Evaluation of Sawah Rice Management System in an Inland Valley in Southeastern Nigeria. II: Changes in Soil Physical Properties ()
1. Introduction
The term sawah is defined as a leveled rice field surrounded by bunds with inlet and outlet connections to irrigation and drainage canals. It originated from MalayoIndonesian term ‘paddi’ which means rice plants. However, the term ‘paddy’ refers to rice grain with husks in the whole of West Africa. Wakatsuki et al. [1] therefore used the term sawah to distinguish between rice grain with husk, rice field and rice plant. Establishment of effective sawah management system for increased rice production in southeastern Nigeria involves the manipulation of certain soil physical properties in form of ecological engineering works. This manipulation of soil physical properties may involve deep earth movement and tillage to achieve a better topographic setting and optimal soil physical condition. Wakatsuki and Masunaga [2] remarked that ecological engineering of the inland watershed by the local people are required to increase agricultural productivity. These techniques according to them include leveling, bonding, and construction of canals and head dykes. Most soils in the West African sub-region are highly weathered and very fragile [3-7]. Mbagwu [4] reported that physical degradation of soils in the tropics resulted from soil erosion by water and mechanical land clearing using bulldozers. Lal [8] and Mbagwu et al. [9] showed that this degradation was manifested in high bulk density, low total and macro porosity, reduced water infiltration and transmission rate and low water retention and available water capacity within the root zone.
Rengasamy et al. [10] had earlier indicated that many soils used for irrigated or dry land agriculture are difficult to manage owing to their tendency to develop unsatisfactory structure particularly in their surface layers. Breakdown of aggregates leads to surface crusting, reduced water infiltration, restricted plant establishment and growth. The reason for the breakdown is normally as a result of slaking and dispersion of aggregates. These negative physical conditions of the soils added to poor nutrient status of such soils according to Mbagwu [4] resulted in poor crop-productivity and often abandonment of such lands leading to reduction in resource base of rural farmers.
Nnabude and Mbagwu [11] had used abandoned biological waste to improve the physical condition of some soils used in rice production in southeastern Nigeria. They insisted that application of rice mill waste on a Typic Haplustult in southeastern Nigeria resulted in significant improvement in bulk density, permanent water wilting point and total porosity. The use organic waste to restore the physical condition of soils solves two problems; one is the removal from the environment which they pollute and secondly supply of soil plant nutrient and eventual amelioration of soil physical properties. The use of biological wastes in the management of degraded soils or soils used for sawah rice management production is sustainable [12,13]. Most of the previous research where biological wastes were used for sustainable management of soils was mainly on upland and rainfed cultivation. None of these uses have been reported in sawah managed cropping system especially in southeastern Nigeria where these wastes are heaped with problems on the disposal. The objective of the study was therefore to 1) compare the influence of sawah and non sawah water managements on the physical characteristics of the soil, 2) to determine the contributions of the amendments on the soil physical properties and 3) the relationships among the soil physical properties.
2. Materials and Methods
2.1. Location and Field Study
The location of the study and the field design of this study are already given in the part I of this study [14]. The mineralogy of the soil is mainly kaolinite and the so-called interlayer minerals [15]. The major physical characteristics of the soil are shown (Table 1).
The field, which was under fallow for more than 5 years, was disk-ploughed and disk-harrowed to a depth of about 20 cm before puddling and treatments. The plot was divided into 2 portions, one part for sawah and the second part for non-sawah water management. In the non-sawah managed field, there was no defined water management and no bunding of plots in the field. Water was allowed to flow in and out as it comes but in the sawah field water was controlled and maintained to an approximate level of between 5 and 10 cm from two weeks after transplanting to the stage of ripening of the grains. In each of the plots the following treatments, arranged as a Split-Plot on a Randomized Complete Block Design (RCBD) were as shown (Table 1); each treatment was replicated 3 times and each plot was 6 m × 2.5 m. The NPK fertilizer consisted of 400 kg/ha as compound fertilizer, poultry dropping was applied at the rate of 5 tons/ha and Rice husk dust applied at 10 tons/ha. The RD on decomposition is widely applied by local farmers as source of plant nutrient. The nutrient contents of these organic amendments were determined. The mature PD and RD were spread on the plots that received them and incorporated manually into the top 20 cm soil depth 2 weeks before planting. All amendments were applied only in 2004 and their residual effect maintained for the 2 years.
The test crop was a high yielding rice variety Oryza sativa var. Tox 3108. This cultivar is widely used by farmers in the area. This was first planted in a nursery field and later transplanted to the main fields after 4 weeks in nursery. At maturity rice grains were harvested dried and yield computed at 90% dry matter content. This was done for the two years (2004 and 2005). At the end of each harvest soil samples were collected from each replicate of every plot for physical analyses.
2.2. Laboratory Methods
Particle size distribution of the less than 2-mm fine earth
Table 1. Some physical properties of the top soil (0-20 cm) before ploughing and amendment.
Table 2. Treatment combinations and their symbols.
fractions was measured by the hydrometer method as described by Gee and Bauder [16]. The clay obtained from particle size analysis with chemical dispersant is regarded as total clay (TC) and silt as total silt (TSilt), while clay and silt obtained after particle size analysis using deionised water only were the water-dispersible clay (WDC) and water-dispersible silt (WDSi). The soil organic carbon was determined by the Walkley and Black method described by [17]. Dispersion ratio which is an index of soil dispersion was calculated as:
Dispersion ratio (DR) = [(WDSi + WDC)/(TSilt + TC)] (1)
The higher the DR, the more the ability of the soil to disperse in water. The soil saturated hydraulic conductivity was measured using Klute and Dirksen method [18]. Soil bulk density was measured by the core method [19]. Total porosity (Tp) was obtained from bulk density (ρb) values with assumed particle density (ρs) of 2.65 Mg/m3 as follows
Porosity = Tp = 100(1 – ρb/ρs)(2)
The soil moisture contents at 0.1 and 1.5 MPa suction were determined by Klute [18] method while the available water capacity was calculated as the difference between moisture retention at 0.1 and 1.5 MPa [i.e. field capacity (FC) and permanent wilting point (PWP)].
The method of Kemper and Rosenau [20] was used to separate the water-stable aggregates (WSA). In this method 40 g of < 4.75 mm air-dried soils were put in the topmost of a nest of four sieves of 2.00, 1.00, 0.50, and 0.25 mm mesh size and pre-soaked for 30 min in deionized water. Thereafter the nest of sieves and its contents were oscillated vertically in water 20 times using 4 cm amplitude at the rate of one oscillation per s. After wet-sieving, the resistant soil materials on each sieve and the unstable (< 0.25 mm) aggregates were quantitatively transferred into beakers, dried in the oven until steady weight is achieved. The percentage ratio of the aggregates in each sieve represents the water-stable aggregates (WSA) of size classes; > 2.00, 2.00-1.00, 1.00-0.50, 0.50-0.25 and < 0.25 mm. Aggregate stability was measured as the mean-weight diameter (MWD) of stable aggregates as equation
MWD = ΣXiWi (3)
where Xi is the mean diameter of the ith sieve size and Wi is the proportion of the total aggregates in the ith fraction. The higher the MWD values, the higher proportion of macroaggregates in the sample and therefore better stability.
2.3. Data Analyses
An analysis of variance of each soil properties between water management systems and amendments was performed on the soil data generated from the laboratory. The differences among the mean values were tested with the LSD. Also correlation coefficients of the relationships between some of the soil properties were determined using the SPSS.10 computer package.
3. Results and Discussion
3.1. The Influence of Water Managements and Amendments on Soil Bulk Density and Total Porosity
During the first year of planting the bulk density was between 1.2 Mg/m3 to 1.46 Mg/m3 in the non sawah water management system and 1.19 to 1.46 Mg/m3 in the sawah system (Table 3). The results indicated that there was a significant difference within the bulk density with amendments. Also the mean bulk density of soils in the sawah system was significantly lower than the corresponding mean bulk density of the non sawah system. Higher bulk density according to Mbagwu et al. [9] signified compaction and undesirable soil structure that affects roots and plant growth negatively. Again, the same trend as was shown for bulk density in the first year was also indicated in the second year of planting. Bulk density varied significantly with amendments while a significant lower bulk density was obtained from the sawah system than the non sawah system. In all cases whether in sawah or non sawah management, rice husk dust reduced the mean bulk density of the soil. Nnabude and Mbagwu [11] showed that rice waste, either burnt or fresh condition could be effective in the improvement of soil properties. The importance of lower bulk density in the soil as portrayed by the sawah managed plots is the improvement of soil aeration, tilt and better water infiltration in addition to unreserved root penetration.
The total porosity also followed the trend in the soil bulk density (Table 3). While total porosity differed significantly with soil amendments in both first and second year of planting, it also differed significantly with water managements. In both years total porosity were always significantly higher in sawah managed system than in non sawah managed system (Table 3). The results here also showed the beneficial contribution of the organic amendments in improving the soil total porosity. Furthermore, sawah managed system could provide management strategies as to the improvement of soils liable to compaction and other negative physical properties when puddle for rice production.
3.2. The Influence of Water Managements and Amendments on Moisture Content at Field Capacity (FC) and Wilting Point (WP)
While amendments showed no significant differences with moisture content at field capacity (FC) in the first year, there was non significant difference in the FC values in the same year (Table 4). However, the value of FC in sawah system is higher when compared to non sawah. Also in the second year the FC did not differ sig nificantly with amendments and with water managements. Again the trend showed that although non significant, relatively higher value of FC was obtained in sawah than in non sawah managed plot. The inference that could be drawn from this is that sawah managed plots may hold water more at the level of field capacity than the non sawah managed. This hypothesis may be exploited in the restoration of these soils occurring within the inland valleys of the agro ecological zone in the area of water management for sustainable production.
In the first year just like the FC, the moisture content at wilting point (WP) was significant with amendments but not with water management. However, the trend was that higher average value was obtained in the sawah managed plots more than the non sawah managed plot (Table 4). In the second year of planting, it was significant both for the amendment and water management. In most cases the amendments improved the moisture content at WP while sawah water management improved significantly the WP (Table 4). This result further confirms the superiority of sawah in soil moisture reserve over non sawah. In these soils which discharge its moisture contents very quickly, it will be an advantage that with sawah practice, more moisture may be reserved at WP, than other practices.
3.3. The Influence of Water Managements and Amendments on Soil Water-Stable Aggregates and Mean-Weight Diameter
Table 5 presents Water-stable aggregate (WSA) sizes > 2.00 mm and < 0.25 mm. These two aggregate sizes were chosen because of their extreme vales and sizes. While the WSA > 2.00 mm were the large aggregates, the < 0.25 mm are the smallest aggregate sizes. The WSA > 2.00 mm are not significant with neither amendment nor