Transitional Red-brown Earth Research Articles

Effect of raised beds, irrigation and nitrogen management on growth, water use and yield of rice in south-eastern Australia

To remain economically and environmentally sustainable, Australian rice growers need to be able to readily respond to market opportunities and increase cropping system productivity and water productivity. Water availability is decreasing whereas its price is increasing. Alternative irrigation layouts and water management approaches could contribute to reduced water use and increased irrigation efficiency. This paper reports results for the first crop (rice) in a cropping system experiment to compare permanent raised bed and conventional layouts on a transitional red-brown earth at Coleambally, New South Wales. The performance of conventional ponded rice grown on a flat layout was compared with rice grown on 1.84-m wide, raised beds with furrow and subsurface drip irrigation. In addition, deep and shallow ponded water depth treatments (15 and 5 cm water depth over the beds) were imposed on the rice on beds during the reproductive period. A range of nitrogen (N) fertiliser rates (0–180 kg N/ha) was applied to all treatments. The traditional flat flooded treatment (Flat) achieved the highest grain yield of 12.7 t/ha, followed by the deep (Bed 15) and shallow (Bed 5) ponded beds (10.2 and 10.1 t/ha, respectively). The furrow (Furrow) irrigated bed treatment yielded 9.4 t/ha and the furrow/drip (Furr/Drip) treatment yielded the lowest grain yield (8.3 t/ha). Grain yield from all bed treatments was reduced owing to the wide furrows (0.8 m between edge rows on adjacent beds), which were not planted to rice. Rice crop water use was significantly different between the layout–irrigation treatments. The Flat, Bed 5 and Bed 15 treatments had similar input (irrigation + rainfall – surface drainage) water use (mean of 18.3 ML/ha). The water use for the Furrow treatment was 17.2 ML/ha and for the Furr/Drip treatment, 15.1 ML/ha. Input WP of the Flat treatment (0.68 t/ML) was higher than the raised bed treatments, which were all similar (mean 0.55 t/ML). This single season experiment shows that high yielding rice crops can be successfully grown on raised beds, but when beds are ponded after panicle initiation, there is no water saving compared with rice grown on a conventional flat layout. Preliminary recommendations for the growing of rice on raised beds are that the crop be grown as a flooded crop in a bankless channel layout. This assists with weed control and allows flooding for cold temperature protection, which is necessary with current varieties. Until we find effective herbicides and other methods of weed control and N application that do not require ponding, there is little scope for saving water while maintaining yield on suitable rice soil through the use of beds.

Autecology of Bradyrhizobium japonicum in soybean-rice rotations

The effect of rice culture on changes in the number of a strain of soybean root-nodule bacteria, (Bradyrhizobium japonicum CB1809), already established in the soil by growing inoculated soybean crops, was investigated in transitional red-brown earth soils at two sites in south-western New South Wales. At the first site, 5.5 years elapsed between the harvest of the last of four successive crops of soybean and the sowing of the next. In this period three crops of rice and one crop of triticale were sown and in the intervals between these crops, and after the crop of triticale, the land was fallowed. Before sowing the first rice crop, the number of Bradyrhizobium japonicum was 1.32×105 g−1 soil. The respective numbers of bradyrhizobia after the first, second and third rice crops were 4.52 ×104, 1.26×104 and 6.40×102 g−1 soil. In the following two years the population remained constant. Thus sufficient bradyrhizobia survived in soil to nodulate and allow N2-fixation by the succeeding soybean crop. At the second site, numbers of bradyrhizobia declined during a rice crop, but the decline was less than when the soil was fallowed (400-fold cf. 2200-fold). Multiplication of bradyrhizobia was rapid in the rhizosphere of soybean seedlings sown without inoculation in the rice bays. At 16 days after sowing, their numbers were not significantly different (p<0.05) from those in plots where rice had not been sown. Nodulation of soybeans was greatest in plots where rice had not been grown, but yield and grain nitrogen were not significantly different (p<0.05). Our results indicate that flooding soil has a deleterious effect on the survival of bradyrhizobia but, under the conditions of the experiments, sufficient B. japonicum strain CB 1809 survived to provide good nodulation after three crops of rice covering a total period of 5.5 years between crops of soybean.

Effects of gypsum slotting on the hydraulic properties of a red-brown earth

In situ measurements of unsaturated hydraulic conductivity ( k) and sorptivity ( S) were made on a transitional red-brown earth at two sites. The first site was a non-ameliorated soil (control). At the second site the soil had been modified by mixing the soil with gypsum to a depth of 0.4 m using a rotary digger (gypsum slot). Measurements were made using a disc permeameter (radius 100 mm) at the surface of both sites and at depths of 0.15, 0.4 and 0.6 m at the control site, at various potentials. At the control site, the values of k and S decreased with depth and potential, except at the depth of 0.6 m. The range of S measured was within one order of magnitude. The values of k ranged over two orders of magnitude. The values of k and S measured for the surface horizon were similar to previous measurements by other workers. However, the values of k for the subsoil (0.4 and 0.6 m depth) were greater than previously published values. The values of k and S for the slot soil were intermediate between values for the surface horizon and subsoil of the control soil, and similar to the values at 0.15 m depth of the control soil. The steady state infiltration rate into a slot was calculated using a model developed for vertical slot mulches and compared with values from previous infiltration experiments. The calculated and measured values of the steady state infiltration rate were in agreement. An equation was derived for predicting the increase in steady state infiltration rate, of a soil containing slots, from the slot depth and spacing. The slot width has only a small effect on the steady state infiltration rate. The maximum steady state infiltration rate occurs when the depth of the slot is 0.837 m. The relationship between the increase in the maximum steady state infiltration rate and the slot spacing is presented. This and the relationship between the increase in steady state infiltration rate, slot spacing and depth were used in examples to predict the increase in steady state infiltration rate for various slot spacings and dimensions.

Effects of saline irrigation water on soybean yield and soil salinity in the Murrumbidgee Valley

The effect of saline irrigation water on soybean (Glycine max cv. Chaffey) growth and yield is not known for southern Australia. Soybeans were grown on hills, on a transitional red-brown earth, with moderately saline irrigation water (0.25, 0.5, 1.0, 1.5, 2.0 dS/m) for 2 seasons. Grain yield was significantly reduced when soybeans were irrigated using water with salinity &gt;0.25 dS/m in the first year and &gt;1.5 dS/m in the second year. The salinity of the upper 75 cm of the soil profile increased with increasing salinity of applied treatments. Salinity levels at depth indicate increased flow beyond the rootzone with increasing salinity of applied water. The use of water &gt;0.5 dS/m is not recommended for soybean cv. Chaffey on sites where soybeans have not previously been grown.

Modelling leaching and recharge in a bare transitional red-brown earth ponded with low salinity water in summer

A modelling study was conducted to evaluate water and salt movement within a transitional red-brown earth with saline B horizon soil when such waters are used for ponding in summer. The model was calibrated using previously published experimental data. The calibrated model was used to evaluate the effect of depth to watertable, saturated hydraulic conductivity, and ponding water salinity on infiltration, water and salt movement within the soil profile, and recharge. The study showed that when initial soil water content and the saturated hydraulic conductivity (Ks) are low, infiltrating water will be stored within the soil profile even in the absence of a shallow watertable. Once the soil water content is high, however, recharge will be significant in winter, even if there is no net infiltration at the soil surface. Infiltration rates depend more on Ks than the depth to watertable if it is at, or below, 1.5 m from the soil surface. When Ks is high, recharge under ponding will be higher than that under winter fallow. Subsequent ponding in summer and fallow in winter tend to leach salts from the soil profile, the leaching rate dependent on Ks. During winter fallow, due to net evaporation, salts tend to move upwards and concentrate near the soil surface. In the presence of shallow watertables, leached salts tend to concentrate at, or near, the watertable.

Effect of soil loosening on the hydraulic properties of a duplex soil

In a duplex, transitional red-brown earth, the relationships between hydraulic conductivity K, diffusivity D, soil water potential (�) and volumetric water content (�) in different soil layers were evaluated. Changes in these relationships and in the derived parameters; macroscopic capillary length and characteristic microscopic pore radius due to soil loosening were examined. Replicated large undisturbed and repacked cores of 0.75 m diameter and 1.35 m deep, located in a lysimeter, were used in this study. The K- � relationships were measured during steady upward flow from a watertable at different depths. The � � relationships were measured during a soil drying cycle. The undisturbed soil showed similar soil water characteristics (�-� curves) in all soil layers, at depths greater than 0.25 m. In contrast, K at high � was significantly lower in the upper subsoil, 'throttle' layer with high clay content, compared with the deeper soil layers. However, previous studies on a similar soil have also shown that factors other than pore size distribution, such as tortuosity and pore connectivity, markedly influence the hydraulic conductivity properties of clay soils. Coefficients in the log � on log� and in the log K(�) on log � relationships for all soil layers combined differed markedly from the coefficients for individual layers, indicating the need to measure these properties separately for different subsoil clay layers of this duplex soil. The repacking of the soil to a lower bulk density caused an increase in K at high � values. This was accompanied by a significant decrease in slope of the log � -log � relationship, from 17-21 to around 7, in the subsoil layers. The slope of the regression of log � - K(�) on log� changed from 24 in undisturbed soil to 9 in the repacked soil. This was mainly due to large changes in the � -�relationship, in combination with smaller changes in the K- � relationship. The decrease in diffusivity D with reduction in 0 was less marked in the repacked soil compared with the undisturbed soil. Thus, soil repacking shifted D(�) towards a constant value, for the range of � measured. This was mainly due to a slower rate of change in K with decrease in � on the repacked soil, combined with a compensating increase in d � /d� values. The calculated values of macroscopic capillary length decreased and characteristic pore radius increased, reflecting the changes in pore sizes and hydraulic conductivities due to soil loosening. However, at any given supply and antecedent �, the sorptivity was greater in the loosened soil due to larger changes in water content and hydraulic conductivities. The potential application of these soil properties in evaluating water flow in undisturbed or loosened soils is briefly discussed.

Flood irrigation of wheat on a transitional red-brown earth. II. Effect of duration of ponding on availability of soil and fertilizer nitrogen

Grain yield of wheat growing on a transitional red-brown earth was reduced by long periods of ponding during irrigation. To determine whether fertilizer N loss was a major cause of this yield decline, 15N-labelled urea was applied to microplots at the same time and rate as the crop was topdressed with urea (end of tillerfng, 100 kg N ha-1). In addition, 15N-labelled nitrate was used to assess denitrification potential during and after each irrigation. Prior to the first irrigation (19days after urea application), 76% of the urea N was immobilized in the plants (33%) and soil (43%), 15% was present as soil mineral N, and 9% was not accounted for. The majority (85%) of the urea-derived mineral N was present as ammonium in the 0-0.1 m soil layer. After the first irrigation, amounts of mineral N in the soil remained very low at 3-6 kg N ha-l in the 0-0.2 m layer, with only 5-15% of this in the nitrate form. There was no significant effect of duration of ponding on plant or soii recovery of urea N after any irrigation. Mean plant recovery increased to 52%, over the first 55 days following urea application, while soil recovery declined to 22%. Beyond this stage changes in plant and soil recoveries were negligible. Plant total N content increased throughout the season due to ongoing mineralization of native soil N; however, there was negligible net mineralization of recently immobilized 15N beyond day 55. At physiological maturity, 46% of the N acquired by the plants was derived from the fertilizer. Losses of urea N increased to 25% over the first 55 days, and appeared to be due to nitrification-denitrification. Field studies with labelled nitrate indicated that denitrification potential in the soil was high throughout the experimental period. After the final two irrigations there were some significant (P &lt; 0.05) effects of duration of ponding on N loss from urea, with lowest losses in the sprinkler, 1 h and 12 h treatments. The data indicate that, on the transitional red-brown earth, the adverse effect of long periods of ponding on wheat yield was not due to decreased availability or uptake of fertilizer or soil N in the longer term.

Flood irrigation of wheat on a transitional red-brown earth. I. Effect of duration of ponding on soil water, plant growth, yield and N uptake

Long periods of waterlogging during irrigation are thought to adversely affect wheat growth and yield. This work quantifies these effects for wheat (Triticum aestivum L. cv. Bindawarra) growing on a transitional red-brown earth. The crop was irrigated on four occasions during spring. At each irrigation six ponding treatments were imposed involving sprinkler irrigation (SP) or flooding for 1, 12, 24, 48 or 96 h. Each plot always received the same treatment. Nitrogen fertilizer was applied at sowing (23 kg N ha-1 as ammonium phosphates) and at the end of tillering, 3 weeks before the first irrigation (100 kg N ha-1 as urea). There was also a 1 h ponding treatment which did not receive urea at the end of tillering. Grain yield on the 48 h and 96 h ponding treatments was significantly lower than on SP, 1 h and 12 h. Yield declined by 69 kg ha-1 for each day that water was ponded on the surface, or by 55 kg ha-1 for each day that the estimated soil water depletion was below 25 mm. The wheat crop suffered stress due to lack of aeration for periods varying from 42% (1 h and SP) to 68% (96 h) of the time interval between early stem elongation and physiological maturity. The grain yield and N uptake data indicate that the site was highly responsive to N fertilizer. However, there was no significant effect of ponding treatment on total N uptake, suggesting that the yield differences between the ponding treatments were not caused by differences in N availability. The results demonstrate that yield loss through waterlogging can be minimized by changing to irrigation management practices that minimise the period of ponding.

Subsoil macropore space of a transitional red-brown earth after either deep tillage, gypsum or both .II. Chemical effects and long term changes

The management of sodic texture-contrast soils of the riverine plains of S.E. Australia for crops other than rice is difficult because improved subsoil structure often deteriorates after a few seasons of irrigated cropping. This research aimed to improve understanding of the processess which cause deterioration of macropore space (volume and permeability of air-filled soil at -10 kPa matric potential) as well as providing some guidelines for improved management. A transitional red-brown earth was ameliorated by eight different treatments and compared with an unameliorated control treatment in a field experiment. The treatments included surface gypsum application, deep ripping after gypsum application, five designs of gypsum-enriched slots and traffic-free beds after addition of gypsum and deep ripping. The macropore space of the subsoil after initial winter cropping and summer fallow is described from measurements on soil cores collected from the experiment. An index of electrochemical stability (EC/ESP) is proposed which explains the loss of macroporosity of cultivated subsoil in the absence of significant soil stressing by wheels. Considerable loss of macropore volume and permeability after the first summer crop could be associated with loss of soluble gypsum, some increases of exchangeable sodium and compression by lateral swelling of the uncultivated soil from between the slots. These changes were compensated by an increase in the number of vertical fissures, directly observed and deduced from measurements of water entry into whole plots. Thus, management of the balance of soluble salts is essential for the long-term productivity and profitability of such systems of amelioration using deep tillage zones and gypsum application. This is in the absence of significant compaction under wheels or opportunities for addition or organic matter and formation of biopores by roots and soil animals. The relief of lateral swelling of the uncultivated B horizon by the loose slots and formation of extra vertical fissures is a process which may provide long-term benefits and deserves further research.

Subsoil macropore space of a transitional red-brown earth after either deep tillage, gypsum or both .I. Physical effects and short term changes

This research aimed to improve understanding of the processes which cause deterioration of macropore space (volume and permeability of air-filled pores at -10 kPa matric potential), as well as providing some guidelines for improving management. This paper reports the effects of amelioration treatments on soil fragment sizes and horizon-of-origin, as well as the distribution of solid gypsum in the profile immediately after deep cultivation. The macropore space of the subsoil after initial winter cropping is described from measurements on soil cores collected from the experiment. The finer fragments from slotting at low speed resulted in smaller macroporosity than in the ripped soil. Macroporosity, permeability and organization of pore space generally increase with the amount of gypsum initially mixed with the soil. In spite of the smaller volume of soil disturbed by slotting than by ripping, the reduction in runoff was similar to the ripped soil when the gypsum application rate to the slotting was twice that applied to the ripped soil. Where no gypsum was applied to the slotted soil, slumping and swelling led to considerable loss of macroporosity formed by the initial deep tillage, as well as surface sealing by dispersed clay. Hence runoff from this treatment was similar to the unameliorated soil.

Effect of saline water on rice yields and soil properties in the Murrumbidgee Valley

Rice was grown on a transitional redbrown earth for 3 consecutive seasons and irrigated with saline groundwater of salinities 0.25, 0.5, 1.0, 1.5 and 2.0 dS/m. A 4.0 dS/m treatment was applied in the final year. Grain yield was only affected by the 4.0 dS/m treatment, where a 25% yield reduction occurred. Soil salinities increased with increasing salinity of applied treatments, reaching equilibrium after 3 years. The higher salinity treatments resulted in significantly increased rice field water consumption, attributed to increased infiltration rates. The presence of high electrolyte levels in the soil surface prevented any detrimental effect of increased soil sodium levels on soil properties. However, the communal supply system with potential cultivation of salt-sensitive species such as soybean, along with the potential for higher groundwater accessions, suggests that the salinity of irrigation water should be maintained as low as possible.

Use of the neutron method in assessing the changes in soil strength of undisturbed and ameliorated transitional red-brown earths during soil drying cycles

The relationships between penetrometer resistance (qp) and volumetric moisture content (�v) measured using the neutron method in an undisturbed transitional red-brown earth and after an~elioration by application of surface gypsum and slotted gypsum were examined. A very highly significant (P &lt; 0.001) negative correlation was obtained between qp and �v in all treatments. The low r2 values of the regressions were attributed to heterogeneity in strength characteristics of the soil matrix, due to presence of cracks and macropores and the associated wetting patterns. The qp at any given e, was significantly reduced in the slots with lower bulk density compared to the undisturbed soil. The differences in qp- �v relationship of the undisturbed part of the soil under different ameliorative practices were attributed to changes in the swelling characteristic, and hence in the bulk density at any given �v of the undisturbed soil, caused by the presence of gypsum and the slots. Regression equations between qp and neutron count rate (n) for the undisturbed soil and for the slots were developed by combining the qp on �v relationships with the neutron meter calibration for �v measurements. The use of these regression equations and measured n values to predict changes in soil strength profiles during a wheat crop drying cycle in an undisturbed and ameliorated transitional red-brown earth was evaluated on another experimental site. There were no significant differences between the predicted and measured qp values in the non-ameliorated soil and the gypsum-slotted soil. Significant differences were observed between the predicted and measured qp values in the surface gypsum applied soil. The study shows the potential for using the neutron method as a convenient in-situ field technique to predict qp profile changes, preferably using qp on n relationships developed at the experimental site.

The effect of speed of wheeling on soil stresses, rut depth and soil physical properties in an ameliorated transitional red-brown earth

Soil stress measurements were made in an ameliorated transitional red-brown earth (Natric Palexeralf) during one pass of a heavily loaded (4 t) wheel. Different speeds (up to about 8 km h −1 and soil water contents (close to the lower plastic limit) were used for measurements of total stress, final bulk density, air permeability and rut depth. Effective stresses appeared to decrease with speed. Increases of total stress caused by extra fluid pressure (i.e. pore-water pressure) at 20-cm depth were associated with fast deformation rates of the soil (up to about 3 m s −1) just behind the leading edge of the tyre contact area. Shear stress changed very little with speed. Equations relating speed and water content to rut depth and resultant bulk density were also found. These predicted approximately 10% less mean rut depth or bulk density at 20-cm depth by increasing speed from 0.5 to 10 km h −1 over this soil at water contents near the lower plastic limit.

Effects of amelioration on infiltration characteristics of a transitional red-brown earth

Abstract Infiltration into a transitional red-brown earth was analysed for the following ameliorative treatments: no treatment (C), surface gypsum at 8 t/ha (G), 8 t/ha of gypsum concentrated into slots 0.4 m deep and 0.66 m apart (SGI) or 1.0 m apart (SG2). Analysis consisted of fitting the infiltration data to the following equations: At short time, i = i*i + S* t ½ + K* t ; at long time, i = i*i + i*0 + K*t where i is the cumulative infiltration, i*i is a short-time storage term to account for cracks, i*0 is a long-time storage term, K* is the steady-state infiltration rate, t is time, and S* is the sorptivity. S* was similar for all treatments. The increase in i*0 for the slotted treatments resulted from the increased surface area of B horizon exposed to the wetting front. K* increased with treatment in the order C<G<SG2<SG1. The higher values of K* for the slotted treatments were attributed to the increased rate of flow through the slot into the surrounding soil. An equation which predicts K* for ame...

The introduction of earthworms to an ameliorated, irrigated duplex soil in south-eastern Australia and the influence on macropores

Two species of lumbricid, Aporrectodea caliginosa and Aporrectodea longa and a megascolecid, Eukerria saltenis were introduced, during the winter, to a duplex soil (transitional red-brown earth) ameliorated by either gypsum-enriched slots or beds formed after deep ripping and gypsum application. Some were introduced to areas enclosed by mesh within the slots, others were released in adjacent unenclosed areas. The enclosed areas had the equivalent of 0, 5 or 10 t ha-l of wheat straw on the surface, the unenclosed areas had 5 t ha-1. The enclosed areas were excavated in the spring, 17 weeks after earthworms were introduced. Proportionally more Eukerria adults were recovered than Aporrectodea adults (4040% compared with 10-30%). At 20-45 cm depth, Eukerria adults were four times more common than Aporrectodea. Cocoons and immature forms of Aporrectodea were very common, especially near the surface. Nine months after the introduction, even after a drier than average summer fallow, earthworms were still present. More Eukerria than Aporrectodea adults were found in the sample from the unenclosed areas. The smaller species, Eukerria, (about 1 mm diameter and 30 mm long) may be better adapted to survive in these ameliorated clay soils, but further research is needed. In the enclosed areas, soil macroporosity was unaffected by the 17 weeks with earthworms, but air permeability through the macropores was significantly increased, probably due to the burrows increasing vertical pore connectivity within 0 . 3 m depth.

Fragment size distribution within slots created in a duplex soil by a prototype rotary slotter

The relationship between the sizes of fragments produced by slotting with a prototype rotary slotter and the tractor speed and the moisture content of a transitional red-brown earth soil was examined. In this poorly-structured soil, the three rotors spaced 1.2 m apart, working in narrow (0.12 m wide) parallel bands, produced well-defined 0.40 m deep slots. The slots were filled with platy-structured fragments, formed by the slicing action of the rotors on the slot face and smaller blocky fragments produced by subsequent vigorous mixing within the rotor shrouds. The geometric mean diameter of these fragments and the percentage of different sized fragments were closely related to the thickness of cut, which is determined by the forward speed of the tractor and the rotor speed. More than 90 and 70% of the fragments had diameters < 25 mm and 11 mm, respectively, at the tractor speeds (0.36–0.70 km h −1) and the rotor speed (94 rpm) used in the experiment. Regression equations to describe the relationships between the thickness of cut and the fragment size distribution were derived. The fragment sizes were only slightly affected by the soil moisture content. The vigorous cutting and mixing of the soil within the shrouds shattered the fragments derived from the original A horizon to diameters mostly < 1 mm. Within the slots, the percentages of the smaller-sized fragments increased with depth, possibly owing to interference in the deposition of the larger sized fragments by the rotors.

The size and horizon of origin of fragments produced by deep ripping texture contrast soils

Two texture contrast soils were cultivated by deep ripping when they were drier than their lower plastic limits. The size distribution and soil horizon-of-origin of the resulting fragments were measured. One soil, a transitional red-brown earth, had either been previously uncultivated below the A horizon or had been deep ploughed and gypsum added two years previously. There was much fragmentation and mixing of soil from both of the horizons. Fine soil (&lt;2 mm diam.) from the A horizon reached the lower depths of the trough made by the ripping and coarse soil from the B horizon (&gt;50 mm) was brought to near the surface. The fragment size distributions were characteristically bimodal. Fragments of the fine mode (&lt;2 mm) came mainly from the A horizon, fragments of the coarse mode (11-25 mm or larger) came mainly from the B horizon. In the laboratory, clods from the deep ripped soil were crushed at the same low water potential (air dry). The crushing energy per unit mass (specific crushing energy) was inversely proportional to the normalized geometric mean diameter of the fragments produced. Suggestions are made for modelling the effects of deep ripping.

Effect of changes in moisture profiles of a transitional red-brown earth with surface and slotted gypsum applications on the development and yield of a wheat crop

The low productivity of transitional red-brown earths for flood irrigated upland cropping is associated with their low infiltration rates and inadequate aeration of the root zone. The effect of the measured changes in the moisture and aeration profiles with surface and slotted gypsum applications on growth and development of a wheat crop was evaluated in a preliminary field study, on non-replicated plots.The patterns of changes in moisture profiles in the gypsum slotted plots were similar to those observed in the previous season, namely, deeper preferential wetting and faster internal drainage through the slots. This resulted in lower volumetric moisture contents in the slots and in the surface and subsurface layers between slots, compared to the non-slotted plots.The critical moisture contents were defined for each soil depth at which an air-filled porosity of 0.08 mm3 mm-3 was reached. For the transitional red-brown earth used in this study, air-filled porosity needs to be larger than 0.08 mm3 mm-3 to provide a soil pathway for oxygen flow to roots. The moisture profiles in the no-gypsum, surface gypsum and slotted gypsum plots measured throughout the cropping season indicated the period of time when oxygen flow through different soil layers was likely to be restricted. The moisture contents were higher than the critical value in the surface and subsurface layers of the non-slotted plots, particularly in the plot with no gypsum applications, during a period in winter with prolonged rainfall and low evapotranspiration rates. This resulted in reductions in the rates of phasic development, tillering, canopy closure and dry matter production and finally lower yields in the non-slotted plots, especially in the plot without gypsum. Differences in grain yields were mainly due to differences in the number of spikes m-2.During the second half of the growing season higher potential evapotranspiration, lower rainfall and accurate irrigation scheduling resulted in the moisture contents being maintained below the critical limits at all depths in all plots. Consequently, the two yield components which were determined during this period, namely, the number of kernels per spike and kernel weight, showed only slight variations between plots.

The nature of swelling in soils

While existing swelling models adequately describe the swelling in extensively-swelling soils, their applicability to moderately-swelling soils has not been previously tested. The swelling of different layers of a transitional red-brown earth measured in situ using swelling gauges could not be described using existing swelling models. Values of �, defined as the ratio of change in bulk volume of the soil to change in gravimetric moisture content, were less than 0.33 over the whole of the measured moisture range in all layers of this soil, except the 0.25-0.55 m depth. This indicates that the extent of swelling was less than the normal, three-dimensional swelling described by existing models. Modifications to these existing models are proposed to describe the swelling in soils with moderate swelling characteristics. The compressibility factor (�), defined as the ratio of the change in void ratio to the change in moisture ratio, measures the same soil property as the values. Values of � determined in previous field studies and on soil cores of several clay soils are also less than the 0.33 specified for normal swelling, in keeping with the expectations of the modified swelling model. However, the need for further field testing of this model for moderately-swelling soils is emphasized. The limitations of existing methods for determining swelling characteristics of soils using core samples are discussed and several alternative procedures for in situ measurements are suggested.

Measuring air-filled porosity changes in an irrigated swelling clay soil

An in-situ method for measuring air filled porosity (�a) of soils using a neutron meter was developed and evaluated for a layered, swelling clay soil. Bulk density-volumetric moisture content relationships of soil layers were used to calculate the regressions of volumetric moisture content on �a. These regressions were combined with calibration equations of the neutron meter to obtain relationships between �a and neutron counts. The �a values predicted from measured neutron count ratios show good agreement with �a values calculated from the measured bulk densities and moisture contents in a transitional red-brown earth. The method provides a rapid and non-destructive measurement of �a. Soil aeration in a transitional red-brown earth under two irrigation treatments was assessed using neutron counts to monitor the changes in �a profiles. Large soil cores which were flooded had lower profile �a values than did cores which were not flooded. However, differences were small, owing apparently to the very low rates of infiltration and redistribution. The potential use of this method to continuously monitor changes in �a profiles allows frequent calculation of aeration stress indices, which may be used to predict crop responses and yield losses due to poor aeration.