIs. Because livestock densities in the moist savanna are generally low, the applicability of animal manure for crop production is limited (Fernandez-Rivera et al., 1993). The production of crop residues under low-fertility conditions is also low. Furthermore, crop residues are often removed from the field and used for purposes other than soil fertility maintenance, especially in the Northern Guinea savanna (Manyong et al., 2001). Alternative sources of OIs produced in situ or brought to the field in cut-and-carry systems, such as tree prunings in agroforestry systems or legume biomass in legume–cereal rotations, could also be considered for use in cropping systems in the moist savanna zone. These organic materials cover a wide range of residue qualities, leading to varying C and N mineralization rates (Cadisch and Giller, 1997). Palm et al. (1997) stated that essential information on the quality of the applied organic materials is lacking in most of the trials evaluating organic and inorganic inputs and suggested that the direction and the extent of the interaction between organic and inorganic N sources may depend on the quality of the OIs.

The objectives of this work were to (i) quantify the impact of possible interactions between OIs (applied at the soil surface or incorporated) and urea N on maize grain yield for a series of sites representative of the West African moist savanna zone, (ii) assess the impact of initial soil fertility status and climatic conditions on the possible interactions between OIs and urea N, and (iii) assess the role of OI quality in improving maize yield in the treatments receiving only OIs or both OIs and urea.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Experimental Site Characteristics
Trials were established in 1997 at experimental research stations in Bouaké, Côte d'Ivoire (7°40'N, 5°50'W; Southern Guinea savanna zone) on an Alumi-Ferric Acrisol (FAO, 1998) or a Kanhaplic Haplustult (USDA, 1998); in Glidji, Togo (6°15'N, 1°36'E; derived savanna) on a Rhodi-Acric Ferralsol or a Rhodic Kandiustalf; in Sékou, Benin Republic (6°37'N, 2°14'E; derived savanna) on a Rhodi-Acric Ferralsol or a Rhodic Kandiustalf; and in Zaria, Nigeria (11°11'N, 7°38'E; Northern Guinea savanna) on a Chromi-Epiferric Luvisol (Petroferric) or a Typic Haplustalf. The length of the fallow period before trial establishment was more than 5 yr in Glidji and Sékou and more than 10 yr in Bouaké. The fallow in Sékou consisted of sparsely growing Chromolaena odorata (L.) R.M. King & H. Rob. shrubs that were burnt yearly while the fallow in Glidji consisted mainly of Azadirachta indica A. Juss. and Pithecellobium dulce (Roxb.) Benth. trees growing at a low density and undergrown mainly by Tephrosia villosa (L.) Pers., Andropogon gayanus Kunth, and Panicum maximum Jacq. The fallow in Bouaké consisted of tall grasses (mainly An. gayanus and Pa. maximum). The Zaria site had been used for legume and cereal seed multiplication before the establishment of this trial. The overall rainfall received during the maize season varied between 249 mm in Bouaké and 755 mm in Zaria (Fig. 1) . In Sékou and Glidji, a relatively long dry spell occurred at about 65 d after planting (DAP) while in Bouaké, rainfall was minimal during the first 28 DAP (Fig. 1).



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  		Fig. 1. Cumulative rainfall during maize growth in Bouaké, Sékou, Glidji, and Zaria.

 
Organic Inputs and Quality Determination
Senna siamea (Lam.) H.S. Irwin & Barneby and A. indica are common tree species in the moist savanna zone while prunings from L. leucocephala were harvested from alley cropping trials under fallow. The P. phaseoloides residues used in Glidji were taken from an Elaeis guineensis Jacq. plantation near the rock phosphate treatment plant in Kpémé, which may explain its very high P concentration. The farmyard manure used in Zaria was collected from a nearby experimental farm. All tree-derived organic materials were harvested from trees near the trials.

A few days before residue application, at least three subsamples were taken from all organic materials. The dry matter and Kjeldahl total N content were determined and based upon this information, the fresh matter application rates of the different organic materials were calculated to obtain the amount of N in organic form needed for the specific treatments. These materials were also analyzed for total P (IITA, 1982), lignin (Van Soest, 1963), and polyphenols (tissue/extract ratio of 0.75:50 g/mL) (Anderson and Ingram, 1993). Properties of the organic materials are shown in Table 1.


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  		Table 1. Selected characteristics of the organic inputs used in the different trials in West Africa. All tree-derived organic inputs consisted of leaves and twigs, except the Leucaena leucocephala applied in Zaria, which consisted only of leaves.

 
Trial Design, Establishment, and Sampling
Before planting, soil samples were taken on a diagonal transect of the plot (eight soil samples) on each plot at 0 to 10 and 10 to 30 cm and bulked to form one composite sample per plot and per depth. The samples were further bulked per replicate and analyzed for organic C (Amato, 1983), total Kjeldahl N, Olsen P (Okalebo et al., 1993), pH(H2O) (soil/water ratio of 1:2.5), pH(KCl) (soil/1 M KCl solution ratio of 1:2.5), effective cation exchange capacity (IITA, 1982), and texture (IITA, 1982). Selected topsoil characteristics are shown in Table 2.


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  		Table 2. Selected topsoil properties for four research sites in West Africa.{dagger}

 
The treatments were arranged in a randomized complete block design with three (Sékou and Zaria) or four (Bouaké and Glidji) replicates. Plot size was 6 by 6 m. Strips of treated plots were alternated with plots that were left untreated with N and grown with a uniform maize crop receiving 30 kg P ha-1 as triple super phosphate (TSP). A set of treatments was randomly allocated to the different plots: (i) a N response curve (0, 22.5, 45, 67.5, and 90 kg ha-1 urea N), (ii) incorporated L. leucocephala and A. indica residues applied separately at a rate of 90 kg N ha-1 and applied at a rate of 45 kg N ha-1 mixed with 45 kg ha-1 urea N, (iii) surface-applied L. leucocephala residues applied at a rate of 90 kg N ha-1 and applied at a rate of 45 kg N ha-1 mixed with 45 kg ha-1 urea N, and (iv) a set of site-specific incorporated or surface-applied organic residues applied at a rate of 90 kg N ha-1 and applied at a rate of 45 kg N ha-1 mixed with 45 kg ha-1 urea N (Table 3). Hereafter, the treatments that received only OIs are referred to as the organic treatments while those that received both OIs and urea are referred to as the mixed treatments. The treatments that received 45 and 90 kg ha-1 urea N and no OIs are referred to as the 45-urea-N and 90-urea-N treatments, respectively, or the sole urea treatments. All treatments received a blanket application of 30 kg P ha-1 as TSP.


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  		Table 3. Maize planting and harvesting dates (1997), fertilizer management, and treatment structure at the sites in West Africa.

 
In all of the treatments with surface-applied residues, the residues were weighed out per line and spread over the entire plot surface. In the treatments with incorporated residues, the plots were ridged (furrow depth about 10 cm below the original soil surface), and the organic residues were applied in the furrows. Fertilizer management in the treatments with incorporated and surface-applied residues reflected the local practices or recommendations and is described in Table 3. Maize was planted at a density of 75 cm between the lines (eight lines per plot) by 25 cm within the line and thinned to one plant per pocket. The variety used was Oba Super II, which is a long-duration (110–120 d), drought tolerant, and N efficient hybrid (Heuberger, 1998). The plots were regularly weeded to minimize any impact of weed pressure on maize performance. In Sékou, Cyperus sp. weed pressure was very high, necessitating up to about six weedings. At 4, 8, and 12 wk after planting (WAP), four plants were harvested from a 1-m-long stretch randomly selected within both rows neighboring the border rows, and the total dry weight of the stover biomass was recorded after oven drying (65°C). At harvest, the total ear and stover fresh weight was determined in the net plot (four lines of 4-m length). The number of barren maize plants (plants without ears or with ears containing only a few kernels) was also quantified. Ear and stover subsamples were taken, weighed, and oven-dried (65°C). After drying, the ears were separated in grains and cobs, and the total dry matter accumulation of the grains, cobs, and stovers was calculated. The grain and stover yield data were corrected for number of missing plants close to physiological maturity to account for damage done by small rodents (the proportion of missing stands averaged between 2% in Zaria and 8% in Bouaké). Subsamples of all maize yield components (grains, cobs, and stover) were analyzed for Kjeldahl N content (IITA, 1982).

Mathematical and Statistical Analyses
All maize-related data were submitted to analysis of variance for each site separately with the MIXED procedure of the SAS system (SAS, 1992). Replicate was included as a random effect while treatment was used as a fixed effect. Maize yield data in the strip of untreated plots were used as a covariate (paired observations). Because the type of OI did not significantly influence maize yields at any of the sites, the data are presented for the following groups of treatments: surface-applied sole OIs, surface-applied OIs mixed with urea, incorporated sole OIs, and incorporated OIs mixed with urea. The LSMEANS statement was used to calculate the means while the PDIFF option was used to separate significantly different means. Nitrogen response curves were calculated with the REG procedure of the SAS system (SAS, 1985).

The impact of eventual interactions between the applied urea and OIs in the mixed treatments on maize grain yield and total biomass production, or added benefits, were calculated as:

[1]
where Ymixed and Ycontrol are the maize grain yield or total biomass production in the mixed and control treatments, respectively, and Y'organic and Y'urea are the response in maize grain yield or total biomass production to OIs and urea, respectively, applied at 90 kg N ha-1. The latter values are calculated as:

[2]

[3]
where Yorganic and Y90ureaN are the maize grain yields or total biomass production in the organic and 90-urea-N treatments, respectively. For Eq. [2] and [3], the yield difference was divided by 2 to account for the application rate of 45 kg N ha-1 as OIs or urea. Using Eq. [1], [2], and [3], the added benefits can also be calculated as:

[4]

The N recovery (NR) of applied fertilizer or OI-N was calculated with the difference method as follows:

[5]

For the mixed treatments, the total amount of OI-N recovered by the maize was presumed to be equal to half of the total amount of OI-N recovered in the organic treatments. Arguments supporting this assumption are presented in the first paragraph of the Discussion section.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
Maize Stover Production
The control treatment produced significantly lower amounts of stover biomass than the other treatments at 12 WAP in Bouaké and at 8 and 12 WAP in Sékou (Fig. 2a and 2b) . Differences in stover biomass production between the other treatments were not significant at any sampling times. In Glidji, all treatments produced similar amounts of stover biomass at all sampling times (Fig. 2c). In Zaria, the 90-urea-N treatment produced significantly more stover dry matter at 8 and 12 WAP than all other treatments (Fig. 2d). The mixed treatments produced more stover dry matter than the organic and control treatments only at 12 WAP (Fig. 2d).



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  		Fig. 2. Maize stover dry matter accumulation in (a) Bouaké, (b) Sékou, (c) Glidji, and (d) Zaria as affected by the application of urea, organic materials, or the combination of both. The lower and upper error bars are the minimal and maximal standard errors of the difference, respectively. Values in the legend are expressed as kg N ha-1.

 
In Bouaké and Glidji, no significant differences in maize stover yield at maturity were observed between the different treatments (on average, 2.0 and 2.8 Mg ha-1 in Bouaké and Glidji, respectively) (Fig. 2a and 2c). In Sékou, the stover yields in the mixed treatments (2.2 Mg ha-1) were significantly higher than in the control (1.2 Mg ha-1) and the organic treatments (1.8 Mg ha-1) (Fig. 2b). Differences between other treatments were not significant. Stover yields in Zaria were significantly higher in the 90-urea-N treatment (4.5 Mg ha-1) than in all other treatments (Fig. 2d). Stover yields in the mixed treatments (3.3 Mg ha-1) were significantly higher than in the organic treatments (2.2 Mg ha-1) (Fig. 2d).

Maize Grain Yield
Both maize grain yield (Y in Mg ha-1) and total biomass responded significantly to urea application (N in kg ha-1) in Sékou (Y = 0.35 + 0.013N; R2 = 0.95; P < 0.01) and Zaria (Y = 0.10 + 0.034N; R2 = 0.96; P < 0.01). In Glidji, only maize grain yield responded significantly to urea application (Y = 1.79 + 0.027N; R2 = 0.91; P < 0.05) while in Bouaké, no response to applied urea N was observed.

In Bouaké, differences in maize grain yield between treatments were not significant (average = 2.3 Mg ha-1) (Fig. 3a) . In Sékou, maize grain yields in mixed treatments with both surface-applied and incorporated OIs (average = 1.6 Mg ha-1) were significantly higher than the two organic treatments (average = 0.6 Mg ha-1) and similar to the yield (1.6 Mg ha-1) obtained in the 90-urea-N treatment (Fig. 3b). Yields in the organic treatments were similar to the yield in the control plot (0.3 Mg ha-1). Differences in maize yield between the treatments with surface-applied and incorporated OIs were not significant. In Glidji, similar trends were observed as in Sékou, except that the overall yields were higher (1.6 Mg ha-1 in the control treatments, 2.0 in the organic treatments, 3.7 in the mixed treatments, and 4.1 in the 90-urea-N treatment) (Fig. 3c). In Zaria, maize yields in the mixed treatments (average = 1.8 Mg ha-1) were significantly higher than those in the organic (average = 0.7 Mg ha-1) and no-N control (0.3 Mg ha-1) treatments and significantly lower than the yield of the 90-urea-N treatment (3.4 Mg ha-1) (Fig. 3d). At this site, the incorporated mixed treatments yielded significantly more maize grain than the surface-applied mixed treatments while this was not true for the organic treatments.



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  		Fig. 3. Maize grain yields in (a) Bouaké, (b) Sékou, (c) Glidji, and (d) Zaria as affected by the application of urea, organic materials, or the combination of both. The left and right error bars are the minimal and maximal standard errors of the difference. SF, INC, and OI are mean surface-applied, incorporated, and organic inputs, respectively. Numerical values for treatments are expressed as kg N ha-1.

 
In Bouaké, the proportion of barren stands in the harvested area was similar for all treatments (average = 17%) (Fig. 4a) . In Sékou, both the mixed and 90-urea-N treatments (average = 8%) had a lower proportion of barren stands than the organic treatments (average = 20%) and the no-N control treatment (39%) (Fig. 4b). In Glidji, the mixed and 90-urea-N treatments (average = 12%) contained a drastically lower number of barren plants compared with the organic and control treatments (average = 45%) (Fig. 4c). In Zaria, both organic treatments (average = 37%) had a larger proportion of barren plants than the mixed treatments (average = 24%) (Fig. 4d). The 90-urea-N treatment had the lowest proportion of barren plants (6%).



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  		Fig. 4. The number of barren stands at maize harvest in (a) Bouaké, (b) Sékou, (c) Glidji, and (d) Zaria as affected by the application of urea, organic materials, or the combination of both. Barren indicates no ear or ears with only a few kernels. The left and right error bars are the minimal and maximal standard errors of the difference, respectively. SF, INC, and OI are mean surface-applied, incorporated, and organic inputs, respectively. Numerical values for treatments are expressed as kg N ha-1.

 
Contribution of Organic and Inorganic Inputs to Maize
Added benefits in terms of extra grain yield generated in the mixed treatments averaged 0.49 Mg ha-1 (P < 0.001) in Sékou and 0.58 Mg ha-1 in Glidji although the latter value was not significantly different from zero (P < 0.15) (Table 4). No extra grain yield was observed in Zaria (Table 4).


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  		Table 4. Calculation of the added benefits generated by mixing organic inputs (OIs) with urea at three sites in West Africa.

 
The recovery of applied fertilizer N in the 90-urea-N treatment ranged from 15% in Glidji to 22% in Sékou and 42% in Zaria (Table 5). Urea NR in the mixed treatments was higher than in the 90-urea-N treatment in Sékou (44%) and Glidji (32%), although not significantly in the latter location, and similar in Zaria (43%). The recovery of applied OI-N was low (maximally 10% in Zaria) (Table 5).


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  		Table 5. Recovery of applied fertilizer and organic input (OI) N by the complete maize crop (grain, cob, and stover), as calculated with the difference method at three sites in West Africa.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
Added benefits as a result of positive interactions between OIs and urea were observed in two (Glidji and Sékou) of the four sites (Table 4). Although several mechanisms could have generated these positive interactions, some are more likely than others. Application of urea could have directly improved uptake of OI-N by maize by enhancing the OI decomposition–mineralization process through the supply of N to the soil decomposer community or indirectly by improving maize root development and, consequently, maize NR. Because residue quality did not significantly influence the added benefits, the first process is not likely to have occurred, as this would have led to different added benefits in view of the wide range of residue qualities used at the various sites (e.g., the C/N ratio varied between 9 and 17 in Sékou). The similar vegetative biomass accumulation for the organic, mixed, and 90-urea-N treatments between 0 and 12 WAP indicates that the second process was not likely a major cause for added benefits. Consequently, the observed added benefits were more likely caused by improved urea N utilization of the maize crop by applying OIs, rather than vice versa.

Each growth-limiting factor alleviated by applying OIs may theoretically have led to added benefits in the mixed treatments. However, the basal application of soluble P fertilizer surely excluded significant effects of OI-derived P on maize growth. Because the exchangeable Ca and Mg content of all soils is reasonably high (Sanchez, 1976) and TSP also contains a considerable amount of Ca, no extra maize production is expected in the mixed treatments through application of Ca and Mg contained in the OIs. Even the total exchangeable K content of the soil (deficiency if < 0.2 cmolc kg-1) and the relative contribution of K to the total amount of exchangeable bases (deficiency if < 2%) are high enough to exclude K deficiency (Sanchez, 1976) in Sékou and Glidji (Table 2), the two sites where added benefits were observed. This leaves N and water availability as the two limiting factors likely to have been positively altered by applying OIs. Furthermore, as the two sites on which added benefits were observed experienced absence of rainfall during most of the maize grain-filling period, it is logical to first of all evaluate the potential benefits to soil water retention of OI application.

The observed added benefits likely occurred during ear formation and grain filling because differences in stover biomass production between the various treated plots were relatively small or not significant both during the growing season as well as at maize harvest (Fig. 2b and 2c). Maize is known to be very susceptible to drought during flowering and the first weeks of grain filling (Heisey and Edmeades, 1999). One of the most likely causes for the observed added benefits was reduced water stress in the mixed treatments compared with the sole urea treatments because of the presence of organic materials. The OIs were applied at the soil surface or subsurface at about 10-cm depth. While it is well known that surface mulch improves soil water retention by reducing evaporation and runoff and improving infiltration (Duley and Russel, 1939; Steiner, 1994), subsurface placement of residues, as was done in the current trials, can also reduce water loss from the soil surface (Minhas and Gill, 1985; Sembiring et al., 1995).

The potentially higher availability of soil water in the mixed treatments compared with the sole urea treatment during the grain-filling period in Sékou and Glidji has likely improved the efficiency of applied urea N. While the recovery of the total amount of applied N was similar in the 90-urea-N and mixed treatments in Sékou and Glidji, these recoveries were much higher than in the organic treatments (Table 5). This clearly shows that, in these sites, mixing urea with OI has improved the N use efficiency of urea, OI, or both. However, as we argued above, the recovery of OI-N is less likely to have been affected by applying urea N than vice versa. As such, the higher urea NR in the mixed treatments (Table 5) is possibly a consequence of improved soil water conditions in the mixed treatments. In Zaria, where rainfall was sufficient during the complete growing season, the similar urea NRs in the sole urea and mixed treatments further suggest that the mechanisms that have led to added benefits in Sékou and Glidji were not important in Zaria. Expressed differently, in Sékou, e.g., the addition of OIs increased the N response from 13 to 24 kg grain kg-1 fertilizer. Eck (1982) reported a slope of 33 kg grain kg-1 fertilizer for a drought-tolerant maize variety and a N application range varying from 0 to 90 kg ha-1 inorganic N even though water stress was imposed during the grain-filling period. Under unstressed conditions, the slope increased to 44 kg grain kg-1 fertilizer.

Although in view of above arguments that soil water dynamics appear to have driven the occurrence of added benefits, direct interactions between the OIs and urea in terms of temporary immobilization of fertilizer N and subsequent improved synchrony between N supply and demand cannot be completely excluded. Sakala et al. (2000) observed net N immobilization of NH4–N after mixing NH4 with maize stover and the length of the immobilization phase varied between <10 and >50 d, depending on the mineral N application rates. Temporary immobilization of fertilizer N could have reduced the movement of fertilizer N down the soil profile, which can happen quite fast. Vanlauwe et al. (2001) showed that already at 21 DAP, 11% of the applied urea N was recovered between 120- and 150-cm depth in a trial in the derived savanna. However, this mechanism leading to reduced urea N loss in mixed treatments could have led to added benefits in Zaria, which experienced the highest rainfall.

The range of residue qualities used in the trials would have most likely led to differences in added benefits between residues, which was obviously not the case. The general absence of residue quality effects, both in the presence and absence of urea, may have been the result of the range of residue qualities considered. Although this range was not necessarily narrow—e.g., in Zaria, the C/N ratio varied between 11 and 22—more extreme values were not included in the design as the OIs were required to be readily available. Including a wider range of qualities, which would more likely reveal significant influences of residue quality, may prove difficult in the West African moist savanna.

The list of possible processes leading to added benefits in treatments with combined applications of OIs and fertilizer so far considered is not exhaustive. In the short term, application of OIs may alter the soil-related pest spectrum. Akhtar (2000), for example, found that application of A. indica based organic products, sole or in combination with urea, significantly reduced the total number of plant-parasitic nematodes. In the longer term, continuous application of OIs may improve soil physical and chemical characteristics such as soil structure, bulk density, porosity, and nutrient retention among others, and consequently lead to better crop growth. Quantification of fertilizer NR is needed in trials with significantly different contents of soil organic matter between treatments to evaluate the validity of the longer term hypothesis.

While water was the main limiting factor for maize growth in the treatments with sole application of urea, N availability is more likely to be the limiting factor in the organic treatments. The high N stress in the organic treatments is clearly visualized by the relative large number of barren plants, a common feature of severely stressed maize, compared with the mixed or 90-urea-N treatments (Fig. 4b and 4c). Lemcoff and Loomis (1994) reported that N stress led to a fewer number of grains per ear. In a trial with supplementary irrigation, Sidhu and Sur (1993) observed 27% barren plants after applying Vigna mungo (L.) Hepper straw residue in absence of N fertilizer. This value decreased to 16% after applying 100 kg N ha-1 as fertilizer.

In Zaria, the effects of applying OIs together with fertilizer were additive, indicating that OIs can substitute part of the applied N fertilizer. However, the contribution of the applied OIs to the maize yield in the mixed treatments was low compared with the contribution made by urea (Table 5). This clearly shows that the N supply capacity of OIs is low and that OI application rates exceeding the rates used in this work would be necessary to substitute a substantial part of fertilizer N.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
Although it could not be proven beyond doubt, the added benefits in terms of extra grain yield observed in Sékou and Glidji were most likely caused by an improvement in water availability in the mixed treatments relative to the treatments with sole application of urea due to the application of OIs. In the short term, OIs appear to have the potential to alleviate constraints to crop production other than N deficiency and to consequently improve the use efficiency of mineral N. Even in absence of short-term added benefits, as observed in Zaria, the impact on maize yield was at least additive. Long-term application of OIs can subsequently improve the overall soil fertility status and lead to added benefits caused by more efficient use of N fertilizer in the long term.

Evaluating the occurrence of positive interactions and understanding the mechanisms creating them for a wider range of soil fertility and climate conditions in both the short and long term would further substantiate their agronomic importance. This could eventually lead to site-specific recommendations related to the combined use of OIs and fertilizer N. Finally, socio-economic evaluation is required to assess the short- and long-term profitability of substituting part of the recommended fertilizer rates with OIs.


   ACKNOWLEDGMENTS
 
The authors are grateful to ABOS, the Belgian Administration for Development Cooperation, for sponsoring this work as part of the collaborative project between KU Leuven and IITA on Balanced Nutrient Management Systems for Maize-Based Farming Systems in the Moist Savanna and Humid Forest Zone of West-Africa. Messrs K. Chinwo and M. Kelani are acknowledged for the plant and soil analyses. This is IITA manuscript IITA:00/JA/36.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 




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Long-Term Fertilizer and Water Availability Effects on Cereal Yield and Soil Chemical Properties in Northwest China
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