Soil fertility changes and their effects on ginger ( Zingiber officinale Rosc . ) yield response in an ultisol under different pigeon pea hedgerow alley management in South Eastern Nigeria

Short term changes that occur in soil properties of an ultisol under different pigeon pea hedgerow alley populations and inter-hedgerow alley spacing and the effect of such changes on ginger rhizome yield response were evaluated in a two-year (2010 and 2011) field study in South Eastern Nigeria. Treatments comprised pigeon pea hedgerow alley populations of 20,000, 33, 333 and 66, 667 plants/ha in factorial combinations with three inter-hedgerow alley width spacing of 1, 2 and 3 m in a randomized complete block design with three replications. A plot having no pigeon pea component but planted to sole ginger constituted the control. Relative to the control, growing ginger in-between pigeon pea hedgerow alleys resulted in significant improvement in soil exchangeable Ca, Mg, and K, base saturation, organic carbon and available P in addition to reducing soil dry bulk density. Highest response in soil available P, organic carbon, dry bulk density, exchangeable Ca and Na and pH was achieved using pigeon pea population of 66, 667 plants/ha, while highest response for soil total N, base saturation and exchangeable K was achieved with 20,000 pigeon pea plants/ha. Optimum ginger rhizome yield response comparable with the control was achieved using pigeon pea hedgerow alley population of 20,000 plants/ha spaced 3 m apart. Increasing pigeon pea hedgerow alley population beyond 20,000 plants/ha and decreasing pigeon pea inter-hedgerow alley width below 3 m resulted in rhizome yield reduction due to probable nutrient competition. Apart from its capacity to continuously maintain the fertility of the fragile and infertile soils of South Eastern Nigeria, planting ginger in-between pigeon pea hedgerow alley will also serve as additional source of revenue to smallholder resource-poor ginger farmers in Nigeria.

renewable soil resource base.Efforts to minimize excessive use of inorganic fertilizers in tropical agriculture have led to the development of alternative farming systems such as alley cropping which is believed to have the capacity to minimize environmental C and N losses (Ansgar et al., 2009).Alley cropping entails growing food crops between hedgerows of planted shrubs and trees, preferably leguminous species.The underlying scientific principle of this technology is that, by continually retaining fast-growing, preferably nitrogen-fixing, trees and shrubs on crop-producing fields, their soil-improving attributes (such as recycling nutrients, suppressing weeds, and controlling erosion on sloping land) will create soil conditions similar to those in the fallow phase of shifting cultivation (Nair, 1993).
Literatures are replete with the advantages of alley cropping over the use of inorganic fertilizers.Several studies (Kang and Ghuman, 1991;Simpson and Wickham, 2007;Nwite et al., 2008) have demonstrated significant positive effects of alley cropping on soil fertility parameters such as organic C levels, total N and extractable P levels over a range of climatic and soil conditions.
Despite the largely documented contributions of alley cropping technology in soil fertility maintenance in the tropics, the system performance is known to be location specific and greatly influenced by the choice of tree species and the type and level of management practices adopted (Mugendi et al., 1999;Pander and Rai, 2007).Tree orientation and layout, hedgerow manipulation and crop husbandry practices are other important factors determining the biological effectiveness and outcome of any alley cropping system (Gopal, 2002).
Information from the literature on the effect of hedgerow alley spacing on the productivity of an alley cropping system is inconsistent.While some authors (Karim et al., 1993;Gopal, 2002) reported that close spacing of hedgerow alleys helps to favour leaf production over stem, provides a more effective barrier to soil movement on sloping lands and creates a better microenvironment for crop growth thereby allowing for improved distribution of nutrients to a greater proportion of the intercropped alley system, other researchers (Koech and Whitbread, 2000;Rao et al., 1990) maintained that close spacing between the hedgerows reduces the amount of land available for the crop and can result in increased competition for the growth factors of light, moisture and nutrients between hedgerow and crop.In addition, the geometry and population of individual plants making up an alley cropping system, have an important bearing on the growth and yield of the crops grown within the alley as they have a regulatory or compensation capacity over resource use and availability (Drinkwater and Snapp, 2007).
In South Eastern Nigeria where most of the alley cropping studies conducted involved use of pigeon pea hedgerow alleys (Egbe and Idoko, 2009), no particular hedgerow population is maintained by farmers.This practice makes it difficult to have a quantitative data relating pigeon pea population with soil fertility or crop yield enhancement.The unique combinations of optimal nutritional profiles, high tolerance to environmental stresses, high biomass productivity and efficient nutrient and moisture contributions to the soil (Damaris, 2007) are some of the pigeon pea qualities that make farmers in South Eastern Nigeria to prefer it over other legumes in their choice of crops for hedgerow establishment.Pigeon pea is reported to have greater N fixation rate, compared to other legume species available in the tropics (Chikowo et al., 2004).It is nodulated by a wide range of Rhizobia strains including Bradyrhizobium (cowpea group), and fast-growing rhizobia.In South Eastern Nigeria, pigeon pea is generally intercropped with cereals, other legumes, cotton (Gossypium sp), and castor (Ricinus communis).
Despite its high potential as a good vehicle for nutrient cycling in soils (Yeboah et al., 2004), detailed empirical data on the effects of pigeon pea alley population on soil properties and crop yield in Nigeria are lacking.
The objectives of this study therefore are (i) to quantify the short term changes that occur in soil properties under different pigeon pea hedgerow alley population and interhedgerow alley spacing adjustments and (ii) to access the effect of such soil fertility changes on the rhizome yield response of ginger in a humid tropical Ultisol in South Eastern Nigeria.
Determination of appropriate pigeon pea hedgerow alley population and inter-hedgerow alley width is a necessary step towards minimizing competition for moisture, nutrients and light encountered by annual crops in the system.

MATERIALS AND METHODS
The study was conducted in 2010 and 2011 cropping seasons at the research farm of the National Root Crops Research Institute, Umudike (Latitude 05° 29 1 N; and Longitude 07° 33 1 E) typical of South Eastern Nigeria.The soil used for the study was of sandy loam characteristics with N, P and K values of 0.14%, 9.3 mg/kg and 0.10 cmol/kg, respectively.These values are of low fertility and are therefore, inadequate for ginger production in the rainforest ecology of Nigeria according to soil fertility classification for ginger production by Njoku et al. (1995).The soil was strongly acidic with a pH (in water) value of 5.3 thus indicating the possibility of high concentrations of exchangeable Al (Lal, 1994).The soil had exchangeable bases (Ca, Mg and Na) contents of 0.80, 0.40 and 0.02 cmol/kg, respectively.These values were low and fell below the critical levels set by Enwezor et al. (1990) for sustainable root and tuber crops production in the South Eastern Nigeria.
Treatments consisted of three pigeon pea hedgerow populations of 20,000, 33,333 and 66,667 plants/ha in factorial combinations with three pigeon pea inter-hedgerow alley spacing of 1, 2 and 3 m.A plot containing no pigeon pea hedgerow alley was also included as a control.Hedgerow alleys were established by transplanting 6 weeks old pigeon pea plants grown from the nursery onto 1 m x 6 m tractor-slashed, harrowed and ploughed seed beds.Survival rate of the transplanted pigeon pea plants was monitored and dead transplants were re-supplied with fresh plants from the nursery within two weeks.This was done in other to ensure that the desired population density of the plant was achieved.Rhizome sets weighing about 20 g were cut from large, healthy and disease-free mother rhizomes of UG 1 ginger variety and planted up on the seed beds.The rhizome seeds were sown at an intra-row spacing of 0.20 m and inter row spacing of 0.20 m.The inter-plot distance was maintained at 2 m to reduce excessive shading from the pigeon pea.The treatments were laid out in a randomized complete block design with three replications.Planting of ginger was done on 19 th April in the first year (2010) and on 10 th May 2011 for the second year.
Determinations of relevant physico-chemical properties of the soil were undertaken using standard methods.Bulk density was determined by the core sampler method as reported by Black and Hartge (1986).Total porosity of the soil was calculated from bulk density assuming a particle density of 2.65 mg/m 3 with the following formula: Total Porosity = 1 -Bulk density/Particle density Total N was analyzed using the semi-micro Kjeldahl method as reported by Bremner and Mulvany (1982).Available P was determined by Bray and Kurtz -2 as modified by Olsen and Sommers (1982).
Exchangeable K, Ca, and Mg were measured by the 1N neutral NH 4 OAc saturation method of Grant (1982).Exchangeable K was measured flame photometrically using atomic absorption spectrophotometer; Ca and Mg were done using the EDTA complexometric titration method.Organic matter was estimated by the Walkley and Black wet oxidation method of (1934).Soil pH was determined by the combined glass electrode pH meter method of Mclean (1982) at a soil:solution ratio of 1:2.5.Particle size distribution measurement was carried out using the Bouyoucous hydrometer method as reported by Tel and Hagarthy (1984) using sodium hexametaphosphate as the dispersant.
The plots were fertilized with NPK 15:15:15: fertilizer at the recommended rate of 300 kg/ha.Fertilizer application was by broadcasting in two split doses: half dose during ginger planting and half 12 weeks after ginger planting.The plots were mulched two days after ginger planting using 20 t/ha of mature and wilted Panicum maximum grass.The plots were kept weed-free throughout the duration of the study.Weeding of the plots was by rouging.
The effect of application of treatment on soil fertility was evaluated by examining the changes in nutrient status in the topsoil (0 to 20 cm) at three growth intervals of 3, 6 and 8 months after planting of ginger (MAP) in each season.In addition to soil fertility parameters, data on fresh rhizome yield at harvest were also collected.The data generated from the study were analyzed using analysis of variance (ANOVA) according to the procedures out lined by Gomez and Gomez (1984) for Randomized Complete Block Design.Differences among treatment means with significant effects were detected using Least Significant Difference at 5% probability level.

RESULTS AND DISCUSSION
Compared to plots without pigeon pea treatment, alley cropping of pigeon pea with ginger resulted in significant reductions in soil dry bulk density (Table 1).The magnitude of reduction in soil dry bulk density relative to the control increased with increasing pigeon pea population up to 66, 667 plants/ha but decreased with increasing inter-hedgerow alley width irrespective of time after ginger planting.Among the three inter-hedgerow alley widths evaluated, highest significant mean dry soil bulk density reductions of 44 and 36% relative to the control were obtained on plots that had pigeon pea alleys spaced 1 m apart for 2010 and 2011, respectively.Increasing pigeon pea alley width from 1 to 3 m significantly increased soil dry bulk density by 44% in 2010 and by 32% in 2011.
The lower soil bulk density values generally observed in plots alley-cropped with pigeon pea relative to the control was attributed to increased soil porosity arising from extensive pigeon pea root penetration to deeper soil layers.Pigeon pea is reported to have a deep-rooting taproot that grows up to 2 m deep into the soil (Makumba et al., 2009).The deep root system helps to break hardpans, improves water infiltration by maintaining organic matter in the soil and by increasing the waterconducting pores formed by decayed roots thereby reducing soil bulk density (Mafongoya et al., 2006).Reduction in soil dry bulk density observed in the system in plots alley-cropped with pigeon pea is a very positive development in ginger production.Being a shallow-rooted crop, ginger requires soil bulk density not greater than 1.20 g/cm 3 for optimum and sustainable growth and yield (RMDC, 2005).Bulk density affects plant growth through its effect on soil strength and soil porosity.Increased dry bulk density increases soil strength and reduces soil porosity and both of these factors limit root growth at some critical value (Chan, 2002).
There was an observed tendency of competition for soil nitrogen between the ginger crop and the pigeon pea plant component at 3 MAP which resulted in a significant reduction in soil total nitrogen relative to the control.This competition increased with increasing pigeon pea population density and with reducing inter-hedgerow alley width (Table 2).In 2010, application of pigeon pea population of 66, 667 plants/ha reduced soil total nitrogen by 71% against 75% reduction recorded in 2011 relative to the control.Similarly, at 3 MAP reducing pigeon pea alley width to 1 m gave the highest soil total nitrogen reduction of 57 and 58% relative to the control for 2010 and 2011, respectively.
The reduction in soil total nitrogen at 3 MAP following application of pigeon pea treatments is attributed to the fact that at this growth period, the pigeon pea component of the system was still drawing up nutrient for physiological development.Nitrogen is the most critical plant nutrient affecting ginger growth and is needed in large amount for physiological development of the crop (Njoku et al., 1995).It is suspected that active biological atmospheric nitrogen fixation may not have commenced at 3 MAP and therefore, all the nitrogen nutrient needs of both crops (pigeon pea and ginger) had to be satisfied from the soil applied N and hence causing a depression in the amount of the nutrient existing in the soil system.Although, pigeon pea is a Nitrogen-fixing plant, its ability to extend N to the soil environment for the benefit of a companion crop is only possible when all the nitrogen requirement needs of the pigeon pea plant have been satisfied.This fact is validated by the report of Yeboah et al. (2004) who worked with pigeon pea to ameliorate the degraded ultisol in Ghana, found no difference in soil total N content between soil cultivated with pigeon pea and the uncultivated control after one year cropping.
At 6 and 8 MAP, both pigeon pea population and interhedgerow alley width tended to increase soil total N content compared to the control.For pigeon pea population, such increases in soil N were significant at 8 MAP only in 2010.The sudden increases in N observed at 6 and 8 MAP in plots treated with pigeon pea population relative to the control was attributed to biological N fixation by pigeon pea.Nodulated by a wide range of Rhizobia strains including Bradyrhizobium (cowpea group), and fast-growing rhizobia (Chikowo et al., 2004), pigeon pea has been reported to have greater N fixation rates compared to other legume species available in tropical Africa (Valenzuela, 2011).In an African research study conducted by Mafongoya et al. (2006), the Nitrogen fixation rate of pigeon pea was estimated to range from 40 to 97 Kg N/ha.Other research trials conducted across Africa and India also showed that the N contributions from pigeon pea to the soil environment and for the following crop in the rotation range between 40 to 60 Kg N/ha (Chauhan et al., 2004;Odeny, 2007).
There is ample research evidence based on the results of this study indicating that alley cropping pigeon pea with ginger is a promising crop management technology capable of sustaining available P capacity of the highly degraded soils of South Eastern Nigeria.There were significant increases in soil available P following the inclusion of pigeon pea alley treatments in ginger-based system compared to sole-cropped ginger.Both pigeon pea hedgerow alley population and inter-hedgerow alley width significantly affected phosphorus response of the soil at 3, 6 and 8 MAP (Table 3).Relative to the control, soil available P increased significantly with increasing pigeon pea population up to 66, 667 plants/ha.Highest P response for both years was achieved using pigeon pea population of 66, 667 plants/ha.Also, inter-hedgerow alley width of pigeon pea significantly affected available soil P response.Relatively higher soil P levels were generally recorded on plots that received inter-hedgerow alley width treatments than in the control.Across the three inter-hedgerow alley width spacing evaluated, highest response in soil available P was recorded when the pigeon pea alleys were spaced 2 m apart.Other researchers elsewhere have reported similar increases in soil available P with application of pigeon pea treatment (Shibata and Yano, 2003;Sinclair, 2004).Research study conducted in India with rotation systems showed that pigeon pea not only increased the Nitrogen status of the soils but also increased the amount of Phosphorus available for the follow-up crops in the rotation (Sinclair, 2004).
Increase in soil available P observed in this study following application of pigeon pea treatments confirms the research findings of Ae et al. (1990) that cultivation of pigeon pea increases total phosphorus availability in cropping systems with low available phosphorus.They emphasized that Pigeon pea was more efficient in utilizing iron-bound phosphorus (Fe-P) than several other crop species.They attributed such increase in soil P after pigeon pea treatment to root exudates, particularly piscidic acid and its p-O-methyl derivative, which release phosphorus from Fe-P by chelating Fe (3+).
Phosphorus is an important factor for consideration in the design and choice of any crop management practice in South Eastern Nigeria.This is so because the predominance of aluminum and iron oxides in the soils of the zone makes P to form insoluble complexes with cations thereby making it unavailable for plant uptake.Furthermore, soil P is incorporated into organic matter by microbes and this increases its unavailability for plant uptake (McLaughlin et al., 1988b).
At 3 MAP, soil organic carbon content followed a trend opposite to that of available P (Table 4).Soil organic carbon was slightly higher in the control treatment than in plots that received pigeon pea treatments at 3 MAP.This effect was however, not significant in both years of study.This was despite the observed tendency of increase in soil organic carbon response with increasing pigeon pea population at 3 MAP.This was contrary to the trend observed with the application of hedgerow alley width treatment where there was a decrease in organic carbon with increase in pigeon pea inter-hedgerow alley width spacing.
Averaged over the three pigeon pea hedgerow alley populations tested, the result of this study indicated that growing ginger sole without pigeon pea hedgerow inclusion resulted in significantly lower soil organic carbon content at 6 and 8 MAP compared to when it was grown in alleys formed by pigeon pea.This result portrays pigeon pea as being a potentially valuable crop for soil fertility conservation and maintenance especially in the erosion-prone and nutrient-poor soils of South Eastern Nigeria where rainfall is heavy with high nutrient leaching propensity (Okigbo, 2000).In South Eastern Nigeria where environmental challenge consists of reducing soil erosion and increasing soil organic matter content (Mbagwu, 1992b), development of a cropping system that increases organic matter storage in cultivated soils will have far reaching significant implications not only in terms of crop productivity but also on the global carbon balance and thus on climate change (Craswell and Lefroy, 2001).Among other documented roles, soil organic carbon is known to influence many processes that increase the cation exchange capacity, microbial activities of the soil as well as the degree of soil aggregation and by extension, the air: water ratio all of which influence plant growth and yield (Karl and Kotschi, 1997).
Soil exchangeable Ca, Mg, K and base saturation were significantly increased by both pigeon pea hedgerow alley population and inter-hedgerow alley width treatments relative to the control.Such increases were attributed to reductions in soil acidity as a result of increase in pH following pigeon pea alley cropping with ginger.Relative to the control, reductions in soil acidity due to the application of pigeon pea population treatment were significant only at 6 MAP (Table 5).At 3 and 8 MAP such reductions were not significant.Also, at 6 and 8 MAP soil acidity reduced by 27 and 19%, respectively in 2010 in pigeon pea alleys spaced 1 m apart relative to the control.At 3 MAP, inter-hedgerow alley width spacing of pigeon pea had no significant effect on soil pH Increase in soil pH (reduction in soil acidity) arising from pigeon pea hedgerow alley inclusion in ginger system probably explains the relatively higher values of base saturation recorded on plots that had pigeon pea treatments compared to those that received sole ginger treatment (Table 6).Highest improvement in terms of base saturation was recorded on plots alley-cropped with 20, 000 pigeon pea plants/ha spaced 3 m apart (Table 6).
Low soil acidity reduces P availability and also restricts Ca and Mg uptake by crop plants.Aluminium hydrolyses under acidic conditions forms hydroxyl aluminium which can be fixed on the cation exchange site.Hydroxyl aluminium is said to be non-exchangeable and reduces the CEC of clay and soils thus resulting in nutrient deficiencies and optimal restriction of nutrients absorption with subsequent decrease in crop yield.Therefore, for the geologically old and highly weathered soils of South Eastern Nigeria, use of pigeon pea alleys capable of reducing soil acidity and increasing P availability appears to be an important low input resource approach for sustainable crop production particularly for a high nutrient demanding crop such as ginger.
Both hedgerow alley population and inter-hedgerow alley width spacing significantly affected fresh rhizome yield response of ginger (Table 7).Relatively higher rhizome yield values were obtained in the control than in the pigeon pea treatments.Rhizome yield was higher at lower hedgerow alley populations than at higher.Optimum rhizome yields of 13.1 and 14.5 t/ha were obtained in 2010 and 2011, respectively with the application of 20, 000 pigeon pea plants/ha (Table 7).These yield values were comparable to the yield obtained in the control treatment for both years.Beyond this population density, a significant gradual reduction was recorded in the fresh rhizome yield response of ginger.This was attributed to a possible reduction in photosynthetic active radiation due to shading from the pigeon pea crop component.Although, ginger is reported to tolerate light shading of about 21% for optimal field performance (Meerabai et al., 2001), excessive shading however is bound to reduce photosynthetic active radiation (PAR) thereby negatively affecting plant growth and yield responses (Fageria, 1992).Rhizome yields were significantly lower in plots treated with alley width spacing than in the control.Growing ginger in between pigeon pea alleys spaced 3 m apart produced the highest mean rhizome yield response.At lower alley width spacing of 1 and 2 m, yield was reduced possibly due to increased intercrop competition for nutrient resources between the ginger crop and the pigeon pea crop component.
The relatively lower rhizome yields obtained in the pigeon pea-treated plots compared to the control indicated that perhaps the atmospheric N fixed by the pigeon pea was not probably made available to ginger crop within its critical period of nutrient demand (tuberization).The pigeon pea variety used for this study had a gestation period of about 6 to 7 months while ginger has a gestation period of 7 to 8 months (Njoku et al., 1995;Yeboah et al., 2004).It is therefore likely that greater part of the N that was fixed by the pigeon pea crop in the course of its growth would be more beneficial to the succeeding crop planted in rotation after pigeon pea.The result of this study therefore, implicates the need for further research to determine the best time of introduction of ginger in a ginger-pigeon pea cropping system in the rainforest ecology of Nigeria.

Conclusion
Growing ginger in between pigeon pea hedgerow alleys resulted in significant improvement in the nutrient status of the highly degraded soils of South Eastern Nigeria.Soil exchangeable Ca, Mg, K, base saturation, organic carbon and available P increased significantly following application of pigeon pea alley treatment compared to plots without pigeon pea.This is in addition to significantly reducing soil dry bulk density.The magnitude of improvement in these soil fertility parameters relative to the control was a combined function of pigeon pea population and inter-hedgerow alley width as well as the time after planting.While highest response in terms of soil available P, organic carbon, soil dry bulk density, soil exchangeable Ca and Na and pH were achieved using pigeon pea population of 66, 667 plants/ha, highest response for soil total N, base saturation and exchangeable K was achieved with 20,000 pigeon pea plants/ha.Although, fresh rhizome yield was relatively higher in the control than in the pigeon pea treatments, application of 20,000 pigeon pea plants/ha spaced 3 m apart produced statistically same rhizome yield values with the control.

Table 1 .
Effect of pigeon pea hedgerow population and inter-hedgerow spacing on soil bulk density (g/cm3) of an Ultisol in South Eastern Nigeria grown with ginger.

Table 2 .
Effect of pigeon pea hedgerow population and inter-hedgerow alley width on soil total Nitrogen (%) of an Ultisol in South Eastern Nigeria grown with ginger.

Table 3 .
Effect of pigeon pea hedgerow population and inter-hedgerow spacing on soil available Phosphorus (mg/kg) of an Ultisol in South Eastern Nigeria grown with ginger.

Table 4 .
Effect of pigeon pea hedgerow population and inter-hedgerow spacing on soil Organic Carbon (%) content of an Ultisol in South Eastern Nigeria grown with ginger.

Table 5 .
Effect of pigeon pea hedgerow population and inter-hedgerow spacing on soil pH of an Ultisol in South Eastern Nigeria grown with ginger.

Table 6 .
Effect of pigeon pea hedgerow population and inter-hedgerow alley width spacing on soil exchangeable K, Ca, Mg, Na, total exchangeable acidity and base saturation contents (Measured at 4 MAP) of an Ultisol in South Eastern Nigeria grown with ginger (Average of 2010 and 2011 Cropping Results).

Table 7 .
Effect of pigeon pea hedgerow alley population and inter-hedgerow alley width on the fresh rhizome yield (t/ha) of ginger in a tropical humid Ultisol in South Eastern Nigeria.