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Conservation agriculture (CA), with reduced tillage, permanent soil cover and diversified cropping systems, is advocated in southern Africa to improve soil quality, reduce input costs and mitigate climate-induced risks. However, improvements in terms of yield and soil organic carbon (SOC) under CA are slow and variable and many small-scale farmers are unable to buffer themselves against potential short-term financial losses. In this study we examined the effects of CA-related management practices on SOC sequestration and productivity at two medium-term sites on a sandy soil (eight year trial) and clay soil (six years) in maize producing areas of South Africa. Using field data, current input costs and market prices for crops, we calculated the gross margin for each system. Treatments compared conventional ploughing under maize monoculture with reduced tillage, intercropping and crop rotation. On the clay soil, SOC was increased under reduced tillage (57.6 t C ha −1 ) compared to conventional tillage (54.9 t C ha −1 ) while there was no difference for the sandy soil (19.7 t C ha −1 average across treatments). Profitability was most strongly influenced by seasonal rainfall, but was higher on the sandy soil than the clay soil, with an average gross margin of R11,344 ha −1 and R5,686 ha −1 , respectively. This study has demonstrated that while certain CA practices can create site-specific benefits for farmers, it is highly dependent on local weather and soil conditions. For the clay soil an additional payment scheme would be required to reward farmers in southern Africa for C-sequestration to make CA profitable and achieve increased C-mitigation through soil sequestration. [ABSTRACT FROM AUTHOR]

The determination of soil hydraulic properties is laborious and expensive, especially in large-scale applications. One often used substitute for measured hydraulic properties are pedotransfer function (PTFs) estimates. Most PTFs, however, are statistical models that tend to produce biased results for data outside their –often limited– calibration databases. In addition, most PTFs have been established on data derived from temperate regions causing the question whether such models are applicable to soils in tropical regions. This work aimed to evaluate the performance of the Splintex PTF to predict the hydraulic functions for sandy and clayey soils from several tropical and subtropical Brazilian datasets. Splintex is somewhat unique in that it is based on physical principles using a modification of the Arya-Paris method while allowing the estimation of van Genuchten parameters from limited data. In addition, Splintex has an option to include measured soil water retention points, in principle allowing it to produce accurate estimates for a variety of soils. Estimates by Splintex were compared with the empirical Rosetta PTF, which also has an option to use one (or two) retention points. Estimates by both PTFs were compared to observed retention data and field capacity, available water capacity, hydraulic conductivity, and diffusivity using metrics such as Pearson correlation ( r ), mean absolute error (MAE) and root mean square error (RMSE). Both Splintex and Rosetta yielded similar results and sometimes produced significant biases in estimated quantities. In the majority of cases it appears that Splintex produced somewhat better estimates than the 2001 version of Rosetta , indicating that Splintex is a viable, physically-based, alternative to estimating hydraulic properties. [ABSTRACT FROM AUTHOR]

Abstract: In the humid tropics, conversion of forests into croplands has the potential to alter the flux of dissolved organic matter (DOM) and influence C balance and N availability in soil. Sites located on a clay-rich Ultisol in Thailand and a highly acidic and sandy Ultisol in Indonesia were used to quantitatively evaluate effects of land use change on fluxes of dissolved organic C (DOC) and N (DON) in cropland and adjacent forest. The DOC flux from surface soil horizons was lower in cropland (poultry manure-amended) than in the forest on the sandy Ultisol (Indonesia), while DON flux increased in cropland due to release of DOM enriched in N from the decomposed SOM and manure. In contrast, DOC and DON fluxes in cropland (non-fertilized) and forest on the clay-rich Ultisol (Thailand) were consistently low due to high adsorption of DOM and rapid DON mineralization. Total dissolved N (TDN) in soil solution was dominated by nitrate for both cropland soils; DON represented a major fraction of TDN in the sandy cropland soil due to lower DOM adsorption capacity and slower DON mineralization. The conversion of forests to croplands could enhance SOM decomposition and produce DOM enriched in N, but leaching losses of DOC and DON were small under low-input agriculture in tropical region. The effects of agricultural use on DOC and DON fluxes could vary depending on application rates of manure and adsorption and mineralization capacities of soils. [Copyright &y& Elsevier]

Abstract: The fragmentation of larger soil aggregates to smaller fractions is a fundamental physical process that may occur naturally or be imposed. Soils that naturally re-structure themselves following structural damage from a homogeneous mass into a heterogeneous matrix of fine, granular and stable aggregates and pores are of great benefit. A key driving force is shrink–swell processes, particularly wet–dry cycles. Models of dynamic fragmentation have been devised. A promising model based on a threshold limiting size distribution of aggregates, with temporal- and scale-invariance, has been developed recently. We sought to assess this model with soil fragmentation test data. We sampled 12 soils from a 2km transect (Bedfordshire, UK), subjected them to physical damage by shearing and kneading, and then imposed up to 3 or 5 wet–dry cycles. We measured the aggregate size distribution and mean weight diameter (MWD) of both the whole soil fraction and the fine-scale (<5mm) aggregate fraction, selected as a key threshold size, as they fragmented. A subset of four soils was examined in more detail to assess the model predictions. Clayey soils fragmented considerably more than sandy soils, and resulted in an accumulation of fine-scale aggregates. This suggested that fragmentation was induced by shrinkage and swelling stresses impacting upon the clay fraction and that the wet–dry cycles were not able to reverse the structural damage imposed on the sandy soils. In the clayey soils the form of the distribution of fine-scale aggregates showed clear signs of temporal invariance over the wet–dry cycles. In addition, as the cumulative distributions of the fine-scale aggregates followed a power law relationship, this suggested invariance in scale. In fragmenting soils, the MWD of the fine-scale aggregates was a convincing asymptote for the MWD of the whole soil fraction. We demonstrated temporal- and scale-invariance in clayey soils that fragment upon wetting and drying and the accumulation of a resistant set of fine-scale aggregates. Such soils are likely to be better-able to recover naturally from any physical damage to their structure and will require less remedial work, and hence a lower energy input, compared to non-fragmenting sandy soils. [Copyright &y& Elsevier]

Abstract: Subsoil compaction is a recognised threat in the European Soil Strategy and knowledge concerning the vulnerability of subsoils to compaction in Europe and the Netherlands is information required for the determination of priority areas (or risk areas) in the future European Soil Framework Directive. In Europe two risk assessment methods (RAM) are used in more than one country to determine the subsoil compaction risk. The first one (RAM-A) initially determines the susceptibility of soils to compaction as a function of texture and packing density. In the second step the vulnerability to compaction is determined as a function of susceptibility and climate. The second RAM (RAM-B) is a mechanistic model in which the soil mechanical strength determines whether a subsoil is susceptible to compaction. The RAMs are used to produce maps presenting the susceptibility and vulnerability to compaction of Dutch subsoils (RAM-A) and maps with the compression strength and maximal allowable wheel load of a Terra Tire to prevent compaction of Dutch subsoils (RAM-B). Both RAMs have weaknesses. RAM-A is an expert model and can be rather arbitrarily with results that are not in agreement with our experience. RAM-B suffers from lack of good data and probably underestimates subsoil strengths. Results of both RAMs are compared to each other and to a map showing the probability that the subsoil is already overcompacted. This probability map is based on bulk density data in the Dutch Soil Database (BIS). There is a good match between the results of both RAMs, however, the match with the probability map (presumed to be “reality”) is not good. In both RAMs sand and loamy sand soils are indicated as more vulnerable than clay soils, while in the probability map sand subsoils suffer less of subsoil compaction than clay soils. Of concern is that, according to the probability map, about 50% of the most productive and fertile soils of The Netherlands have overcompacted subsoils. [Copyright &y& Elsevier]

Abstract: The study investigated the performance of three model tillage tools (tines). The experimental tillages were made from flat 8mm plain carbon steel. They were designated T1, T5, and T20, corresponding to tine widths of 1, 5, and 20cm respectively. Experiments were carried out in a soil bin filled with sandy clay loam soil at average moisture content 11.5% (dry basis) and 600kPa average cone index. The plastic limit and liquid limit and plasticity index of the soil are 20%, 31% and 11% respectively. Tests were conducted at forward speeds of 0.28, 1.0, and 2.5m/s. Depths of operation considered were 35, 70, 150, 200 and 250mm. Draught measurements were made for the different tines and were also calculated using soil mechanics equation. There was reasonable agreement between measured and predicted draught forces. The effects of depth of operation on draught force of the tines were studied and evaluated. It was observed that draught increased at an increasing rate with depth; the relationship was a curvilinear one best fitted by exponential function. The soil disturbance created as a result was also evaluated and reported in this paper. The parameters used to define soil disturbance of a single tine were: ridge-to-ridge distance (RRD), maximum width of soil cut (WFS), maximum width of soil throw (TDW), after furrow depth (df), height of ridge (hr) and rupture distance (f). They all increased as the depth of operation of the tool increased but less proportionately. The critical depth of the tines was also estimated. The results of analysis of variance showed that tool type and operating depth significantly affected draught at 5% level of significance (p <0.05) and that, there was interaction between the two factors. [Copyright &y& Elsevier]

[Display omitted] • This is the first rigorous assessment of hydrologic soil group (HSG) classification. • The assumption used since 1955 for classification does not hold true. • HSG is governed by sealing and controlled by silt and organic matter. • The CN model works almost perfectly under a wide range conditions. • Spatial variability is pronounced and should not be ignored. The hydrologic soil group (HSG) is required for the determination of curve numbers (CN), which are used in a large number of hydrological models to calculate surface runoff during rain events of ungauged catchments. Despite the paramount importance of HSG, it is still based on an assumption made in 1955. We assessed HSG using 249 rainfall simulation experiments carried out in two sets of experiments examining seedbed conditions. One set analyzed the variation within a small (60 ha) catchment while the other examined 32 benchmark soils selected to cover the full range of arable soils on sloping land in temperate humid areas. The assumption made in 1955 that sandy and stony soils have a low HSG (A), while clayey soils of low permeability have a high HSG (D), cannot be supported. Instead, HSG appeared to be governed by surface sealing because CN increased with increasing silt content in the bulk soil and decreasing soil organic matter content in the fine earth fraction. An equation for calculating CN and, in turn, a HSG from these two parameters is provided. This equation also avoids classification-induced errors. The relation holds true for a wide range of rain properties (e.g., rain amount between 0.2 mm and 103 mm; event intensities between 2 mm h−1 and 80 mm h−1), slope gradients (2 %–24 %) and various soil properties (e. g., stone contents up to 40 %, clay contents up to 61 %, sand contents up to 87 %). The CN and, in turn, HSG remained constant throughout the events from the start of runoff until the end of the event, which was up to 49 h later in 32 cases. Autocorrelation within the catchment was strong and the full variation was found within 600 m. Even small catchments are, hence, heterogeneous and their runoff cannot be assigned to a single HSG or a single CN. Rejecting the assumption developed in 1955, which had not been verified since then, has important implications for hydrological modelling but also for the development, implementation and assessment of water conserving land use. [ABSTRACT FROM AUTHOR]

• Sandy soils reach moderate penetration resistance at field capacity. • In semi-arid climates and sandy soils, low penetration resistance occurs infrequently. • Penetration resistance in clay soils is below 10 MPa even during the dry season. • In seasonally dry climates, penetration resistance follows a beta distribution. • Wet season rainfall affects year-round penetration resistance distribution. In water-controlled environments, because rainfall is one of the primary hydroclimatic forcings, its variability can affect the dynamics of soil water content and other related processes such as soil penetration resistance (PR), plant growth, and food production. The objective of this study was to simulate the relative soil water content (s) and PR year-round considering semi-arid rainfall conditions for different soil classes. An experiment was conducted to obtain relative soil water content (s) and PR data in sandy soil and this information was combined with a data set of four soils from a similar experiment conducted on different soil texture classes. The relative soil water content and PR were simulated at a daily timescale using a stochastic soil water balance model for the five soils. The parameters of the PR - s model can be estimated from the soil texture and bulk density. Under rainfed conditions, there is a short window for tillage (plowing and cultivation) of sandy soils compared with clay soils. During the majority of a typical hydrological year, the PR reaches high values (greater than 5.0 MPa) imposing a moderate to high physical limitation for root growth in sandy soils. In the same soils, limiting PR is reached at s levels very close to the field capacity during the drying process. This indicates that an extra input of water through irrigation may be required during the establishment of crops with a well-developed root system in order to prevent impairments to root growth. The daily PR values modeled over the course of a hydrological year followed a beta distribution with shape parameters (α and β) which were strongly correlated with the frequency of rainfall events during the wet season (λ wet). The modeling framework presented here for simulating relative soil water content and soil penetration resistance can be applied to agricultural systems in order to prevent physical limitations on crop growth and to plan tillage management schedules. [ABSTRACT FROM AUTHOR]

• A two-year field experiment with rooting depth treatment was performed in clay and sandy soil. • Deep roots in maize contribute to increase N uptake and grain yield in sandy soil. • Increasing root length below 40 cm depth can reduce nitrate leaching in clay soil. Increasing rooting depth via subsoiling is suggested to increase nitrogen (N) uptake and reduce nitrate leaching in maize. In a two-year field experiment, this hypothesis was evaluated with two levels of N supply (120 and 240 kg N ha−1) in two soils with different leaching potential (infertile sandy soil and fertile clay soil). Maize rooting depth was limited to either the 0–20 cm (D20) or 20–40 cm (D40) soil layer using a nylon net (30 μm), or was not limited (control). There was no water stress during the growing seasons for both soils. Compared to the control, root length with the D20 and D40 treatments was longer in the 0–20 and 20–40 cm soil layers, respectively. However, total root length per plant differed less among the treatments. In clay soil, the D20 treatment did not affect N uptake, shoot biomass, or maize yield, but increased nitrate leaching. In sandy soil, the D20 treatment greatly reduced plant N accumulation, shoot biomass, and grain yield, while nitrate leaching was similar across the three treatments. It is concluded that deeper roots in maize contribute to increase N uptake and grain yield in sandy soil, but only help to reduce nitrate leaching in clay soil. [ABSTRACT FROM AUTHOR]