Biochar as a strategy to manage plant diseases caused by pathogens inhabiting the soil: a critical review

Phytoparasitica - The current agricultural scenario faces a range of challenges, with phytosanitary ones being paramount. In most cases, plant diseases are treated with chemicals; however, they...     

Yield of rice under water and soil salinity risks in farmers’ fields in northeast Thailand

This study focused on the variability of rice yield under water and soil salinity risks in farmers’ fields in northeast Thailand. A rice plot was monitored in 24, 16 and 11 farmers’ fields during the rice seasons 2005, 2006 and 2007, respectively. The results emphasized that few plots were continuously submerged during the 2005 season, when rainfall was low. Drought significantly affected the rice yield, yield components and the internal efficiency (IE) of the absorbed nutrients, while slight soil salinity had the only significant effect of increasing the IE of potassium (IEK). In the very rainy 2006 and 2007 seasons, most fields were continuously submerged, and in contrast to 2005, the slight soil salinity that was recorded had significant effects not only on IEK, but also on rice yield, spikelet sterility and 1000-grain weight. The yield decrease due to drought was about 87% and that due to salinity was 20%. When neither salinity nor water were limiting, the soil nutrient supply was high enough to achieve about 80% of the maximum yield reported in the literature for the rice cultivar in this area. As both drought and salinity risks are hardly avoided by the current farmers’ management they should be taken into account in the way technical recommendations to farmers are formulated.     

Soil cover and landscape evolution in the Senegal floodplain: a review and synthesis of processes and interactions during the late Holocene

The aim of this paper is to summarize the successive biological, pedological, hydrodynamic, geomorphological and geochemical processes that have occurred in the Senegal valley, and to describe how their interactions during the late Holocene conditioned soil cover formation and landscape evolution. Potential acidity accumulated as pyrite in the floodplain sediment during the last marine transgressions, and was expressed during the following regressions because of oxidation. Soil acidification was mitigated by the soil buffer capacity and by the interaction with the slightly alkaline continental freshwater of the river. Two pedogenetic transformation processes that resulted from the succession of acidic and neutral conditions, transformed unripe muds with pyrite (potential Acid Sulphate soil) to actual Acid Sulphate soils, and then to Vertisols. Geochemical modelling with PHREEQC quantitatively confirmed the feasibility of the processes involved. These two pedogenetic processes also controlled two independent salt accumulation processes: (i) the transformation of shell accumulation beds into gypsum layers and (ii) aeolian deflation and formation of clay dunes. The study shows that pedogenetic effects on alluvial material can lead to contrasting horizons that cannot be explained stratigraphically. It also shows that the presence of saline areas in the Senegal middle valley results from much more complex processes than a simple salt deposition during transgressions.     

Experimental and numerical study of water flow in soil under irrigation in northern Senegal: evidence of air entrapment

Irrigation by surge flooding does not always wet the soils thoroughly, and we have investigated the reasons for this on an irrigated plot in northern Senegal by monitoring the water budget during a rice cropping season (100 days). The amount of water added during each irrigation event was measured, and evapotranspiration and infiltration were estimated with lysimeters and Muntz infiltration rings, respectively. At the same time, piezometric levels, neutron probe values and water tension data were recorded at two stations in the plot. These measurements showed unusual results: infiltration rate was less than 1 × 10^?6 mm s^?1 (less than 0.1 mm a day), there was a constant water deficit during the entire irrigation period, around 50 cm deep, and tensiometers at 40 cm reacted very slowly to water infiltration. The water fluxes in the vadose zone derived from these data showed clearly a discrepancy between fluxes calculated from hydraulic gradients and fluxes calculated from mass conservation. The hydraulic gradients suggested a zero flux plane at 40 cm below the surface, but the calculated values of the fluxes overestimated by several orders of magnitude the infiltration rates determined on the plot, whereas fluxes determined from mass conservation matched far better. These results show that air was entrapped between the shallow water table and the wetting front, and this inhibited water infiltration. Modelling water flow down the soil profile with a computer program for simulating one‐dimensional water movement (Hydrus) confirmed that single‐phase models cannot describe imbibition in this situation. Simple infiltration models based on a modified Green–Ampt equation accounting for air compression and air counter‐flow, however, fit experimental infiltration data much better. We demonstrated that where surge flooding is associated with a shallow water table, as in many large irrigation schemes, one must take into account the presence of air to quantify the flow of water into the soil.     

Characterization of Pterocarpus macrocarpus (pradoo wood) biochar and its effect on the retention properties of sandy soils in Northeast Thailand

Pradoo wood biochar has been tested in order to explore sustainable solutions to the development of agriculture on poor sandy soils in marginal areas in Northeast Thailand. Some basic physicochemical properties of biochar, for example pore size distribution, cation exchange capacity (CEC), specific surface area (SSA), and water and nutrient adsorption, were determined and compared to soil properties in order to determine appropriate biochar application to soil. Pradoo wood biochar showed important adsorption properties with high SSA, CEC and nutrient adsorption. The water retention properties were also improved on the dry end of the water retention curve. Phosphorous and ammonium adsorption–desorption isotherms were established and their respective affinity for the biochar surface was quantified, by the means of a retention index and thermodynamical parameters. We found that despite excellent retention properties, biochar needs to be added in large amounts (between 10 and 70 kg m⁻²) to soil to be able to modify noticeably the resulting soil properties.