Plant Available Water Capacity Research Articles

Plant-available soil water capacity: estimation methods and implications

The plant-available water capacity of the soil is defined as the water content between field capacity and wilting point, and has wide practical application in planning the land use. In a representative profile of the Cerrado Oxisol, methods for estimating the wilting point were studied and compared, using a WP4-T psychrometer and Richards chamber for undisturbed and disturbed samples. In addition, the field capacity was estimated by the water content at 6, 10, 33 kPa and by the inflection point of the water retention curve, calculated by the van Genuchten and cubic polynomial models. We found that the field capacity moisture determined at the inflection point was higher than by the other methods, and that even at the inflection point the estimates differed, according to the model used. By the WP4-T psychrometer, the water content was significantly lower found the estimate of the permanent wilting point. We concluded that the estimation of the available water holding capacity is markedly influenced by the estimation methods, which has to be taken into consideration because of the practical importance of this parameter.

Visual examinations and soil physical and hydraulic properties for assessing soil structural quality of soils with contrasting textures and land uses

This study evaluates the use and the ability of visual examinations for assessing soil structural quality (SSQ) in soils with contrasting textures and under different land uses. The study searched for similarities in SSQ class between visual examinations and soil physical and hydraulic properties (soil organic carbon (SOC), aggregate stability, bulk density, porosity, plant available water capacity (PAWC) and unsaturated and saturated hydraulic conductivity), as well as the statistical relationships between them. The visual examinations used were the visual evaluation of soil structure (VESS), the visual soil assessment (VSA), the visual assessment of aggregate stability and the visual type of aggregates index. The latter is proposed as a new visual index for assessing SSQ. Samples were taken on a sandy loam and a silt loam soil, both under cereal monoculture (CM) and permanent pasture (PP), with conventional tillage and no tillage, respectively. Visual examination methods indicated significant differences between CM and PP in the silt loam soil (0.01<P<0.05), which were confirmed by significant differences in soil porosity and PAWC values. Wet sieving and the visual type of aggregates index were similar in identifying differences between land uses in both soils. Measurements of the visual type of aggregates index and of the hydraulic conductivity at different pressure heads were similar in indicating the soil structure condition of the soils. In the silt loam soil, the visual examinations were most related to properties such as SOC, PAWC, aggregate stability and porosity, whereas in the sandy loam soil they were most associated with water flow properties. The present study demonstrated that visual examinations are reliable semi-quantitative methods to assess SSQ and could be considered as promising visual predictors of soil physical properties (0.33<R2<0.95). Finally, from the dissimilarities in terms of soil quality found with the VSA, VESS and porosity compare to the amount of SOC, SOC should be used cautiously as a sole indicator for soil structural quality as has been proposed in the literature, because SOC per se is not always well related to soil structural quality.

Impact of future climate change on wheat production in relation to plant-available water capacity in a semiaridenvironment

Conceptions encompassing climate change are irreversible rise of atmospheric carbon dioxide (CO2) concentration, increased temperature, and changes in rainfall both in spatial- and temporal-scales worldwide. This will have a major impact on wheat production, particularly if crops are frequently exposed to a sequence, frequency, and intensity of specific weather events like high temperature during growth period. However, the process of wheat response to climate change is complex and compounded by interactions among atmospheric CO2 concentration, climate variables, soil, nutrition, and agronomic management. In this study, we use the Agricultural Production Systems sIMulator (APSIM)-wheat model, driven by statistically downscaled climate projections of 18 global circulation models (GCMs) under the 2007 Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A2 CO2 emission scenario to examine impact on future wheat yields across key wheat growing regions considering different soil types in New South Wales (NSW) of Australia. The response of wheat yield, yield components, and phenology vary across sites and soil types, but yield is closely related to plant available water capacity (PAWC). Results show a decreasing yield trend during the period of 2021–2040 compared to the baseline period of 1961–1990. Across different wheat-growing regions in NSW, grain yield difference in the future period (2021–2040) over the baseline (1961–1990) varies from +3.4 to −14.7 %, and in most sites, grain number is decreased, while grain size is increased in future climate. Reduction of wheat yield is mainly due to shorter growth duration, where average flowering and maturing time are advanced by an average of 11 and 12 days, respectively. In general, larger negative impacts of climate change are exhibited in those sites with higher PAWC. Current wheat cultivars with shorter growing season properties are viable in the future climate, but breading for early sowing wheat varieties with longer growing duration will be a desirable adaptation strategy for mitigating the impact of changing climate on wheat yield.

Changes in soil quality and plant available water capacity following systems re-design on commercial vegetable farms

Loss of ecological functions due to soil degradation impacts viability of crop production systems world-wide, particularly in vegetable cropping systems commonly located in the most productive areas and characterized by intensive soil cultivation. This paper reports soil degradation caused by intensive vegetable farming, and its reversibility after two to five years of drastic changes in soil management on 16 commercial vegetable farms in south Uruguay. Changes in soil management included addition of green manures and pastures in rotations of vegetable crops, use of animal manure, and erosion control support measures (terracing, reducing slope length, re-orientation of ridges). Soil degradation caused by vegetable farming was assessed by comparing soil properties in 69 vegetable fields with values at reference sites located close to the cropped fields. Effects of the changes in soil management in the 69 fields were assessed by comparing soil properties at the start and to those at the end of the project. Compared to the on-farm reference sites, the vegetable fields contained 36% less SOC, 19% less exchangeable potassium, water stable aggregates with an 18% smaller geometric mean diameter, and 11% lower plant-available soil water capacity. Phosphorus availability was 5 times higher under vegetable cropping compared to the on-farm reference. Phaeozems (Abruptic) revealed greater degradation (44% less soil organic carbon (SOC)) than Vertisols (24% less SOC) and Phaeozems (Pachic) (21% less SOC). After two to five years of improved soil management, SOC concentration in the upper 20cm increased by on average 1.53gkg−1 (12%) in the Phaeozems (Abruptic) and 1.42gkg−1 (9%) in the Phaeozems (Pachic). SOC in Vertisols increased only by 0.87gkg−1, most likely due to their greater initial SOC concentration. Topsoil carbon sequestration was on average 3.4Mgha−1 in the Phaeozems. Multiple linear regression showed the quantity of incorporated amendments, the initial amount of SOC and the clay content to explain 77% of the variability in yearly changes of SOC. Available water capacity increased significantly with SOC particularly due to more water retention at field capacity, resulting in an increase in available water capacity in the first 20cm of soil of 8.4mm for every 10gkg−1 of SOC increase. Results are discussed in relation to perspectives of soil degradation reversal in the long term.

Deep drainage and soil salt loads in the Queensland Murray - Darling Basin using soil chloride: comparison of land uses

Increases in deep drainage below the root-zone can lead to secondary salinity. Few data were available for drainage under dryland cropping and pastures in the Queensland Murray–Darling Basin (QMDB) before this study. Modelled estimates were available; however, without measured drainage these could not be validated. Soil chloride (Cl) mass-balance was used to provide an extensive survey of deep drainage. The method is ‘backward-looking’ and can detect low rates of drainage over longer times. Soil Cl and other soil properties were collated for a number of soils, mostly Vertosols and Sodosols, for paired native vegetation, cropped and sometimes pasture sites, from historical data and new soil sampling. Large amounts of salt and Cl had accumulated under native vegetation (Cl mean 25 t/ha, range 6–54, in 2.4 m depth), due to low rates of drainage. Steady-state Cl balances for native vegetation gave average drainage of 1.2 mm/year at wetter, eastern sites and 0.3 mm/year for Sodosols and Grey Vertosols in drier, western areas. Chloride profiles were mostly of a shape indicating matrix/piston flow. One site (Hermitage fallow trial) appeared to be affected by diffusion of Cl to a watertable. The Cl profiles from 14 longer term cropping sites (18–70 years), mainly used for winter cropping/summer fallow, indicate: (i) large losses of Cl since clearing (mean 50%, range 13-85% for 0–1.5 m soil); and (ii) drainage rates from transient Cl balance are a relatively low percentage of rainfall but are considerably higher than under native vegetation. Drainage averaged 8 mm/year and ranged from 2 to 18 mm/year. This variation is partly explained by rainfall (R2 = 0.63) (500–730 mm/year) and soil plant-available water capacity (R2 = 0.77) (80–300 mm). Deep drainage increases with increasing rainfall and with decreasing available water capacity. Drainage under pasture was less than under cropping but greater than under native vegetation. The deep drainage water (leachate) was of poor quality and will increase salinity if added to good quality groundwater. Leachate at nine sites was too saline to be used (undiluted) for irrigation (&gt;2500 mg Cl/L) and was marginal at the remainder of sites (~800 mg Cl/L). Cropping areas in the QMDB have the precursors for secondary salinity development—high salt loads and an increase in drainage after clearing. The Vertosols and Sodosols studied occur in 90% of croplands in the QMDB. Salinisation will depend on the properties of the underlying regolith and groundwater systems.

Modelling shows that the high rates of deep drainage in parts of the Goondoola Basin in semi-arid Queensland can be reduced with changes to the farming systems

Clearing native vegetation and introducing crops and pastures may increase deep drainage and result in dryland salinity. In south-west Queensland, native vegetation of the Goondoola Basin has been substantially cleared for cropping and pastoral activities, resulting in shallow groundwater and localised salinity. Simulation modelling was used to estimate the water balance of a range of vegetation and soil types. Six soils were studied, with plant-available water capacity (PAWC) of 71 mm (a Kandosol) to 198 mm (a Vertosol) for 1200 mm depth. Vegetation types were annual wheat, opportunity cropping, and perennial pastures in poor and good condition, and high quality perennial pasture with deep roots growing on deep (2400 mm) variants of the 6 soil types. Opportunity cropping did not reduce deep drainage. Substantial differences were found in long-term average deep drainage (mm/year) between wheat crops and pastures for all soil types. The differences in deep drainage between wheat cropping and pasture in good condition were greatest for the 2 Kandosols, which had the lowest PAWC (34 and 21 mm/year less deep drainage, reductions of 53% and 62%, respectively), and a Vertosol with intermediate PAWC (23 mm/year less deep drainage). A Chromosol and a Dermosol with intermediate PAWC had smaller reductions in deep drainage (14 and 11 mm/year, respectively). In the case of a Vertosol with high PAWC (198 mm), deep drainage was negligible with all pastures. Due to increased infiltration and reduced soil evaporation, more deep drainage was simulated with pasture in good condition than pastures in poor condition, especially for 2 Kandosols. Pasture with deep roots (2400 mm) growing on deep variants (2400 mm) of the 6 soils had lower rates of deep drainage than the other pastures. Simulated deep drainage and other components of the water balance were in good agreement with field measurements and expectations. These results indicate that large reductions in deep drainage can be achieved in the Goondoola Basin by replacing cropping with pastoral activities. Kandosol soils used for wheat cropping should be the primary target for land use change.

Soil properties and their relationships with crop productivity after 30 years of different fertilization in the Indian Himalayas

We analyzed results of a long-term (30 year) experiment under rainfed soybean-wheat rotation to determine the effects of differences in fertilization on selected soil properties and to investigate the relationship between these properties with crop yield. The results showed that addition of farmyard manure (FYM) with N or NPK fertilizers increased soil organic carbon (SOC) content. FYM application reduced bulk density. Mean weight diameter (MWD), plant available water capacity (PAWC) and water transmission characteristics of soil were highest in the NPK + FYM treatment and lowest in the unfertilized treatments, with the mineral fertilizer treatments showing intermediate results. Combined use of NPK and FYM increased total soil N, Olsen-P, exchangeable Ca2+ and exchangeable Mg2+ by ∼31, 13, 40 and 94%, respectively, in the 0–15 cm soil layer compared to application of NPK. Mean productivity of soybean-wheat system was positively correlated with total SOC stock, TSN stock, Ca2+, Mg2+, NH4OAc-K, MWD, PAWC, and dehydrogenase activity, and negatively related with soil bulk density. Thus, the changes in soil physical and biological properties should also be considered along with soil chemical properties/nutrient status to construe yield sustainability of other long-term field experimental data.

Estimation of water retention and availability in soils of Rio Grande do Sul

Dispersed information on water retention and availability in soils may be compiled in databases to generate pedotransfer functions. The objectives of this study were: to generate pedotransfer functions to estimate soil water retention based on easily measurable soil properties; to evaluate the efficiency of existing pedotransfer functions for different geographical regions for the estimation of water retention in soils of Rio Grande do Sul (RS); and to estimate plant-available water capacity based on soil particle-size distribution. Two databases were set up for soil properties, including water retention: one based on literature data (725 entries) and the other with soil data from an irrigation scheduling and management system (239 entries). From the literature database, pedotransfer functions were generated, nine pedofunctions available in the literature were evaluated and the plant-available water capacity was calculated. The coefficient of determination of some pedotransfer functions ranged from 0.56 to 0.66. Pedotransfer functions generated based on soils from other regions were not appropriate for estimating the water retention for RS soils. The plant-available water content varied with soil texture classes, from 0.089 kg kg-1 for the sand class to 0.191 kg kg-1 for the silty clay class. These variations were more related to sand and silt than to clay content. The soils with a greater silt/clay ratio, which were less weathered and with a greater quantity of smectite clay minerals, had high water retention and plant-available water capacity.

Soil chloride and deep drainage responses to land clearing for cropping at seven sites in central Queensland, northern Australia

Summary Soil cores were taken at seven paired sites (native vegetation and adjacent dryland cropping on cracking clay soils) which had been cropped for 10–65 years in the Fitzroy Basin in central Queensland, northern Australia. Levels of soil chloride (Cl − ) and nitrate nitrogen (NO 3 –N) were determined in 0.3 m increments to a depth of 5 m where possible. The amounts of Cl − in the soil (0–1.5 m depth) under native vegetation were generally high (10–23 t ha −1 at six of the seven sites). The amounts of Cl − that had leached below 1.5 m depth during dryland cropping varied from 2.2 to 16.8 t ha −1 or 19–91% of the original totals at 0–1.5 m. Leaching of salt from the crop rooting zone in combination with higher rates of deep drainage can lead to outbreaks of soil salinisation but can also increase the soil plant available water capacity (PAWC). NO 3 –N had also been leached below crop rooting depth at three sites. Such leaching not only contaminates the groundwater but also wastes crop nutrients. The transient chloride mass balance approach was used to determine mean annual rates of deep drainage below crop rooting depth (1.5 m). At all seven sites annual deep drainage was low under native vegetation (0.2–1.7 mm yr −1 ) but increased under dryland cropping (1.6–27.5 mm yr −1 ). Drainage losses showed an inverse relationship with plant available water content (PAWC). Drainage losses waste the limited supply of water available for dryland cropping but can be reduced by practising opportunity cropping or by growing ley (temporary) pastures in rotation with annual crops.

Plant available soil water at sowing in Mediterranean environments—Is it a useful criterion to aid nitrogen fertiliser and sowing decisions?

In regions where rainfall is low and variable, water stored in the soil profile prior to sowing can alter yield expectation and hence management decisions. Thus, wheat farmers in Mediterranean regions may be able to benefit from knowing the amount of soil water at sowing by optimising their nitrogen (N) fertiliser management and by deciding on whether or not to sow a crop. We used the ASPIM-Nwheat model to explore how levels of plant available soil water (PAW) at sowing, N fertiliser rate, soil, site and season-type (below or above median rainfall) affected wheat yields at sites in the Mediterranean area of southwest Australia. Overall, the greatest influence on yield potential and the consequent N fertilisation requirement was season-type. The additional yield per mm PAW at sowing was generally higher in seasons with below median rainfall, except when yields were severely water-limited by below median rainfall of <222 mm combined with <40 mm PAW at sowing on light clay soil with 109 mm plant available water capacity (PAWC). Sowing was generally warranted; only on light clay soil with <10 mm PAW at sowing and below median rainfall of <222 mm was there an opportunity for a conditional sowing strategy. Scope for varying N fertiliser rates with PAW at sowing was limited to soils with higher PAWC (109 and 130 mm, respectively) in below median rainfall seasons at the wetter site (295 mm mean seasonal rainfall), and in both season-types at the drier site (225 mm mean seasonal rainfall). Only in these combinations, soil water at sowing modified the optimal N fertiliser rate for maximum average yield resulting in significant interactions between PAW at sowing and N fertiliser rates. Similar interactions were found for a site in the Mediterranean Basin and a site in the eastern Australian subtropics on soil with high PAWC (183 and 276 mm, respectively). In contrast, there was no benefit from modifying crop management based on PAW at sowing on soil with low PAWC (i.e. sandy soil) and/or under conditions of high in-season rainfall. The conditional N management approach becomes more viable as the proportion of water stored in the soil prior to sowing increases relative to total crop water use and as the PAWC of the soil increases. Knowledge of PAW at sowing × N fertiliser rate interactions in a particular soil × site × season-type context can help to identify sites where a more targeted N management dependent on amounts of PAW at sowing is potentially profitable.

Use of indicators and pore volume-function characteristics to quantify soil physical quality

The Dexter “ S-value” ( S gi) is a promising new indicator of soil physical quality (SPQ), but it is not well tested against established indicators, such as relative field capacity (RFC), plant-available water capacity (PAWC), air capacity (AC), macroporosity ( P MAC), bulk density (BD), organic carbon content (OC), and structural stability index (SI). Furthermore, all SPQ indicators are direct or indirect expressions of pore volume and/or pore function, but optimal pore volume-function characteristics have not been identified. The objectives of this study were to: i) compare S gi to the other seven indicators for a range of rigid to moderately expansive soils and artificial porous media; ii) use the indicators to propose an optimal pore volume distribution and soil water release curve; and iii) assess the SPQ of a compost-amended soil using indicators, pore volume distributions and water release curves. The indicators were measured in the laboratory on intact soil cores and grab samples collected from 13 soil-management combinations. Soil texture included clay loam, sandy clay loam, loam, sandy loam and sand; management included virgin soil, no-till cropping and mouldboard plough cropping. Also included were two artificial media consisting of glass beads and builders sand. Pore volume distributions and water release curves were determined by fitting the van Genuchten function to desorption data obtained from the soil cores and grab samples. The S gi indicator gave correct SPQ designations for the structured loamy soils, but erroneous designations ( S gi > 0.035) for the structureless sands, glass beads and builders sand. The indicators suggested that four of the 13 soil-management combinations had optimal overall SPQ, and these combinations were used to define an optimal pore volume distribution and water release curve. Non-optimal soils with poor aeration (RFC > 0.7, AC < 0.14 m 3 m − 3 ) and poor structure ( P MAC < 0.07, SI < 7%, S gi < 0.035) had greater proportions of small pores and excessive water retention relative to optimal soils, while droughty soils (RFC < 0.6, PAWC ≤ 0.15 m 3 m − 3 ) had lower proportions of small pores and insufficient water retention. The indicators, pore volume distributions and release curves showed that adding 75 t ha − 1 compost improved the SPQ and maize yield of a clay loam soil, but addition of 300 t ha − 1 compost was required to achieve optimal SPQ and maximum measured yield. It was concluded that the S gi indicator should be used judiciously and in concert with other indicators for assessing SPQ; and that the suite of eight indicators used in conjunction with an optimal pore volume distribution and water release curve are effective for quantifying the physical quality of rigid to moderately expansive agricultural soils.

Advances in precision agriculture in south-eastern Australia. IV. Spatial variability in plant-available water capacity of soil and its relationship with yield in site-specific management zones

Spatial variability in grain yield can arise from variation in many different soil and terrain properties. Identification of important sources of variation that bear significant relationship with grain yield can help achieve more effective site-specific management. This study had three aims: (i) a geostatistical description/modelling of the paddock-level spatial structure in variability of plant-available water capacity (PAWC) and related soil properties, (ii) to determine optimal number of management zones in the paddock, and (iii) to assess if the variability in PAWC and related soil properties is significantly associated with the variability in grain yield across the management zones. Particle size distribution, bulk density (BD), field capacity (FC), permanent wilting point (PWP), and soil water content (SWC) at sowing were measured at 4 soil depths (to 0.60 m) at 50 representative spatial sampling locations across a paddock near Birchip (Victoria). PAWC and plant-available water at sowing (PAWs) were derived from these data. Moderate to strong spatial dependence across the paddock was observed. The magnitude of the structural variation and of range varied widely across different soil properties and depths. The south-east edge and the central areas of the paddock had higher clay content, FC, PWP, PAWC, and lower PAWs. The paddock was divided into 6 potential management zones using combined header yield and normalised difference vegetation index (NDVI). The adequacy of zoning was evaluated using relative variability (RV) of header yield and soil properties. The mean RV for 3 zones differed little from that of 6 management zones for header yield and for each measured soil property, indicating division of the paddock into 3 zones to be adequate. The results from residual maximum likelihood (ReML) analysis showed that low yield zones had significantly higher clay content, FC, PWP, SWC, and PAWC and significantly lower PAWs than both medium and high yield zones. The mean FC, PWP, and PAWC in the low yield zones were, respectively, 25%, 26%, and 28% higher, and PAWs 36% lower than their corresponding values in the high yield zones. Linear regression analysis indicated that 59–96% of the observed variation in grain yield across management zones could be explained by variation in PWP. The practical implications of these results are discussed.

Improving estimates of water-limited yield of wheat by accounting for soil type and within-season rainfall

Rainfall is the main driver of potential yield in the dryland cropping environment of Australia. Rainfall-based empirically derived models such as that proposed by French and Schultz (1984) (F&amp;S) can be used to determine an upper limit of water-limited potential yield, but F&amp;S often overestimates actual yield as it does not account for rainfall distribution, runoff, drainage, or stored soil water. More complex models are available to predict potential yield more accurately; however, farmers and advisors favour easy-to-use-methods to estimate potential yield. To derive a simple yet accurate method for estimating potential yield, several adjustments to F&amp;S were evaluated: (1) accounting for stored soil water at sowing, (2) varying the value of the intercept between yield and growing-season rainfall (GSR), (3) varying the water-use efficiency of the crops (WUE) according to soil type, and (4) adjustments to GSR depending on soil plant-available water capacity (PAWC). The water-limited potential yields predicted from these methods were compared with simulations from the daily time-step simulation model APSIM and observed wheat yields from 146 dryland wheat crops, managed to water-limited potential yield, covering the 1996–2006 seasons in the Mediterranean-type growing environments of Australia. The original F&amp;S method overestimated actual yields, particularly at high rainfall (GSR &gt; 220 mm) when PAWC was low, and underestimated yields at low rainfall (GSR &lt; 220 mm). Significant improvements to the F&amp;S were achieved with a few simple adjustments. With the addition of a variable intercept (dependent upon GSR), accounting for stored soil water at the start of the season and placing a cap on GSR that is a function of the soil PAWC, the predictive performance (RMSE 624 kg/ha) was similar to that gained with the daily time-step model APSIM (RMSE 419 kg/ha). The improved method gave more realistic estimates of water-limited potential yield, particularly at low and high rainfall and on soils of low PAWC.

Quantifying the benefits of accounting for yield potential in spatially and seasonally responsive nutrient management in a Mediterranean climate

Crop yield potential is a chief determinant of nutrient requirements, but there is little objective information available on the gains in profitability that can be made by accounting for the influences of soil type and season on yield potential when making fertiliser decisions. We conducted such an assessment using crop growth simulation coupled to nutrient response curves for wheat-growing at 4 locations in the low-medium rainfall zone of Western Australia. At each location, the yield potential was simulated on 10 soil types with plant-available water capacity (PAWC) ranging from 34 to 134 mm, which represent the major soils types in Western Australia. Soil survey maps were available to quantify soil type variability and the historical climate record (1974–2005) for seasonal variability. The benefits possible for fertiliser (NPK) management that takes account of variation in crop yield potential due to season and soil type by having ‘perfect knowledge’ ranged from $2 to 40/ha. Seasonal variation was more important than soil type for the better soils (high PAWC), providing two-thirds of the benefit of perfect knowledge. On low PAWC soils, knowledge of soils and seasonal influences on yield potential were similar contributors to profit gains. An assessment of one yield forecasting system showed that about 50% of the maximum gains could be captured if seasons could be categorised as below, at, or above average at the time the fertiliser decision is made. In each catchment, 30–40% of fields showed scope for benefits in accounting for within-field variation in soil type due to large variation in PAWC, and therefore yield. Maximum profit gains and reductions in nutrient excess were greater in the low rainfall locations and also on the low PAWC soil types.

Optimal soil physical quality inferred through structural regression and parameter interactions

Soil physical quality (SPQ) is assessed by comparing values of “indicator” soil properties (e.g. bulk density, air capacity) to “ideal ranges” established in the literature. These ideal ranges may not be optimal for any particular soil or field site, however, as they are only “guidelines” based on broad soil types. The objective of this study was to determine if more relevant estimates of optimal indicator ranges can be obtained by using structural (Model II) regression to account for soil/site-specific indicator behaviour and interactions among the indicator parameters. The indicator parameters included relative water capacity (RWC), organic carbon content (OC), bulk density (BD), bulk soil air capacity (AC B), soil matrix air capacity (AC M), macroporosity (POR P), matrix porosity (POR M), field capacity (FC), plant-available water capacity (PAWC), and saturated hydraulic conductivity ( K S). The indicators were determined using intact soil cores collected from the top 10 cm of a uniform, cool, humid clay loam soil under maize–soybean cropping, continuous bluegrass sod, and virgin soil. The structural regression analysis was applied by regressing RWC, OC, AC B, AC M, POR P, POR M, FC, PAWC and K S against BD using the “least squares bisector” method. Structural regression predicted soil/site-specific optimal ranges of 3 ≤ OC ≤ 4 wt.%, 1.10 ≤ BD ≤ 1.23 Mg m − 3 , 0.16 ≤ AC B ≤ 0.22 m 3 m − 3 , 0.07 ≤ AC M ≤ 0.10 m 3 m − 3 , and 0.09 ≤ POR P ≤ 0.13 m 3 m − 3 , which are either narrower than the corresponding literature guidelines, or partially overlapping the guidelines. The regressions also provided information that is not currently available in the literature, including a possible optimal range for FC (0.30 ≤ FC ≤ 0.35 m 3 m − 3 ), and plausible “upper limits” to the optimal ranges for AC B, AC M and POR P (see above). Optimal ranges were not obtainable for POR M and PAWC because they were effectively constant among the cropping and sod treatments. A questionably high optimal range was obtained for K S (8 × 10 − 2 ≤ K S ≤ 7 × 10 − 1 cm s − 1 ), and this was attributed to the perturbing effects of large, highly water-conductive macropores. The overall SPQ at the field site was judged by structural regression to be “poor” for maize–soybean cropping, “good” for bluegrass sod, and “fair” for virgin soil. It was concluded that structural regression is useful for determining soil/site-specific optimal ranges for SPQ indicators.

Estimating Plant‐Available Water Using the Simple Inverse Yield Model for Claypan Landscapes

Plant‐available water (PAW) is one of the fundamental soil factors affecting crop yield, yet quantitative determination of plant‐available water capacity (PAWc) at a field scale has been challenging. A simple inverse yield model (SIYM) has been devised and shown to be successful in estimating PAWc at a field scale for well‐drained soils by matching simulated corn (Zea mays L.) yield with measured yield. For other soils, however, SIYM is yet to be tested. Our objective was to evaluate SIYM performance in estimating PAWc for poorly‐drained claypan‐soil landscapes. Soil PAWc to a depth of 1.2 m (PAW1.2) was measured at 19 and 18 sampling locations for two claypan‐soil fields, Fields 1 and 2, respectively. Corn yield maps of the two fields (nine site‐years between 1993 and 2003) were used with the model to estimate PAWc Yield reduction associated with low precipitation and high vapor‐pressure deficit during corn reproductive stages, and large yield variation in dry years were indications available water was the main yield‐limiting factor. The regression r2 values between SIYM‐estimated PAWc and the measured PAW1.2 were 0.43 for Field 1 and 0.31 for Field 2 with estimating errors of 18 and 50 mm, respectively. In Field 2, SIYM estimated markedly lower PAWc compared with the PAW1.2 at the most‐eroded backslope areas, where claypan characteristics were most prevalent. The SIYM‐PAWc estimates would be more informative in assessing soil productivity because they are based on crop‐water relations and not solely on soil texture.

Tillage and irrigation effects on crop yields and soil properties under the rice–wheat system in the Indian Himalayas

Conservation tillage systems generally improve soil organic C (SOC), plant available water capacity (PAWC), aggregation and soil water transmission. A field experiment was conducted for 4 years (2001–2002 to 2004–2005) to study tillage (conventional tillage (CT) and zero tillage (ZT)) systems. The selected irrigation treatments were at four levels (I 1: pre-sowing (PS), I 2: PS + active tillering (AT)/crown root initiation (CRI), I 3: PS + AT/CRI + panicle initiation (PI)/flowering (FL), and I 4: PS + AT/CRI + PI/FL + grain filling (GF)), applied at the critical growth stages on rice ( Oryza sativa L.) and wheat ( Triticum aestivum L.). Their effects on direct seeded rice productivity and soil properties (SOC and selected physical properties) after rice and wheat harvest were investigated. Soil organic C contents after rice and wheat harvest in the 0–15 cm soil depth were higher under ZT than under CT. Soil organic C increased significantly with I 2 over I 1 for both crops and with I 4 over I 2 for the wheat crop. The PAWC was significantly higher with ZT than CT. Zero tilled and frequently irrigated plots showed enhanced infiltration characteristics (infiltration rate, cumulative infiltration and sorptivity) and saturated hydraulic conductivity. Both direct seeded rice and wheat yields were not significantly different in the plots under ZT and CT. There was a significant increase in both rice and wheat yields in the plots under I 2 over I 1. However, water use efficiency between irrigation treatments was not significantly different. Hence, under direct seeded rice–wheat system in a sandy clay loam soil of the sub-temperate Indian Himalayas, farmers may adopt ZT with two irrigations in each crop for optimum resource conservation.

Optimising maize plant population and irrigation strategies on the Darling Downs using the APSIM crop simulation model

Optimum plant population and irrigation strategies for maize grown in the Dalby district of the Darling Downs in Queensland, Australia, were investigated using the APSIM crop simulation model. After testing the model against three seasons of experimental data, simulation experiments using different irrigation strategies were conducted across a range of plant populations ranging from 20 000 to 80 000 plants/ha, on two soil types with plant available water capacities (PAWC) of 146 mm and 220 mm. All soil type × plant population × irrigation strategy scenarios were simulated using the historical climate record for Dalby from 1889 to 2004, in order to obtain long-term average yield and gross margins (LGM) for each scenario. Soil water was reset to two-thirds of PAWC at sowing in each year. Plant populations required to achieve maximum LGMs ranged from 50 000 to 80 000 plants/ha across the range of scenarios, and were higher than currently recommended by district agronomists for partially irrigated maize. The use of higher plant populations increased season-to-season variability in grain yield and gross margins and may not be a suitable strategy for growers who do not want to increase their risk of crop failure. Partially irrigated maize achieved substantially higher gross margins in years where a positive Southern Oscillation Index phase was recorded in August, and the use of higher plant populations in such years also increased long-term profitability, but also increased the risk of crop failure. Economic gains were achieved by varying the timing and amount of irrigation within a limited available irrigation volume, with a single 100 mm irrigation giving greater LGMs than two 50 mm irrigation events on both soil types, when the irrigation events were scheduled to fill a soil water deficit equal to the effective irrigation volume. However, under full irrigation the use of smaller irrigation volumes increased LGMs on the 146 mm PAWC soil, demonstrating the importance of timely irrigation scheduling on low PAWC soils.

Long-term effects of organics, fertilizers and cropping systems on soil physical productivity evaluated using a single value index (NLWR)

Effect of different cropping systems, viz. maize–wheat (M–W), rice–wheat (R–W), soybean–wheat (S–W), and perennial grasses (guinea grass and setaria grass), in vogue since 6–32 years, and long-term use of chemical fertilizers (N, NP, NPK and NPK + lime) and organic materials (FYM, wheat straw, lantana biomass) on physical productivity of medium-textured (silt loam and silty clay loams) soils was investigated using non-limiting water range (NLWR) as the soil physical index. Higher the NLWR better is the soil physical condition for crop growth. The sources of N, P and K were urea, single superphosphate and muriate of potash, respectively. The NLWR was highest in S–W (16.8%), followed by grasses (14.4–15.6%) and M–W (13.1–15.4%), and lowest in R–W (7.5–11.0%). Under M–W system (32 years), NLWR was highest in NPK (13.1%), followed by NP (12.2%), NPK + lime (9.4%) and control (9.0%), and lowest in N (7.7%). Application of organics increased the NLWR in both M–W and R–W (6–18 years) systems; the NLWR values with and without organics were 18.0 and 17.1% in M–W, and 14.1–15.9 and 15.7–17.2% in R–W system. The NLWR was linearly, significantly and positively correlated with wheat grain yield ( r = 0.646**, −0.706**). The NLWR:PAWC (plant available water capacity) ratio (higher the ratio, better is the soil physical condition), which was 0.58 in control, decreased with N (0.49) but increased with NP (0.72) and NPK application (0.77); use of organics further improved the ratio. The NLWR:PAWC ratio was highest in S–W (0.97), followed by grasses (0.88–0.91), M–W (0.77–0.86) and R–W (0.54–0.68) system. Thus, long-term use of urea alone deteriorated, while NPK at recommended rates improved soil physical productivity over the control of no fertilizer application; the effect further improved when NPK were combined with organic sources. Among different cropping systems, the soil physical productivity followed the order: S–W > grasses > M–W > R–W system.

Estimating Plant‐Available Water Capacity for Claypan Landscapes Using Apparent Electrical Conductivity

Information on plant‐available water (PAW) capacity (PAWc) variation within a field is useful for site‐specific management. For claypan soils, established relationships between soil apparent electrical conductivity (ECa) and topsoil thickness suggested the hypothesis that profile PAWc could be estimated by assuming a two‐layer soil composition, a silt loam topsoil layer and a silty clay sublayer, with known PAW fraction values for each layer. Objectives were (i) to investigate the direct relationships between ECa and the upper and lower limits of PAWc, and (ii) to test the previously stated hypothesis. Nineteen and 18 soil profile samples were taken from two Missouri claypan fields in October 2005. The lower limit of PAWc was determined at −1500 kPa soil water pressure. Samples were taken again from the same locations in March 2006 to determine the upper limit of PAWc Calculations were on a 1.2‐m basis. The direct relationship between ECa−1 and profile PAW (PAW1.2) was significant, with regression r2 values of 0.67 and 0.87 and RMSEs of 30 and 20 mm for Fields 1 and 2, respectively. The RMSEs for two‐layer‐estimated PAW1.2 were 14 and 16 mm for Fields 1 and 2, respectively, or 7.6 and 8.6% of the respective mean measured PAW1.2 With the two‐layer approach, some underestimates of PAW1.2 resulted from underestimation of topsoil thickness, whereas overestimates were attributed to soil horizons being short of field capacity at sampling due to slow recharge. The resulting field‐scale PAWc information is useful in site‐specific decision making for soil and water management.