Relationship between Aggregate Stability Indices of Four Contrasting Textural Classes of Soils as Influenced by different Periods of Soaking

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1 ISSN: Submitted: 25/10/2017 Accepted: 03/11/2017 Published: 13/11/2017 DOI: Relationship between Aggregate Stability Indices of Four Contrasting Textural Classes of Soils as Influenced by different Periods of Soaking By Nweke I.A. Ijeh A.C.

2 Research Article (DOI: Relationship between Aggregate Stability Indices of Four Contrasting Textural Classes of Soils as Influenced by different Periods of Soaking Nweke I.A.* 1 and Ijeh A.C. 2 1 Department of Soil Science Chukwuemeka Odumegwu Ojukwu University, Anambra State. 2 Nwafor Orizu College of Education Nsugbe Anambra State. ABSTRACT *Corresponding Author s The ability of soil aggregates to resist the forces of water and/or wind, the potential to degrade, crust and/or seal is referred to aggregate stability. Little is known about the effect of soil texture and period of soaking on aggregate stability for cultivated soil of four major textural classes in southeast, Nigeria which was the objective of our study. Soil samples representing a range of agricultural soils of four contrasting textural classes of sandy loam; sandy clay loam; clay loam and loam under five different period of soaking namely; 0, 30, 60, 90 and 120 minutes were studied using wet and dry sieve techniques. The indices evaluated include; mean weight diameter wet and dry, water stable aggregates >2.00mm (WSA1); water stable aggregates 2.00mm 1.00mm (WSA2); water stable aggregates 1.00mm 0.5mm (WSA3); 0.5mm 0.25mm (WSA4); < 0.25 mm (WSA5). Results generated from the study showed that the studied soil types are poor in organic carbon (OC), organic matter and sodium (Na) contents. Their cation exchange capacity (CEC) values are of moderate values of which ranged from cmolkg -1. The relationship study indicated that WSA1 correlated significantly and positively with WSA2 and mean weight diameter wet (MWDW), but negatively with WSA4 and WSA5 with r values of and respectively. WSA2 was observed to have significant positive correlation with WSA3, WSA4 and MWDW. WSA3 and WSA4 did not correlate significantly with MWDW but has positive correlation with each other with r value of WSA5 was found to correlate significantly but negatively withwsa1 WSA5 (2.00mm 0.25mm). Mean weight diameter of wet aggregates correlated positively and significantly with degree of aggregation (DA) and state of aggregation (SA) at minutes, this result was equally true for DA and SA correlation result at times studied. The correlation matrix of mean weight diameter of dry aggregates (MWDD) with MWDW, DA and SA indicated negative correlation in all the time studied, except at 30 minutes where it showed positive correlation with DA with r value of and at 90 minutes were it was not significant with DA and SA. The correlation between aggregate stability indices studied and Na, CEC at various periods of soaking was not significant and OM content was low. Though not statistically significant, hydraulic conductivity (HC) and bulk density (BD) showed positive correlation with MWDW, DA and SA in all the soaking period while field capacity (FC) and total porosity (TP) showed negative correlation but not statistically significant with MWDW, DA and SA, but has positive correlation with MWDD. The findings from this study is of evidence that the period of soaking had some contributions to the stability of soil aggregates and soil properties especially with the trend in the correlation matrix between MWDD and MWDW, DA and SA. Keywords: Aggregate stability, water stable aggregates, aggregate sizes, soaking period. INTRODUCTION The structural stability of soils is an index of aggregate stability. It tend to explain the ability of the soil to maintain its arrangement of aggregates and pore space when exposed to external force such as wind, cultivation, wetting etc. Hence good soil structure or its resilience is considered the ability of a soil to recover its structural form or pore space arrangement through natural process after the removal of stress (force) such as intense rain fall impact and abrupt changes in soil moisture, compaction as well as the ability to resist the effect of cultivation. This is because any change in the structural stability of soil has potential effect on root-soil-air-water relationships and the ability of soils to sustain long-term crop production. The structural stability of soils according to the following authors; Bazzoffi and Mbagwu (1986); Lal (1991); Neves et al. (2003); Kay and Munkholm (2004) plays very influential role in seedling emergence and root development and growth, water retention, infiltration rates, hydraulic conductivity, solute movements and attendants problem, soil porosity, heat transfer processes and gaseous exchange, reduction in potential of soils to erode, environmental quality and agricultural sustainability. 36

3 Many researchers due to the problem of standardization have adopted many methods for assessing soil structural stability. Matkin and Smart (1987) found wet-sieving to be generally applicable to all soils while Diaz-Zorita et al. (2002); Nimmo and Perkins (2002) found aggregate stability and wet-sieving fragmentation of soil matrix and estimation of the distribution of soil aggregate as common method used for assessing soil structure stability. Dry and wet-sieving methods are also widely used by soil scientists for different soil types and land use systems for direct structural assessment (Kemper and Chepil, 1965; Burkke et al., 1986; Nweke and Nnabude 2014; 2015ab). Apart from the aforementioned methods many researches like Matkin and Smart (1987); Chisci et al. (1989); Piccolo and Mbagwu (1994) and Adesodun et al. (2001) have also used various time of soaking in their structural aggregate stability assessment using the wet-sieving technique and the highest time so far used for soaking before sieving according to literature is 30 minutes but precipitation sometimes could be upward of 1 hour, 2 hours and even more in a tropical environment. In south eastern Nigeria some fields could hold water for hours before the water drains and this will have effect on the stability of the aggregates in the field. To stimulate these field conditions four different soil textural classes were selected namely; sandy loam, sandy clay loam, clay and loam and different periods of soaking were used to study the relationship between aggregate stability indices of four textural classes of soils as this has the potential of influencing the productivity of the soils and crop production in the area. MATERIALS AND METHODS Site description and sampling The sandy clay loam (SCL) texture was collected from the University of Nigeria, Nsukka Agricultural Farm. Nsukka is located on Latitude N and Longitude E within the derived savanna zone of eastern Nigeria. It has an average elevation of 447m above sea level and two seasons-rainy and dry seasons. The rainy season last from April to October with a short break in the month of August with an average annual rainfall of about 1550mm and more than 85% of this fall within the rainy season, the temperature of the area ranges from 22 o C 30 o C while the relative humidity is rarely below 60% (Asadu et al., 2002). Generally the soils of the area have high percentage of sand and granular structure at the top and the topsoil is characterized by rapid to very rapid permeability. The sandy loam (SL) texture was collected from Umuzu near Ugwuakukuo River in Lilu, Ihiala local government area of Anambra sate. Lilu is located on Latitude N and Longitude E, with average annual rainfall of about 1500mm-2000mm. The clay loam (CL) texture was collected from Anambra lower river basin (Lower Anambra Irrigation Project Field) at Omor Oyi local government area Anambra state. The soil samples were taken from experimental plot Turnout W4 and Turn-out W8 (CRW4). The irrigation project field is located on N and Longitude E and situated in the southern corner of the Do-Anambra River. According to Asadu (1992) the entire area is generally gently undulating sloping mostly from the North to South and the gradient is on the average approximately 1in 300. Loam (L) texture soil was collected from two locations, the experimental farm W7 at Anambra lower river basin and at Amamputu in Uli across Ogada River in Ihiala local government area which lies on the Latitude N and Longitude E. In each of these locations soil samples were taken with a spade at the depth of 0-20cm. The soil was carefully lifted and placed in polyethylene bags. Three samples were collected from each soil textural class and each served as a replicate. Three undisturbed soil core samples per textural class were taken with the help of core samplers. The soil core samples collected were used to determine the hydraulic conductivity (HC), bulk density (BD) and pore size distribution and total porosity (TP). The physical and chemical properties of the soils used were given in Table 1. LABORATORY METHOD Chemical Properties The organic carbon was determined using the Walkley and Black (1934) method. The exchangeable sodium was measured from ammonium acetate leachate using the flame photometer while the cation exchange capacity (CEC) of the soils was obtained by the extraction method of Jackson (1958). 37

4 Physical properties Particle size analysis was done by the Bouyoucos (1951) hydrometer method. Bulk density was determined by the core method described by the Blake (1986). Pore size distribution was determined using the water retention method and total porosity was estimated from bulk density and particle density thus TP = 1- BD x 100 PD Where BD = Bulk density, PD = Particle density Hydraulic conductivity was determined by the modified constant water head methods of Klute (1986) thus K = Q x dz At x dh x hr Where K= saturated hydraulic conductivity cm per second Q = the steady state volume flow from entire volume cm3/hr dz = length of core sampler (cm) A = cross section area (cm) t = change in time interval (hr) dh = hydraulic head change (cm) Core samples of length 5cm and 5.6 cm diameters were used with hydraulic head changes of 2.5 cm. Aggregate Stability This was determined using wet and dry sieving method of Yoder (1936). The technique involved the use of sieves with diameters 2, 1, 0.5 and 0.25mm. The time of soaking/pre-soaking selected were; 0 minutes, 30 minutes, 60 minutes, 90 minutes and 120 minutes, the textural class evaluated were sandy loam, sandy clay loam, clay loam and loam. Wet sieve aggregate 40g of soil samples (< 4.75mm) from each of the textures was placed on the topmost sieve of the nest of sieves and presoaked at different periods and then oscillated for 2 minutes. The samples remaining in each of the sieves were collected dried and weighed. After weighing and recording the soil samples they were bulked together and soaked for 24 hours in NaOH. The sand was then washed through a 0.5 mm sieve in to an evaporating dish. The water was decanted and the sand oven dried for at least 24 hours and weighed. This was repeated with the 3 replications of the textures used. The following aggregate stability indices were determined: State of aggregation (SA) SA = wt of water stable aggregate wt of sand x 100 wt of sample Degree of aggregation (DA) DA = wt of water stable aggregate wt of sand x 100 wt of sample wt of sand Water stable aggregates (WSA) WSA 1 (> 2.0 mm) = wt of aggregates on 2.0 mm sieve x 100 WSA 2 (2.0 mm 1.0 mm) = wt of aggregates on 1.0 mm sieve x

5 WSA 3 (1.0 mm 0.5mm) = wt of aggregates on 0.5 mm sieve x 100 WSA 4 (0.5mm 0.25 mm) = wt of aggregates on 0.25 mm sieve x 100 WSA 5 (< 0.25 mm) = wt of aggregates that passed through the 0.25 mm sieve x 100 Mean Weight Diameter wet aggregates (MWDW) MWDW = xiwi Where xi = the sum of the product of the means diameter of each size fraction Wi = proportion of the total sample weight wi of each size fraction MWDW = Mean weight diameter of wet aggregates (mm) Dry sieve aggregate (DSA) The dry sieving was done using the FRITSCH laboragetateban analysette (Germany) to shake the 40g soil sample placed on top of a set of sieves, arranged in their descending order of 2.00 mm, 1.00 mm, and 0.5 mm, 0.25 mm and dry stable aggregates DSA1to DSA5 were obtained with the formula stated below; DSA1 (> 2.00 mm) = wt of dry aggregates on 2.00 mm sieve x 100 DSA 2 (2.00 mm mm) = wt of dry aggregates on 1.00 mm sieve x 100 DSA 3 (1.00 mm 0.5 mm) = wt of dry aggregates on 0.5 mm sieve x 100 DSA 4 (0.5 mm 0.25 mm) = wt of dry aggregates on 0.25 mm sieve x 100 DSA 5 (< 0.25 mm) = wt of dry aggregates passed on through 0.25 mm sieve x 100 Dry mean weight diameter (MWDD) = xiwi Where n = total number of size xi = the arithmetic mean diameter of the i = 1 i = sieve opening (mm) w = the proportion of the total weight (uncorrected or sand and gravel) occurring in each size grade Data analysis Comparison of data generated from wet sieve and dry sieve techniques at various soaking/pre-soaking periods with the four selected soil textures and also relating the aggregate stability indices to selected soil properties were done by multiple correlation analysis as outlined in SPSS 16 software statistical package for social sciences. RESULTS AND DISCUSSION Physical and chemical properties of the soils used for the study The result of physical and chemical properties of the soil used for the study showed that the values of bulk density (BD), total porosity (TP), hydraulic conductivity (HC) and field capacity (FC) of the four (4) soils were of the range 1.20gcm gcm -3 ; %; 1.63 cmhr cmhr -1 and % respectively. The value of soil bulk density recorded in sandy clay loam simple suggest soil too compact for rapid root growth because soil bulk density values of 1.55 mgm mgm -3 according to Evanylo and McGuinn (2000) adversely affect root growth and development. Also high BD decrease soil pore values and water availability in soil hence less TP value recorded in sandy clay loam while soil porosity reduces runoff and soil erosion and enhance soil moisture retentions. The lower value of 1.63 cmhr -1 of HC recorded in clay loam can only suggest of soil in which water transmission and nutrient 39

6 recycling will be difficult to achieve. Field capacity have been noted to vary with soil types and the higher the clay content the higher the FC this probable influenced the value recorded in loam and clay loam soil types. MD and MP are micro and macro pores respectively their values vary with the soil types. Soil pores are inevitable soil gabs in packing of soil particles, disturbance due to plant roots, soil animals, cracking/swelling or tillage. Macro-pores (MP) are used for air and water conduction and this tend to be more effective in sandy loam soil type and least in clay loam soil type compared to the other soil types studied. Micro-pores (MD) are used for water retention and storage and from the values obtained sandy loam was observed to be least effective compared to the other soil types. Generally the recorded values of organic carbon (OC), organic matter (OM) and sodium (Na) of the soils were low indicating that the studied soils are poor and deficient in those parameters. The cation exchange capacity (CEC) of the soils varied from 4.67 cmolkg cmolkg -1 and they are of moderate values. Table 1: Physical and chemical properties of the soil before the beginning of the study Texture BD TP FC MD MP HC OC OM CEC Exch Na gcm -3 % % % % cmhr -1 % % cmolkg -1 cmolkg -1 Loam Sandy loam Clay loam Sandy clay loam BD= bulk density; FC = Field capacity; TP = Total porosity; MD = Micro-pore; MP = Macro-pore; HC = Hydraulic conductivity; OC = Organic carbon; OM = Organic matter; CEC = Cation exchange capacity; Na = Exchangeable Sodium Relationship between aggregate stability indices of water stable aggregates and mean weight diameter The result presented in Table 2 showed that WSA1 (> 2.00mm) correlated significantly and positively with WSA2 ( mm) and MWDW with r values of and respectively. Also WSA1 (>2.00mm) correlated significantly but negatively with WSA4, WSA5 with r values of and respectively. The correlation with WSA3 was not significant probable because of the differences in the size of the aggregates. WSA2 correlated significantly and positively with WSA3, WSA4 and MWDW and correlated negatively with WSA5 with r values of 0.542, 0.347, and respectively. Water stable aggregates (WSA3) correlated positively and significantly with WSA4 with r value of though it was not significantly correlated with MWDW, it however correlated significantly but negatively with WSA5 with r value of Water stable aggregates 4 (WSA4) did not correlate significantly with MWDW while WSA5 correlated significantly but negatively with WSA1 to WSA4 (ie mm) and MWDW with r values of 0.695, , , , respectively. This particular trend agrees with the findings of Mbagwu (1992) who found that the WSA < 0.25 mm correlated negatively but significantly with all the macro tests of stability. The results generally may be attributed to the organic carbon content of the textures as water stability of micro aggregates depend on OC binding agents a characteristics of soil independent of the land use management and the extent of tillage. Denef et al and Eynard et al observed that the structural stability of aggregates decrease with OC content and cultivation. Soil structure and aggregates according to Holeplass et al and Khurshid et al are strongly influenced by cultivation related practices. The nature of results obtained might as well be due to differences in the size of the aggregates and clay content of the textures. Aggregates of different sizes have different stability with the highest stability of the lower hierarchical order (Dexter 1988; Amezketa 1999) While Nweke and Nnabude (2015a) observed that the aggregate stability of four Nigerian soils were proportionally increased on the basis of their clay contents. Table 2: Correlation coefficient for the linear relationship between aggregate stability indices of water stable aggregates (WSA) and mean weight diameter (MWD) WSA1 WSA2 WSA3 WSA4 WSA5 MWDW WSA1 WSA ** WSA ** WSA * 0.347** 0.794** WSA ** ** ** ** MWDW 0.976** 0.565** NS NS ** = highly significant; * = significant; ** P < 0.01; * P < 0.05; NS = Not significant at P <

7 Relationship between aggregate stability indices and different time of soaking The correlation coefficient for the linear relationship between aggregate stability indices at different periods of soaking is presented in Table 3. The result showed that at 0 minutes the mean weight diameter wet aggregate (MWDW) correlated positively and significantly with degree of aggregation (DA) and state of aggregation (SA) with respective coefficient r of and Degree of aggregation correlated positively and significantly with state of aggregation with a coefficient r of At time 30 minutes the MWDW correlated positively and significantly with DA and SA with r values of and respectively. DA also correlated positively and significantly with SA with r value of The same trend of result was observed at time 60 minutes, 90 minutes and 120 minutes. The correlation matrixes of Mean weight diameter dry (MWDD) with MWDW, DA and SA showed a negative correlation from time 0 to 120 minutes except at 30 minutes for DA with r value of The result at time 0 minutes showed that MWDD correlated significantly at P < 0.05 with MWDW and DA but not significantly with SA. At time 30 minutes the correlation was not significant with MWDW, but positively and negatively correlated with DA and SA respectively at P < At 60 minutes MWDD showed non-significant correlation with MWDW, DA and SA, while at 90 minutes it was significant with MWDW but not significant with DA and SA. The result of 120 minutes MWDD showed significant correlation at P < 0.05 with MWDW, DA and SA. The scenario of these results clearly showed that the time of soaking contribute strongly to the stability of soils, especially with the trend in the correlation matrix between MWD wet and MWD dry. The development of structural stable aggregates is very important to ameliorate hard setting behavior that imposes restrictions to cultivation and plant growth (Mullins et al., 1990). According to the following authors; Anabi et al. (2008); Madari et al. (2005) and Denef et al. (2004) soil aggregation affect seedling emergence, root development and growth, moisture retention and the dynamics of organic carbon sequestration. Also soil texture and OM accumulation have been observed to influence soil aggregation (Frankel et al., 1978; Nweke, 2015c). Table 3: Correlation coefficient for the linear relationship between MWDW, DA, SA and MWDD at time 0 to 120 minutes Parameter MWDW DA SA MWDD At time 0 minutes MWDW - DA 0.929** - SA 0.863** 0.976** - MWDD * * NS - At time 30 minutes MWDW - DA 0.938** - SA 0.880** 0.972** - MWDD NS 0.639* * - At time 60 minutes MWDW - DA 0.832** - SA 0.864** 0.769** - MWDD NS NS NS - At time 90 minutes MWDW - DA 0.905** - SA 0.860** 0.986** - MWDD * NS NS - At time 120 minutes MWDW - DA 0.951** - SA 0.922** 0.982** - MWDD * * * - MWDW= mean weight diameter wet; MWDD = mean weight diameter dry; DA = degree of aggregation; SA = state of aggregation; ** significant at P < 0.001; * significant at P <

8 Relationship between aggregate stability indices and soil properties The result presented in Table 4 shows the correlation coefficient for the linear relationships between aggregate stability indices and soil properties. The correlation of Na which is a soil aggregate dispersing agent, with MWDW, DA, SA and MWDD at various times of soaking was not significant with the exception of SA index that was significant (P < 0.05) at 0 minutes period of soaking. Generally the result showed that as Na increased the aggregate stability decreased probable due to mineral weathering. The trend of the results from the period 0 to 120 minutes shows that there are differences in r values which decreased as the period of soaking increased while that of MWDD correlated positively and the figures obtained remained the same trend on all soil properties from period 0 to 120 minutes. This is also an indication of the effect of time of soaking on soil aggregate stability and probable due to OM disintegration and dissociation of minerals adsorbed in the clay fraction and their subsequent removal through water solution. The fact that the correlation between the aggregates stability indices and SOM content is low can be explained by the fact that at the most fundamental level of aggregation the organic carbon can act as either an aggregating or disaggregating agent depending on the relative contribution of the other aggregating agents like clay and divalent cations (Mbagwu 1992). Grieve (1980) and Molope et al., (1985), however reported highly significant correlation between organic matter and aggregate stability. Though they worked with soils that varied in OC contents from 5.1 to 11.5%, but the soils used in this study were rather low in OC contents (0.12 to 0.80 %). Bazzoffi and Mbagwu (1986) obtained low and barely significant correlation between WSA and percent OC (0.53 to 1.7%) asserting that it would appear that for these soils the critical level above which OM begins to make a significant contribution to aggregate stability had not been reached. The result could also be attributed to the nature of microbes, clay and OM contents, Fe and Al contents of these soil textural classes. For the work of Miquel and Norton (1994) showed that aggregate stability is mostly an index of OM, clay, Fe and Al oxide contents of the soil Fe and Al oxides were found to be correlated with aggregate stability (Panayiotopoulos and Kostopoulous 1989). The correlation of CEC with MWDW, DA, SA and MWDD at various periods of soaking was not statistically significant probable due to higher amount of the hydrated cations. Igwe et al. (1999) observed that soils with high CEC tend to be unstable and could reflect the influence of their mineralogical composition on soil structural stability. Also when a soil aggregate is immersed in water and then agitated the polyvalent metals forming the bridges between the clay particles and OM are dispersed by Na leading to dispersion (Mbagwu et al., 1993). The results obtained is of evidence that there are changes in the correlation between CEC and MWD, DA and SA at various periods of soaking which shows that stability was affected at various times of soaking. Frequent rainfall and leaching as always observed in soils of south east Nigeria can lead to dispersion and weak aggregation especially when divalent cations like Ca and Mg are leached. Hydraulic conductivity (HC) had positive correlation with MWDW, DA and SA in the all the soaking periods though not statistically significant. This showed that as the HC increased these indices MWD, DA and SA increased. HC correlated significantly but negatively with MWDD with r value of in all the period of soaking. The bulk density (BD) correlated positively though not significantly with MWDW, DA and SA. The field capacity (FC) had negative correlation, though not statistically significant with MWDW, DA and SA implying that when FC increases the indices decreases and vice versa while the correlation between FC and MWDD was highly significantly positive with r value of Total porosity (TP) had non significant negative correlation at almost all the periods of soaking with MWDW, DA and SA but correlated significantly (P < 0.05) and positively with MWDD with r value of The micro porosity (MD) correlated positively though not statistically significant with MWDW, DA and SA in all the periods of soaking and also with MWDD. The macro porosity (MP) correlated positively though not statistically significant, with MWDW, DA and SA but correlated negatively and not statistically significant with MWDD with r value of

9 Table 4: Correlation coefficient for the linear relationship between aggregate stability indices and soil properties Parameter BD FC TP MD MP HC 0C CEC Exch. Na gcm -3 % % % % cmhr -1 % cmolkg -1 cmolkg -1 At time 0 minutes MWDW DA SA * MWDD ** 0.586* * At time 30 minutes MWDW DA SA MWDD ** 0.586* * At time 60 minutes MWDW DA SA MWDD ** 0.586* * At time 90 minutes MWDW DA SA MWDD ** 0.586* * At time 120 minutes MWDW DA SA MWDD ** 0.586* * ** Significant at P < 0.01; * significant at P < 0.05; BD = bulk density; FC = field capacity; TP = total porosity; MD = micro porosity; MP = macro porosity; HC = hydraulic conductivity CONCLUSION The findings from this study have revealed that periods of soaking have effects on the stability of soil aggregates. The MWDW of the clay loam was found to be significantly low under wet sieving among all the soil textural class studied. The correlation of MWDD with MWDW, DA and SA was negative at time 0 minutes, 30 minutes, 60 minutes, 90 minutes and 120 minutes. There was a significant correlation between MWDD, MWDW and DA at time 0 minutes. At time 90 minutes MWDD correlated significantly (P < 0.05) with MWDW and at 120 minutes MWDD also correlated significantly (P < 0.05) MWDW, DA and SA. All the aggregate stability indices used in the study to assess aggregate stability both in wet and dry sieving technique, texture was found to be a significant controlling factor in both the formation and stability of soil aggregation. REFERENCES Adesondun JK, Mbagwu JSC, Oti N, (2001). Structural and carbohydrate contents of an ultisol under different management systems, Soils and Tillage Research 60: Amezketa E, (1999). Soil aggregate stability: A review, J. Sustainable Agric. 14: Anabi M, Houot S, Fracou C, Poitrenandi M, Bissonnais YL, (2008). Soil aggregate stability improvement with urban compost of different materials, Soil Sci. Soc. Am. J. 71:

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