Soil puddling for rice production under glasshouse conditions its quantification and effect on soil physical properties

Transcription

1 CSIRO PUBLISHING Australian Journal of Soil Research, 25, 43, Soil puddling for rice production under glasshouse conditions its quantification and effect on soil physical properties G. Kirchhof A,B and H. B. So A A The University of Queensland, School of Land and Food Sciences, Brisbane, Qld 472, Australia. B Corresponding author. g.kirchhof1@uq.edu.au Abstract. The effect of soil puddling on soil physical properties of 3 different textured soils (clay, loam, and silty loam) and growth of rice (Oryza sativa) on these soils was investigated under glasshouse conditions. Puddling intensity was expressed as the ratio of soil volume subjected to the puddling implement and the total soil volume in the puddled layer, thus integrating the effects of speed and time of the puddling operation. This parameter was well related to soil dispersion, bulk density, and saturated hydraulic conductivity. However, following prolonged periods of submerged conditions during rice growth, saturated hydraulic conductivity decreased with a decrease in soil dispersion, in contrast to an expected reduction in saturated hydraulic conductivity with increased dispersion. There was indication that continuous waterlogging reduced the effect of soil puddling, in particular on heavy-textured soils. Additional keywords: rice, tillage, soil puddling, soil structure, dispersion, hydraulic conductivity. Introduction Management of clay soil in rice-based cropping rotation must deal with opposing soil physical requirements for the rice crop and the subsequent dry season crop. Rice (Oryza sativa) needs impermeable or puddled soil to maintain submerged condition. However, the puddled surface layer becomes very hard during drying after the wet season, which is not conducive to seedling establishment. Root growth of the dry season crop [e.g. legumes such as soybean (Glycine max), mungbean (Vigna radiata), or groundnut (Arachis hypogaea) and cereal such as wheat (Triticum aestivum)] is restricted and soil water stored in the subsoil may not be available to post rice crops. Wet cultivation or soil puddling is the common soil preparation technique used for lowland rice production. The puddled soil is soft and assists in transplanting of rice seedlings. Reduced hydraulic conductivity from puddling is necessary to decrease water and nutrient losses. Weeds are easily controlled through the continuous submerged conditions of the rice cropping cycle (Sharma and De Datta 1985). Puddling breaks down, smears, and disperses soil aggregates. The degree with which this process happens is likely to be affected by the soil type and structure, and the intensity with which puddling is carried out. Soil puddling is generally regarded as beneficial for rice production; however, it is likely to result in adverse soil conditions for dry season crops when the puddled layer dries out after the rice season (Pasaribu and McIntosh 1985). This paper describes a method to quantify puddling intensity and reports on the effect of puddling on soil dispersion and hydraulic conductivity/percolation rates as well as changes in soil physical properties during drying of the puddled layer. Materials and methods Evaluation of puddling intensity on soil cores The laboratory puddling apparatus used steel rods as puddling implements, attached to a disk driven by a controlled constant speed DC-motor (Fig. 1). Puddling intensity is influenced by a number of factors including (i) the duration of puddling, (ii) the speed with which the puddling implement is moved through the soil, and (iii) the number and shape of the implements used. In this paper, puddling intensity is described as the ratio of soil that was exposed directly to the puddling implements to the soil volume in the puddled layer and is called the puddling ratio. If a cylindrical volume of soil is puddled to the height of the cylinder the puddling ratio is the volume inside the cylinder subjected to the puddling implement and the cylinder volume, i.e. the puddling ratio reduces to the area of soil exposed to the implement to the total soil area puddled. It was calculated as followed: P = 2πw pt ω m n=1 r p n (1) A s where P is puddling ratio, w p is diameter of steel rod (puddling implement), t is time of puddling, ω is angular velocity of disk, m is total number of steel rods, r pn is distance from disk centre to steel rod n, and A s is areas of the puddled soil. The puddling ratio ranges from for an unpuddled soil to infinity for a soil that was completely puddled. It incorporates the combined effects of duration and speed of puddling. Although it does not include parameters relating to shape of implement und thus efficiency or energy CSIRO /SR /5/5617

2 618 Australian Journal of Soil Research G. Kirchhof and H. B. So Fig. 1. Schematic diagram of the puddling implement. used for puddling, we consider this simple puddling ratio a suitable first approximation to quantify soil puddling. Four different puddling ratios were used in the laboratory:.4, 1, 2.3, and 2. They were derived from a combination of 3 different angular velocities, ω (2, 5, and 125 rounds per min), different number of rods, m (1, 2, and 4), and puddling times t (2 262 s). Rod width, w p was 6.4 mm. Rods were attached to the rotating disk at 8, 15, 22.5, or 3 mm from its centre (Fig. 1). Three different-textured soils (clay, loam, and silty loam) were puddled after air-dry (>2 mm) soil was placed into 75-mm-diameter and 6-mm-high cores and wetted to saturation. Puddling ratio was related to saturated hydraulic conductivity and the amount of dispersible clay and clay plus silt content. Bulk densities were calculated from saturated soil water content after puddling, assuming zero air-filled porosity and a particle density of 2.65 g/cm 3. Evaluation of soil puddling in mini rice beds Design of the mini rice beds Twelve mini rice beds were constructed of steel reinforced concrete boxes (Fig. 2). Each weighed about 1 kg. The mini rice beds had a wall thickness of 75 mm. Internal dimensions were 1.2 m long,.92 m wide, and.78 m deep. A drainage hole was located in the centre of the front wall. The base had a slope towards the drainage hole. It was 1 mm higher than the drainage hole (5 mm diameter) in the centre at the back of the base and at the front left and right, and.2 m higher than drainage hole in the back left and right corners. Inner walls were coated with cement glue and bitumen emulsion for waterproofing. Prior to packing the mini rice beds with soil, a 3-mm-thick layer of coarse sand was spread in the base of each mini rice bed to facilitate drainage. The drainage hole was blocked with gravel and the gravel covered with a small sheet of PVC (.2 by.2 m). The PVC sheet was attached to the wall. It served to prevent mixing of soil, sand, and gravel that could result in erosion near the drainage outlet. Instrumentation of the mini rice beds Several access holes were located in the sidewalls of the mini rice beds. They were installed into the mini rice beds by placing PVC tubes Fig. 2. soil. Diagram of mini-rice beds with instruments prior to filling with inside the reinforcing steel mesh before the concrete was poured into the mould (Fig. 2). The mini rice beds were packed with 3 different soil types: a grey clay from rice-growing areas in Griffith NSW (clay), an alluvial hardsetting silty soil from Moggill near the Brisbane River (silty loam), and a loamy soil from the Lockyer Valley (loam). Using the Australian Soil Classification System (Isbell 1996), these soils corresponded to a Grey Vertosol, Brown Kandosol, and Brown Dermosol, respectively. The soil was air-dried and crushed to pass a 2-mm mesh sieve. To avoid soil layering and to ensure uniform bulk density, the following method was used. A layer of about.1 m of air-dry soil was filled into the mini rice beds. Water was added through the drainage hole until the soil was saturated. An additional.1 m layer of air-dry soil was added. The mini rice beds were then left to drain and equilibrate overnight before further packing. An axial compression force of about 2 kpa was applied for about 1 s. This was achieved by placing a board (.1 by.4 m) on the soil surface and applying a weight of 8 kg. This procedure was repeated until the entire soil surface had been loaded. The soil surface was loosened, water applied through the drainage hole, and so on until the entire mini rice bed was filled with soil. Instruments were installed while the mini rice beds were filled with soil. Access tubes in the front wall of the mini rice bed were provided for tensiometers and root observation tubes. Punch tensiometers,.5 m long, were used to determine matric potential. They were installed through the access hole sloping downwards at an angle of approximately 13. Holes were located about.4 m from the right wall. Measured from the top of the mini rice bed, they were at approximate depths of.15,.25,.35,.55, and.75 m. The tips of the tensiometers

3 Soil puddling and soil physical properties Australian Journal of Soil Research 619 were approximately.3 m from the aluminium access tube for soil water measurements (Fig. 2). If necessary, tensiometers were topped up with de-aired water at least 24 h prior to measurement. After the mini rice beds were packed, a uniformity crop was sown. Oats were sown after the mini rice beds were left to drain for about 2 weeks. The rice beds were flooded again after the oats had dried the soil to permanent wilting point. At harvest of the uniformity crop, each mini rice beds had undergone 2 drying and wetting cycles. Application of puddling treatments to mini rice beds Puddling treatments were applied using the laboratory puddling apparatus described above. Two steel rods with a diameter of 9.5 mm were attached to the disk and rotated with an angular velocity of 11 rpm at the radius of 58 mm. They were inserted.1 m into the topsoil of the mini rice beds. Two levels of puddling intensity were used. Puddling the soil for 5 min gave low puddling intensity and puddling the soil for 25 min gave high pudding intensity. The times were chosen to correspond to puddling ratios of 3.5 and 17. In contrast to the field condition, the implements of laboratory puddling puddle the soil vertically and not horizontally and do not create a compacted layer below the puddled zone. Although the puddling action can be considered sufficiently similar to field conditions, the absence of a compacted layer may lead to an underestimation of flow rates through the soil profile compared with field conditions. Measurement of percolation rates and hydraulic conductivity Continuous paddy conditions were maintained in the mini rice beds by blocking the drainage outlet. For percolation measurements, the drainage hole was opened and the drop of the water level monitored in 1 3-days intervals, depending on magnitude of percolation rate. Potential evaporation was monitored regularly by weighing a water filled black PVC evaporation box. The box had a size of.31 by.61 m, was.11 m high, and was filled with water 5 8 mm deep. It was placed on top of 2 adjacent mini rice beds. Percolation rates were calculated as the difference between change in water levels in the mini rice beds and evaporation. Percolation rate measurements were carried out 1 week and 2 and 4 months after puddling in the puddling experiment. Suspended soil mass, mean weight diameter, and dispersion Immediately after puddling, the amount of dispersed material in the water was measured; 1 ml of soil suspension was sampled by inserting a pipette to 1 mm depth below the water surface. Suspension mass was measured after oven drying. The mean weight diameter of aggregates in the puddle layer was determined 1 week after puddling using a wet sieving procedure similar to the Yoder (1936) apparatus. Approximately 15 g of saturated soil was collected from the puddle layer. Samples were left to drain overnight on a tension table at 3 Pa. Half of each sample was used to determine soil water contents; the other half was used in the wet sieving procedure. Samples were wet-sieved for 15 min in a nest of 6 sieves (mesh seize 9.5, 5.1, 2., 1.,.5, and.25 mm). Sieving amplitude was 12 mm with an angular velocity of 2 r/min. Dispersible clay and clay plus silt of the puddled layer was measured 1 week and 2 months and 4 months after puddling. The procedure described by Cook (1988) was modified for measurements on wet soil. In this study wet soil, equivalent to approximately 4 g oven-dry soil was sampled from the puddled layer from each rice bed. The saturated soil was placed onto a tension table and left to drain overnight at 3 Pa suction. Half of each sample was used for water content determination. The other half was placed into a settling cylinder and water added to 1 ml. Shaking was reduced to a minimum, but adequate to produce a homogenous suspension suitable for sedimentation. The pipette method was used to measure dispersible clay and clay plus silt. Matric potential and soil water contents Volumetric soil water contents were measured in.1-m intervals from depth.2 to.6 m using a neutron moisture meter. Soil samples for the determination of gravimetric water contents were taken from depth.2,.2.5, and.5.1 m depth during the drying phase when mungbean was grown as the indicator crop. Size of samples was kept as small as possible to minimise disturbance of the soil surface. Each sample had a diameter of approximately.2 m. Matric potential was measured using punch tensiometers. During the first week after drainage, these measurements were taken every second day; time intervals were then steadily increased to 2 measurements per week at mungbean harvest. Particle size distribution changes due to puddling The changes in soil texture within the puddle layer as a consequence of redistribution of soil particles during settling after soil puddling was evaluated. Samples were taken after mungbean harvest at depths.1,.1.2,.2.4,.4.7, and.7.1 m. The pipette and sieving method was used to obtain the particle size distribution at the different depths within the puddled layer. Statistical analysis The effect of puddling ratio on the soil physical properties on the 3 different-textured soils was assessed using linear and multi-variable regression procedures. The mini rice bed study was conducted as a randomised complete block design with 2 replicates on 3 differenttextured soils and 2 puddling ratios given a total of 12 mini rice beds. A general linear model was used to evaluate the data and where appropriate least significant differences were calculated. Results and discussion Laboratory study: Puddling ratio Table 1 shows that the puddling ratio was significantly related to dispersible clay, clay plus silt, saturated hydraulic conductivity, and bulk density. Using multi-variate regression procedures by including additional variables from Eqn 1 did not improve regression coefficients significantly. This indicated that the puddling ratio integrated the effect of time of puddling, angular velocity, and number of rods and was therefore a reasonable measure of the degree of puddling intensity. However, the relatively large variation in soil physical properties is probably due to the inability of the puddling ratio to account for soil that is exposed to the implement more than once, or not exposed to it at all during the puddling operation. Further work will be required to refine the puddling ratio, possibly by including a term that describes the energy applied to the soil during puddling. Table 1. Correlation coefficients (R) between puddling ratio and soil properties Dependent variable R (n = 62) Dispersible clay + silt.649** Dispersible clay.782** log(ksat).677** Bulk density.497**

4 62 Australian Journal of Soil Research G. Kirchhof and H. B. So Soil physical properties of the puddled layer during rice growth Soil properties The properties of the soils used in the mini rice beds are given in Table 2. The ph values were neutral to slightly acid with low electrical conductivities, indicating no soil chemical restriction except the need to apply fertiliser to ensure nonlimiting plant nutrition; ph values changed slightly during the rice phase. The lowest ph and redox potential under submerged conditions were observed for the clay soils but ph and redox potential were not limiting to the growth of rice for any soil (Table 2). Soil water content at pf (.1 kpa) was similar for all soils. The volume of pores that drain at field capacity was largest for the loam and similar for the silty loam and clay. Plant-available water was lowest for the loam, due to its course texture compared with higher water-holding capacities for the heavier silty loam and clay soil. Lower plastic limits were similar for the three soils despite large textural differences. The liquid limit, and concomitantly the plasticity index, followed the texture grades and increased with increasing clay content. Assessment of soil puddling The effect of low and high puddling intensity, or puddling ratios of 3.5 and 17, respectively, was evident in the amount of dispersed soil that remained in suspension after puddling and a decrease in mean weight diameter (Fig. 3). Differences between low and high puddling intensity were large for Table 2. Soil properties of the soils used in the mini rice beds Soil property Silty loam Standard measurement of ph(h 2 O) ph of submerged soil Redox potential of submerged soil [mv] Electrical conductivity (ds/m) Soil water (g/g): θ g 1.5 MPa θ g 1 kpa θ g.1 kpa Texture (%): Coarse sand (.2 2 mm) Fine sand (2 2 µm) Silt (2 2 µm) (<2 µm) Texture grade Silty loam Australian soil classification Grey Brown Brown Vertosol Dermosol Kandsol Organic carbon 1.3% 1.11%.68 Consistency (g/g): Lower plastic limit Liquid limit Sediment mass (g/cm 3 ) 1 h after puddling Mean weight diameter (mm) Puddling intensity Low High l.s.d. (P =.5) Soil (a) l.s.d. (P =.5) Silty loam Fig. 3. Effect of low and high puddling intensity on (a) suspended soil mass and (b) mean weight diameter. the fine-textured soil but not significantly different for the loamy soil. In terms of dispersion, this showed a fairly high structural stability of the loam soil. Several days after puddling, suspended soil had settled and measurement of suspended soil was no longer possible. Wet sieving was not repeated at later stages because of insufficient sensitivity for the detection of significant differences in puddling intensity and time required for analysis. Dispersed clay was measured 3 times during the rice phase (Fig. 4). Dispersed clay was higher under high puddling intensity. Between soil types it followed the clay content of the soil and was therefore lowest in the loam soil and highest in the clay soil. It was observed was that amount of dispersed clay decreased with increasing length of inundated conditions for the 3 different soils, although the decrease was not significant for the loamy soil. In addition, percolation rates also tended to decrease with time of submergence. These effects were significant for the 2 heavier textured soils puddled with low intensity (Fig. 4). This appeared to be in contrast to observations by So and Cook (1993), who compared dispersion between clay soils and found that hydraulic conductivities decreased as (b)

5 Soil puddling and soil physical properties Australian Journal of Soil Research 621 Dispersible clay (g/g) (a) High puddling Low puddling Low & high combined Silty loam r 2 =.89** r 2 =.999** r 2 =.999** r 2 =.74 ns Percolation rate (mm/day) (b) Days after puddling Silty loam High puddling intensity r 2 =.96 ns r 2 =.89 ns r 2 =.79 ns r 2 =.99** r 2 =.7 ns Low puddling intensity r 2 =.99** Fig. 4. Change of (a) dispersible clay and (b) percolation rates during prolonged periods of flooding. Depth (m) Gravimetric water content (g/g) Silty loam End of paddy (drainage) 3 days after drainage 7 days after drainage 1 days after drainage 17 days after drainage 2 days after drainage clay dispersion increased. They attributed the decrease in hydraulic conductivity to an increase in exchangeable sodium percentage, which resulted in clay particles effectively blocking pores for water-flow. Puddling, however, cannot be expected to affect exchangeable sodium contents, and the mechanism leading to dispersion is of mechanical nature, rather than being related to physio-chemical processes. While mechanical energy is applied to saturated soil, particles may be held in suspension similar to dispersion due to the input of mechanical energies. If this energy input ceases, particles may slowly re-flocculate and re-consolidate even if submerged conditions prevail. At the same time, coarser particles are likely to settle faster than finer particles and over time a seal may develop on the surface of submerged soil that limit hydraulic conductivity by blocking surface pores. Analysis of the particle size distribution profile clearly showed an increase in clay content close to the soil surface (Fig. 5). However, the analysis was not sufficiently sensitive to differentiate between low and high puddling intensity. The formation of a clay surface seal due to puddling also affects water movement following the rice phase, in particular during the period before the seal ruptures due to crack formation. Water contents at the soil surface remained considerably higher for up to 7 days after drainage for the 2 finer textured soils and >17 days on the loam soil (Fig. 5). The latter corresponded with the strongest particle size redistribution (Fig. 6). However, differences Fig. 5. Soil water profiles the 3 different soils (average of high and low soil puddling) in the puddling experiment after drainage. Silty loam % Particles Depth (m) (<2 µm) (<2 µm) Silt (2 2 µm) Silt (2 2 µm) Fine sand (2 2 µm) Coarse sand (2 2 µm) (<2 µm) Silt (2 2 µm) Fine sand (2 2 µm) Coarse sand (2 2 µm) Fine sand (2 2 µm) Coarse sand (2 2 µm) <2 µm <2 µm <2 µm Fig. 6. Texture profiles (of high and low soil puddling) of the puddled layer in the puddling experiment.

6 622 Australian Journal of Soil Research G. Kirchhof and H. B. So Table 3. Gravimetric water content and matric potential of the clay soil at.8 m depth at 3 different times after drainage in the puddling experiment Stage/date θ g (g/g) ψ m (cm) Puddling intensity: High Low High Low End of paddy (16 Dec. 1992) Sowing (23 Dec. 1992) Establishment (3 Dec. 1992) due to puddling intensity between soils could not be detected except for the clay soil (Table 3). Close to the bottom of the.1-m deep puddled layer (at.8 m depth), water contents were greater under high intensity puddling in particular at the end of submerged conditions. However, compared with differences in matric potential these differences were relatively small. During the drying phase while mungbean seedlings established, matric potential dropped below 1 cm in the low intensity puddling treatment but remained considerably higher in the high intensity puddled soil. Due to the close relationship between soil strength and matric potential, root growth of establishing seedlings may have been restricted and resulted in the observed lower seedling weight of 32 and 24 mg/plant, for the high and low puddling treatment, respectively. Conclusion 1. The puddling ratio is a reasonable expression of the degree of puddling intensity and reflects changes in soil physical properties. 2. In the absence of a potentially impeding compacted layer, percolation rates and the amount of dispersion in the puddled surface soil decreased with period of submerged conditions due to the formation of a clay surface seal and possibly continuous ongoing consolidation process of soil in the puddled layer in the absence of overburden pressure and swelling and shrinking conditions. 3. The formation of a clay seal due to puddling results in decreased soil drying rates in particular on coarse-textured soils. Acknowledgment This project was funded by the Australian Centre for International Agricultural Research (ACIAR) as part of a study on management strategies to improve dry season crop yields after rice. References Cook GD (1988) Degradation of soil structure under continuous cultivation on the Darling Downs, Queensland. PhD thesis, The University of Queensland, Australia. Isbell RF (1996) The Australian Soil Classification System. (CSIRO Publishing: Collingwood, Vic.) Pasaribu D, McIntosh JL (1985) Increasing tropical soybean production with improved cropping systems and management. In Soybean in tropical and subtropical cropping systems. (Eds S Shanmugasundaram, EW Sulzberger) pp (Asian Vegetable Research and Development Centre: Taiwan) Sharma PK, DeDatta SK (1985) Effects of puddling on soil physical properties and processes. In Soil physics and rice. pp (International Rice Research Institute: Los Banos, The Philippines) So HB, Cook GD (1993) The effect of slaking and dispersion on the hydraulic conductivity of clay soils. Catena Supplement 24, Yoder RE (1936) A direct method of aggregate analysis of soil and a study of the physical nature of soil erosion losses. Journal of the American Society of Agronomy 28, Manuscript received 17 May 24, accepted 24 March 25