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Mean Weight Diameter (mean + weight_diameter)
Selected AbstractsSoil detachment and transport on field- and laboratory-scale interrill areas: erosion processes and the size-selectivity of eroded sedimentEARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2006O. Malam Issa Abstract Field- and laboratory-scale rainfall simulation experiments were carried out in an investigation of the temporal variability of erosion processes on interrill areas, and the effects of such variation upon sediment size characteristics. Poorly aggregated sandy soils from the semi-arid environment of Senegal, West Africa, were used on both a 40 m2 field plot and a 0·25 m2 laboratory plot; rainfall intensity for all experiments was 70 mm h,1 with a duration of 1 to 2 hours. Time-series measurements were made of the quantity and the size distribution of eroded material: these permitted an estimate of the changing temporal balance between the main erosion processes (splash and wash). Results from both spatial scales showed a similar temporal pattern of runoff generation and sediment concentration. For both spatial scales, the dominant erosional process was detachment by raindrops; this resulted in a dynamic evolution of the soil surface under raindrop impact, with the rapid formation of a sieving crust followed by an erosion crust. However, a clear difference was observed between the two scales regarding the size of particles detached by both splash and wash. While all measured values were lower than the mean weight diameter (MWD) value of the original soil (mean 0·32 mm), demonstrating the size-selective nature of wash and splash processes, the MWD values of washed and splashed particles at the field scale ranged from 0·08 to 0·16 mm and from 0·12 to 0·30 mm respectively, whereas the MWD values of washed and splashed particles at the laboratory scale ranged from 0·13 to 0·29 mm and from 0·21 to 0·32 mm respectively. Thus only at the field scale were the soil particles detached by splash notably coarser than those transported by wash. This suggests a transport-limited erosion process at the field scale. Differences were also observed between the dynamics of the soil loss by wash at the two scales, since results showed wider scatter in the field compared to the laboratory experiments. This scatter is probably related to the change in soil surface characteristics due to the size-selectivity of the erosion processes at this spatial scale. Copyright © 2006 John Wiley & Sons, Ltd. [source] Yield, Boll Distribution and Fibre Quality of Hybrid Cotton (Gossypium hirsutum L.) as influenced by Organic and Modern Methods of CultivationJOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 4 2006D. Blaise Abstract India is the largest cotton-growing country (8.9 million hectares) in the world and most of the area is rain-dependent. Large amount of pesticides are used for the control of sucking pests and lepidopterans. Increasing demand for clean organic fibre has led to an interest in organic cotton. However, information on the effects of organic cultivation on fibre quality is limited. Seed cotton yield and fibre quality (length, strength, micronaire and uniformity) were determined for an organic and modern method of cultivation during 3 years (2002,2003 to 2004,2005) of a 11-year (1994,1995 to 2004,2005) study. Vertical and horizontal distribution of bolls on a cotton plant was also determined in 2003,2004 and 2004,2005. At the end of year 11, soil samples were collected and analysed for soil organic carbon content, water-stable aggregates (%), and mean weight diameter. Averaged over 3 years, an additional 94 kg seed cotton ha,1 was produced in the organic over the modern method of cultivation and the difference was significant. The year × treatment interaction was significant. Seed cotton yield in the organic plots was significantly greater than the modern method of cultivation plots in 2003,2004 because of a well-distributed normal rainfall and low pest incidence. The main stem nodes 13,22 accounted for the largest numbers of bolls present on the plant. Plants of the organic plots had significantly (37,71 %) more bolls on nodes 13,27 than those for the plants of the modern method of cultivation. Lateral distribution of bolls on a sympodial (fruiting) branch, was noticed up to fruiting point 11. However, treatment differences were not significant. With regard to fibre quality (length, strength, fineness and uniformity), differences between years were significant. Inferior quality fibre was produced in 2004,2005 because of delayed planting and early cessation of rain. On average, cotton grown under organic conditions compared with the modern method of cultivation had significantly better fibre length (25.1 vs. 24.0 mm) and strength (18.8 vs. 17.9 g tex,1). Soil samples of the organic plots had significantly greater C content, water-stable aggregates and mean weight diameter than the modern method of cultivation plots. Differences were restricted to the top layers (0,0.1 and 0.1,0.2 m). Yield benefits of growing cotton in an organic system over the modern method of cultivation are expected to be greater in years receiving normal rainfall and having low pest incidence. [source] Effect of water and nitrogen management on aggregate size and carbon enrichment of soil in rice-wheat cropping system,JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 2 2004Rojalin Tripathy Abstract A study was carried out on a silty clay loam soil (Typic Haplustept) to evaluate the effect of farmyard manure (FYM) vis-à-vis fertilizer and irrigation application on the soil organic C content and soil structure. The fertilizer treatments comprised of eight different combinations of N and FYM and three water regimes. The results indicated that the application of FYM and increasing N rate increased soil organic carbon (SOC) content. Addition of FYM also increased the percentage of large sized water stable aggregates (> 5,mm) and reduced the percentage of smaller size aggregates. This was reflected in an increase in the mean weight diameter (MWD) and improved soil structure. The organic carbon content in macroaggregates (> 1,mm) was greater compared to microaggregates, and it declined with decrease in size of microaggregates. This difference in organic C content between macro- and microaggregates was more with higher N dose and FYM treated plots. The effect of residual FYM on MWD and organic C content of the soil after wheat harvest was not significant. The effect was less in deeper layers compared to surface layers of the soil. MWD was significantly correlated with the SOC content for the top two layers. [source] Soil-aggregate formation as influenced by clay content and organic-matter amendmentJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 1 2007Stephen Wagner Abstract Naturally occurring wetting-and-drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic-matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting-and-drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1,t,ha,1, 6.2,t,ha,1, and 12.4,t,ha,1) on the development of water-stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water-stable aggregates in those soils with 34% and 38% clay. Repeated wetting-and-drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1,t,ha,1 and 12.4,t,ha,1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2,t,ha,1 were required. However, rapid wetting of straw-amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement. [source] | ||||||||||