Water use and grain yield response of rainfed soybean to tillagemulch practices in southeastern Nigeria

Transcription

1 554 Obalum et al. Water use and grain yield response of rainfed soybean to tillagemulh praties in southeastern Nigeria Sunday Ewele Obalum *, Charles Arizehukwu Igwe, Martin Eze Obi, Toshiyuki Wakatsuki 2 University of Nigeria Dept. of Soil Siene Nsukka 4000, Nigeria. 2 Kinki University/Shool of Agriulture Nara , Japan. *Corresponding author <ijewelle@yahoo.om> Edited by: Edmilson José Ambrosano ABSTRA: Despite the agronomi, eonomi and food values of soybean (Glyine max L. Merrill), there is still dearth of information on the tillage need and the impliations of surfae mulh for the rop in the eastern part of the forestsavanna transition zone of Nigeria. This study was therefore arried out on a sandy loam Ultisol at Nsukka with a subhumid limate, during 2006 and 2007 ropping seasons. Our objetive was to devise an appropriate tillage method for the rop from evaluated effets of notill (), onventional tillage () and mulh on seleted key agronomi indies. Eah of the and the was either unmulhed (U) or mulhed (M) in a splitplot, giving four treatments/tillage methods (U, M, U and M) randomized in four bloks. Rainfall was more favorable in the first than in the seond season. The mean seasonal soil water storage (range, 9909 mm) within 0.5m soil layer differed among the treatments (U < U < M = M). However, for the first and seond seasons, both water use (58266 and mm respetively) and grain yield ( and.22.9 Mg ha respetively) were not different. Mulh lowered the rop water use but had no influene on grain yield. Water use effiieny was enhaned with mulh only in the seond season. Although either of the two mulh treatments (M/M) would be suitable for growing soybean espeially in years of unfavorably distributed rainfall, M is a more rational hoie than M. Rainfall adequay at the ritial reprodutive stage of the rop showed to be a more important yield fator than the tested tillage methods. Keywords: tillage system, surfae mulh, soil moisture, soybean grain yield, water use effiieny Introdution The ultivation of soybean (Glyine max L. Merrill) is, ompared to other vegetable legumes, on the inrease in Nigeria as the rop has lately been introdued to other parts from the southern Guinea Savanna region owing to its nutritive and eonomi values (Dugje et al., 2009; Kamara et al., 2007; Lasisi and Aluko, 2009). In various loations in the southwestern part of the forestsavanna transition zone, researhbased tillage requirement of the rop has been taken are of (Ahaneku, 2006; Lasisi and Aluko, 2009; Onwualu and Ahaneku, 200). Similar guide is yet to be reported anywhere in the southeastern part of the transition zone inluding the Nsukka agroeology, whih has more features of a derived savanna than any other part of the zone. Although the urrent mean seasonal rainfall in the derived savanna region appears to be adequate with respet to the water requirement for soybean, the region is haraterized by hydrolimati flutuations (Chukwu, 999; Odurukwe et al., 995) and poor soil water retention of the predominant Ultisols. These onstraints often result in not meeting the water requirements of key rops and, hene, poor yields. It is neessary at this early adoption stage of the soybean rop to study its response to soil moisture indued by the routine tillage operations in this rainfed system. Agele et al. (2002) opined that suh a relationship is basi to understanding adaptation and yield stability. The notill () and the onventional tillage () systems have been found to have large influene on soil moisture and rop yields (Agriulture and Rural Development, 2004). However, they are typially inonsistent in their agronomi effets, and seem to be dependent on loation and rop. Speifially for soybean, the effet of tillage methods on yield is inonsistent. It has been reported to be variable among years (Norwood, 999; Singer et al., 2008; Thiagalingam et al., 996) or higher with (Pederson and Lauer, 2003; Temperly and Borges, 2006) or higher with (Feak et al., 200; Lasisi and Aluko, 2009; Onwualu and Ahaneku, 200). There are also studies showing only marginal differenes in soybean yields between and (Alvarez and Steinbah, 2009; Koga and Tsuji, 2009; Rodrigues et al., 2009). Conversely, the effet of surfae mulh is almost always preditable. Surfae mulh has been reported to have positive effet on the soil hydrothermal regime and rop yield (Thiagalingam et al., 996). However, it has been hypothesized that ombination of or with mulh modifies the soil surfae and may have muh greater impat on the soil water balane and evapotranspiration; and so would ultimately affet how effiiently rops use the rainwater input (Hatfield et al., 200). Sine water is a primary limiting fator and, hene, an important management onern in soybean prodution (Deosthali et al., 2005), any hosen tillage method for the relatively new soybean rop in the Nsukka plains should Si. Agri. (Piraiaba, Braz.), v.68, n.5, p.55456, September/Otober 20

2 Tillage and mulh praties in soybean in Nigeria 555 aim at maximizing the rainwater resoure for the rop. The objetives of this study were to establish the effets of and systems with and without surfae mulh on the soil moisture status, water use (WU), grain yield and water use effiieny (WUE) of soybean grown in a sandy loam soil under rainfed ondition at Nsukka, southeastern Nigeria. Materials and Methods The experiment was arried out at Nsukka (06º52 N, 07º24 E, 400 m a.s.l.) in southeastern Nigeria. Generally, the limate is haraterized by mean annual total rainfall of about,600 mm. The entire wet season lasts from April to Otober, with a short break inbetween two phases as the rainfall is bimodally distributed; whereas the dry season lasts from November to Marh. On the average, the potential evapotranspiration (PET) normally exeeds rainfall in a year. The rainfallpet dynamis as well as the prevailing air temperature during the months of the growing seasons is depited in Figure. The soil belongs to the Nkpologu series and was lassified as a Typi Paleustult. It is essentially a mineral soil that is deep and welldrained, with an usti soil moisture regime and an isohyperthermi soil thermal regime. Runoff rarely ours on the experimental site not only beause of its gentle slope (only about 2 %), but also due to high infiltration rate enouraged by its oarse texture and unfavorable aggregation. Steady state infiltration rate ranges from 240 to 750 mm h (Obi and Nnabude, 988). The initial haraterization of the soil based on seleted physiohemial properties is shown in Table. Being in the forestsavanna transition vegetation zone, grasses dominate the natural vegetation, with few interspersing leguminous weeds. Some of the grass speies identified at the site inluded Andropogon gayanus, Celosia trigyna, Table Some physiohemial properties of the top 0. m soil at the start of the study. Physial Soil Properties Chemial Sand (g ) 752 ph in H O ilt (g ) 60 SOM (g ) 8. 6 S Clay (g ) 88 Total N (g ). 3 Bulk density (Mg m ).46 Available P (mg ) 28 Total porosity 0.57 CEC (mmol ) 70 Maroporosity 0.2 Exhangeable bases ( mmol ) Miroporosity 0.45 Ca (mmol ) 7 MWD (mm) 2. 3 Mg (mmol ) 2 K sat ( m h * AWC (mm 0.5m ) 8. 3 Na (mmol ) 4 ) 9 0 K (mmol ) K sat saturated hydrauli ondutivity; MWD mean weight diameter. AWC available water apaity *(moisture held between 0.06 and 5bar tension). SOM soil organi matter. CEC ation exhange apaity 34 Emilia sonhifolia, Pennisetum polystahion, and Spermaoe vertiillata. Leguminous weeds were represented by Calapagonium muunoides and Muuna urens; broad leaf weeds by Asystasia gangetia. In 2006 when the study was initiated, the field had been under a mixedspeies fallow for about ten years. Land learing was manually ahieved at the site with minimal soil disturbane at the beginning of eah ropping season in both years (2006 and 2007) of the study. Prior to the preplanting tillage operations in eah year, thoroughly mixed organi manure (poultry droppings) was uniformly applied in the entire field at a rate of 5 Mg ha ; no inorgani fertilizers were used. This was based on the reommended rate of poultry manure that would serve as a substitute for inorgani fertilizer for soybean (Kratohvil et al., 2006). Treatments onsisted of a fatorial setup with and as the main plots, and unmulhed and mulhed onditions as the subplots. This 2 2 splitplot arrangement yielded four treatments:, unmulhed (U);, mulhed (M);, unmulhed (U); and, mulhed (M). The plots were just leanweeded flat beds; the plots were seedbeds manually prepared by ploughing to the depth of about 0.2 m. Treatments were repliated four times in a randomized omplete blok design (RCBD). In the plots, the only soil disturbane ourred during seeding and oasional weeding, both operations of whih were arried out with aution. The subplot size was 4.2 m 2. m. Raised earthen bunds with separating pathways (width, 0.4 m) were used to demarate the bloks. The entire field, measuring 8 m 8.4 m, was fened round using earthen bunds. In line with the reommended agronomi praties for soybean (Dugje et al., 2009; ICSNigeria, 2003), an earlymaturing ultivar (SAMSOY2) was treated with Apron Star and manually sown at three per hill in 24 m depth on 3 July, 2006 and 7 June, Crop stands were spaed 0.6 m between and 0.3 m within rows, resulting a population of 55,555 plants per hetare. Seedlings Figure Some limati variables for the months in the 2006 and 2007 growing seasons Potential evapotranspiration, alulation of whih was based on BlaneyCriddle equation (Blaney and Criddle, 950). Si. Agri. (Piraiaba, Braz.), v.68, n.5, p.55456, September/Otober 20

3 556 Obalum et al. were later thinned down to one per stand 4 days after sowing (DAS). Appliation of mulh followed immediately after thinning. The mulh material was omposed mainly of dry leaves of Paspalum notatum, and was applied at the rate of 5 Mg ha. All plots were kept free from weeds using a hand hoe or by hand piking throughout the study; no herbiides were used. The three omponents of water balane needed for the omputation of rop WU were preipitation (P), drainage (D) below the root zone, and hange in storage (ΔS) during the growing season. The daily values of P for the entire period were obtained from the University Meteorologial Station, loated about 50 m away from the study site. To obtain ΔS, the soil was sampled immediately after sowing, on the assumption that the profile had been wetted homogenously. Subsequent soil sampling for determination of the profile soil water storage was started two weeks after mulh appliation and ontinued at 0 ± day intervals until harvest. This sampling interval was based on the postulation that plant water need over periods of about ten days would usually be met by soil water storage (Stern et al., 982). Sampling was limited to the 0.5 m depth zone, the zone of greatest root density of soybean (FAO, 2002; Willatt and Olsson, 982). On eah sampling oasion, the approah used by Hulugalle and Lal (986), Moitra et al. (996) and Zougmore et al. (2004) in determining the storage in a 0.5 m layer in similar studies with soils of the same textural lass (sandy loam) as the present soil, was adopted. This involved the use of a graduated tube auger to sample down to the 0.5 m depth in a single swoop and partitioning the soil sample into top(00.3 m) soil and sub( m) soil, before determining their water ontents gravimetrially. Using predetermined mean soil bulk densities, the gravimetri water ontents were onverted to volumetri basis. The volumetri water ontent was multiplied by the thiknesses of the respetive soil layers (in mm) to express on depth basis. The values from the two soil layers were summed up to obtain the root zone moisture storage (in mm). During the sampling period, two of the four repliates of eah treatment were seleted for monitoring the soil water storage S. Designated portions, entrally loated within the plots and away from the border rows, were permanently marked for the repeated water ontent measurements. To maximize the hanes of sampling on days when differenes did appear, the priniple of not monitoring immediately after rains was adopted, hene the ± day allowane in the sampling intervals. With this priniple, the aim was not to ahieve an exat piture of the rop s daily WU during the growing season, but to identify differenes among treatments (Tilander and Bonzi, 997). The experimental field was sampled nine and eight times before harvest in the first and seond years, respetively. The ΔS was alulated as the differene between the total value of water storage on all the sampling dates and the orresponding value on all respetive preeding dates. D was simulated as outflow from a nearby (about 30 m away) welldesigned nonweighing drum lysimeter, buried at the soil surfae level over 25 years ago. The outflow was measured on daily basis. Based on the assumption that the drainage proess of deep perolation took plae only when the theoretial field apaity (simulated at 0.6mwater tension) was exeeded (Oluwasemire et al., 2002), water storage and D under the treatments were regarded as variables that exhibited an inverse relationship. Thus, the values of water storage were used to adjust D for the different treatments. A simple water balane equation was used to ompute rop WU or evapotranspiration (ET): ET = P D ΔS. In order to examine the effet of water defiit onditions on grain yield during the growing seasons, an analysis of water stress at different growth stages was arried out using the Moisture Availabilty Index (MAI), the ratio of available water to PET (Srivastava et al., 996). PET was alulated using the BlaneyCriddle Model (Blaney and Criddle, 950). A MAI value of 00 % implies that the dependable rainfall equals PET. The intensity of water stress for soybean was determined after Srivastava et al. (996), as the lowest required MAI values at various stages of development of the rop; 75 % during the seedling stage, 00 % during the vegetative/reprodutive stage, and 50 % during the maturity stage. Soybean pods were harvested at maturity and this took plae on 27 Otober, 2006 (6 DAS in the first year) and 24 Otober, 2007 (38 DAS in the seond year). Grain yield was assessed using 6 plants (onsisting of four four rows of plants) from the designated entral portion in eah subplot. Dry pods were threshed to separate the seeds from the haff, and weights of the seeds (grain yield) taken thereafter. The YieldWU relationship was assessed by WUE, expressed as the measured grain yield () per land area (ha) per water onsumptive use (mm). Analysis of variane (ANOVA) proedure for a splitplot in RCBD was used to test the data under the treatments for differenes. The separation of treatment means (for statistial omparison) was ahieved by the proedure of Fisher s least signifiant differene (FLSD) as desribed by Obi (2002). Results and Disussion The pattern of rainfall in the two growing seasons (Figure ) learly depits the exat nature of the hydrolimati variability that haraterizes the study area. By the traditional features, rainfall is bimodally distributed and reahes its peak in the months of July and Otober in the longer and shorter subwet seasons respetively. This was observed in A point of deviation in 2007 worth mentioning was that the peak in the longer subwet season, instead of ourring in July, ourred in August when ordinarily a period of short break with the least rainfall would be expeted. On the other hand, analyses of the daily rainfall showed that drizzles (individual rain events < 0 mm in depth) ourred 55 and 45 times, amounting to 236 and 82 mm respetively, in the first and seond seasons respetively. For rainstorms (individual rain events 40 mm), the orresponding values were two and six times and 92 and 267 mm. Rainfall distribution was, therefore, more favorable in 2006 ompared with Si. Agri. (Piraiaba, Braz.), v.68, n.5, p.55456, September/Otober 20

4 Tillage and mulh praties in soybean in Nigeria 557 Figure 2 Water ontents of the 00.3 and m layers under the treatments aross the sampling dates U Notill, unmulhed; M Notill, mulhed; U Conventional tillage, unmulhed; M Conventional tillage, mulhed. Error bars are standard deviations of the measurements for eah treatment. Figure 3 soil water storage in the 0.5m profile aross the sampling dates in both years of the study. U Notill, unmulhed. M Notill, mulhed. U Conventional tillage, unmulhed. M Conventional tillage, mulhed. The vertial error bars represent the LSD (0.05) among the treatments. Overall, the M and the M appeared to have the most influene on soil water in the 00.3 and m layers respetively (Figure 2). Apart from refleting the depth of tillage, this trend also buttresses the importane of mulh in soil water onservation. The mean seasonal values of S in the entire monitored profile differed (p < 0.05) among the tillage methods (Figure 3). Averaged over the two years, the soil water status dereased from the two mulh treatments (M and M) through the U to the U. Higher S under the mulhed ompared to the unmulhed treatments was evident in espeially the top(0.3 m) soil layer during the early phase of the growing season, when deterioration of the organi mulh material had not set in (Figure 2). The better soil water ondition of the mulhed plots relative to the unmulhed plots was, therefore, due to the water onserved under mulh during the early growth phase. Rathore et al. (998) reported similar results with straw mulh in the profile of a hikpea field. Moreover, the overall soil water relation was more favorable under the mulhed than the unmulhed plots, as indiated by the moderate and lower saturated hydrauli ondutivity in the former ompared to the rapid and higher values in the latter, at the end of the study (Obalum and Obi, 200). On the other hand, the better soil moisture status under the U relative to the U was attributed to the enhaned rainwater infiltration into the former due to the presene of large lods and larger surfae area (Ali and Talukder, 2008; Hatfield et al., 200). The fat that this prevailed only in the unmulhed treatments is indiative of the overriding influene of the mulh on the effets of and on soil moisture. In both growing seasons, the tillage systems had no influene on the total WU of the rop (Table 2). There was a redution (p 0.05) in the total WU due to the applied mulh in the two years of the study. Higher evaporation from the surfae and topmost part of the soil may have taken plae under the unmulhed treatments espeially early in the season. This was probably not ompensated for by an inrease in transpiration following better vegetative growth under mulh at a later stage, hene the differenes in the total WU. Similar to these results, the WU of hikpea grown under tillagemulh treatments did not differ between the and the, but was lower under mulh ompared to unmulhed plots (Rathore et al., 998). In the present study, the WU values indiated higher water onsumptive use in the seond than in the first season. Considering the dependeny of the water requirement of the soybean rop on length of growing season (FAO, 2002), higher WU would be expeted in the seond season, in whih the rop remained 22 days more in the field, ompared to the first season. Treatments had no effets on the grain yield in either of the two years (Table 3). Soybean is nonresponsive to tillage systems (Alvarez and Steinbah, 2009; Koga and Tsuji, 2009; Kramer and Alberts, 988; Rodrigues et al., 2009; Wilhelm and Wortmann, 2004; Yusuf et al., 999). Kramer and Alberts (988) onluded, after a sixyear study, that tillage systems had no effet on grain yield of soybean. Soybean in exhibited ompensatory reprodutive growth to the early season plant development Si. Agri. (Piraiaba, Braz.), v.68, n.5, p.55456, September/Otober 20

5 558 Obalum et al. Table 2 Total water use of the soybean rop as omputed from the omponents of the water balane in the two years of the study. Final storage (S f i rop Treatment mm M M M M LSD( 0.05) ns, 30.0*, ns LSD( 0.05) ns, 28.3*, ns LSD ns, 27.5*, ns ( 0.05) The field reeived 836 and 923 mm of preipitation (P) between sowing and harvest in 2006 and 2007 respetively. S f and S i represent the total values on all the sampling dates from sowing to harvest; S f S i = ΔS. P ΔS D = ET rop (rop evapotranspiration). Notill; Conventional tillage; Unmulhed; M Mulhed. Given for tillage system, mulh pratie, and tillage x mulh in that order; ns stands for not signifiant at the hosen level of probability. Table 3 Grain yield and water use effiieny of soybean. Grain yield Water use effiieny Treatment Mg ha ha mm M M LSD( 0.05) ns, ns, ns ns, ns, ns LSD( 0.05) ns, ns, ns ns, 0.69*, ns LSD 0.05) ns, ns, ns ( ns, 0.48*, ns Notill; Conventional tillage; Unmulhed; M Mulhed. Given for tillage system, mulh pratie, and tillage mulh in that order. ns stands for not signifiant at the hosen level of probability. in, thereby minimizing yield differenes between the two tillage systems (Yusuf et al., 999). Growth attributes suh as plant height and number of leaves measured at three stages followed a similar trend as grain yield, i.e., omparable values among the treatments (data not shown). Si. Agri. (Piraiaba, Braz.), v.68, n.5, p.55456, September/Otober 20

6 Tillage and mulh praties in soybean in Nigeria 559 Although the tillageindued differenes in grain yields were not appreiable, outyielded by 6.2 % in the seond year whih was impliated with the unfavorable rainfall pattern. Singer et al. (2008), who reported inonsistent results over a threeyear period, equally observed that years with unfavorable rainfall favored. The result of the present study differs from the higher yield with reported for soybean by Lasisi and Aluko (2009) in southwestern Nigeria, the similarity in the soil textural attribute of the present loation to theirs notwithstanding. The disparity is attributed partly to the fat that, unlike the manual in this study whih rarely exeeded the top 0.2 m, Lasisi and Aluko (2009) used tratormounted implements to ahieve deeper and more intensive ploughing and harrowing. The disparity ould have also resulted partly from the differenes in loal soil and limati onditions. In spite of the higher mean seasonal water storage observed under the mulhed (M and M) ompared to the unmulhed (U and U) treatments (Figure 3), the grain yields were only marginally higher in the former than the latter (Table 3). This weak auseeffet relationship between soil moisture and grain yield was further highlighted in the ontrasting trends of the two parameters under the U and the U in the seond season and in the seasonweighted means. The inonsistent effets of the tillage systems on water ontents of the topsoil layers (data not shown), was presumably the reason why the effet of tillage methods on soil moisture ould not reflet on rop yield. Hakansson and von Polgar (984) observed similar situation and showed how suh differenes ould eliminate variations in yield responses on seedbeds. Some other authors equally indiated that hanges in soil water ontent in response to tillage were not of the magnitude to influene rop yield (Aboudrare et al., 2006; Anikwe and Ubohi, 2007; Tessier et al., 990). Rather than the soil moisture status, the effets on other key soil properties would help to explain the yield results. Organi matter ontent of the soil has, among a range of soil properties inluding P and K onentrations, been shown to have the greatest influene on grain yield of soybean (Kravhenko and Bullok, 2000). At the end of the present study, organi matter ontents as well as the strutural properties of the soil maintained same values under the various treatments (Obalum and Obi, 200). Similarly, available P was the only fertility parameter that differed (p 0.05) under the unmulhed plots (8.6 mg soil) and their mulhed ounterparts (2.9 mg soil). Soybean is a high Pdependent rop in the savanna region of Nigeria (Chiezey and Odunze, 2009; Kamara et al., 2007; Mahamood et al., 2009). So, the more favorable P status of the mulhed than the unmulhed treatments may likely be the reason for the numerially higher yields under the former than the latter. Variations in yields of the rop were more between the two years of the study than among the treatments. The grain yields were higher (p 0.00) in 2007 relative to 2006 (Table 3), in spite of the water defiit ondition that prevailed in 2007 during July when PET was appreiably below rainfall (Figure ). This was probably beause the water defiit between 36 and 54 DAS, took plae during the R2 (full bloom) stage, when rop water demand was lowest. Soil water defiit during this stage has similarly been reported not to ause grain yield deline of soybean (Deosthali et al., 2005; Foroud et al., 993; Karam et al., 2005). Moreover, the analysis of water stress based on MAI during the growing season (Figure 4) shows that the values were onsistently above the 75 % threshold for that stage (Srivastava et al., 996). The MAI data reveal instead that the rop atually experiened more yieldrelevant water stress in 2006 ompared with A perusal of the figure would show that the MAI was lower than 00 % during the R3R4 (pod development) stage (4680 DAS) in This orresponded to the period of normal short break in rainfall. Suh a situation has been found to ause appreiable yield deline (Srivastava et al., 996). Conversely, there was no suh senario during this ritial reprodutive stage and the maturity stages in the 2007 growing season. Although soybean yield is relatively insensitive to shortage of water during the vegetative growth, WU must not be less than the potential demand during the podfilling stage (Agele et al., 2004; FAO, 2002; Feak et al., 200; Foroud et al., 993; Rasiah and Kohl, 99). The view that a high MAI (> 60 %) would result in soybean yield redution did not apply in the present study beause the soil was moderately welldrained with fairly good aeration and therefore supports a good soybean growth under high MAI onditions (Deosthali et al., 2005). Moreover, the high MAI was shortlived. The present results suggest that, unless the drought and assoiated water stress to soybean does not oinide with the ritial stage of development when ET is supposed to be at optimum rate, the seasonal rainfall pattern may have Moisture Availability Index (Available water/pet) (%) Days after sowing 2007 Figure 4 Water stress index of the soybean rop during 2006 and 2007 growing seasons. ounted as from 3 July in 2006 and 7 June in Si. Agri. (Piraiaba, Braz.), v.68, n.5, p.55456, September/Otober 20

7 560 Obalum et al. strong influene on grain yield of the rop. Feak et al. (200) also reported, following a similar study, that limati ondition was the most dominant fator that influened the grain yield of soybean in Milhostov, Slovak Republi. Suh other fators as ropping history may also be partially impliated for the relatively poor yield in the first season. Soybean performs poorly in the first year of establishment on a plot not ropped to it for some three years, due to the absene of residual effet of nodulation (Kratohvil et al., 2006). Another possible fator is that the soil ph dropped from 6.6 before ropping in the first year (Table ) to 5.3 after the study. These ph values suggest a more favorable soil reation for the rop in the seond than in the first season, sine the desirable ph for optimum soybean yield has been shown to be in the range of 56 (FAO, 2002; Kratohvil et al., 2006). As with the WU, there were no tillageindued differenes in WUE in both growing seasons (Table 3). In the first season, mulh did not result in signifiantly higher WUE, despite the lower seasonal WU under the mulh plots. This observation suggests that the higher soil moisture status under mulh in suh a good rainfall season was not due to the evaporationreduing attribute of the mulh, neither was the lower WU with mulh due to a more benefiial onsumptive use of soil water under mulh. Instead, these benefits assoiated with mulh in the first season were oneivably due to better enouragement of infiltration of the trapped and transiently restrained rainwater by the mulh material (Adekalu et al., 2007). In ontrast, mulh (p 0.05) enhaned WUE in the seond year that was haraterized by errati rainfall during the growing season. Similar enhanement of WUE under mulh ompared to unmulhed plots was reported for hikpea in a semiarid environment (Rathore et al., 998). In the present study, the range of WUE values for the soybean rop in the seond season (when the yield was higher) was ha mm. This ompares favorably with the range ( ha mm ) reported for the rop under similar management and dryland onditions elsewhere (Norwood, 999; Varvel, 995). Conlusions The and the for the growing of soybean in this sandy loam Ultisol produed omparable agronomi effets, evident in the soil moisture status, onsumptive use of water and grain yield of the rop. Although mulh enhaned the soil moisture status and lowered the rop WU, it had no influene on grain yield. Hene, M and M onserved soil water better than U and U, but there were no differenes in WU, grain yield and WUE among these four tillage methods. Surfae mulh proved to be useful for enhaning soybean WUE espeially in a bad rainfall season. Besides, the mulh treatments (M and M) have the potential benefit of bequeathing the soil water onserved under them to the sueeding rop. Considering the relative requirements of the two tillage methods, it would be preferable to grow soybean with notill under mulh (M) ondition. 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