Capítulo 4. Resultados y análisis de investigación
4.4 Resultados implementación Segundo ciclo
4.4.1 Resultados Aula 1.
Soil shear strength differs from one soil type to another, not only because of the differences in cohesion and the internal friction angle (Ohu et al., 1985), but also due to its moisture content. John et al. (1986) developed a shear strength model in consideration of soil moisture content, organic matter and compaction. The relation between soil shear strength and moisture content was observed in each of clay, clay loam and sandy loam soils with organic matter of 3%, 10% and 17% respectively. The soils were subjected to three levels of compaction (5, 15 and 25 blows of standard compaction hammer). Increasing gravimetric moisture content up to 55% of the liquid limit of each soil type resulted in an increase in soil shear strength, before it started to reduce with further increasing moisture content (Figure 2.5, 2.6, 2.7). This is because an increase in moisture (limited) will result in an increase in soil cohesion before creating bigger water pores in the soil, hence less cohesion and accordingly reduced shear strength. Further reduction in soil shear strength was observed with incorporating more organic matter to soil (John et al., 1986).
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Figure 2.5. Shear strength vs. moisture content of clay soil at different organic matter andcompaction, adapted from John et al. (1986).
Figure 2.6. Shear strength vs. moisture content of clay loam soil at different organic matter and compaction, adapted from John et al. (1986).
Figure 2.7. Shear strength vs. moisture content of sandy loam soil at different organic matter and compaction, adopted from John et al. (1986).
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Likewise, in addition to the soil shear strength, the study of Ayers (1987) also included soil cohesion and angle of internal friction in between soil particles in relation to a range of soil densities and moisture content in two sandy loam soils (Ruston and Fuquay). The author reported that soils with lower clay content have lower shear values because of smaller cohesion force between the particles. This is similar to the finding of John et al. (1986) and Ayers (1987) who reported that at low moisture content, soil shear strength increases with an increase in moisture content. However, further increase in moisture content will decrease soil shear strength components (cohesion and friction angle) in case of given soil density (Figure 2.8). Soil shear strength components at given soil moisture responded differently to an increasing soil density in the two soil types (Figure 2.9) (Ayers, 1987). These results were consistent with later results Figure 2.10a, 2.10b reported by Ekwue and Stone (1995).15
Figure 2.8. Moisture content vs. soil friction angle and soil cohesion in two sandy16
Figure 2.9. Bulk density vs. soil friction angle and soil cohesion in two sandy loam17
Figure 2.10. Variation of bulk density, shear strength and penetration resistance with moisture content at different frame yard manure applications, (a) in sandy loam soil, (b) inclay soil, adopted from Ekwue and Stone (1995).
The relationship between soil bulk density, soil water content and penetration resistance was investigated by Aggarwal et al. (2006) under two different soil practices (bed planting and conventional system) when wheat crop was sown into a sandy loam soil. They concluded that irrespective of soil bulk density, when volumetric water content increased from 5% to 16%, penetration resistance decreased from more than 2 MPa to about 0.8 - 1.6 MPa. Also irrespective of water contents, when bulk density increased from 1.3 to 1.5 Mg m-3, penetration resistances increased up to 2 MPa which is the limit at which root growth and elongation are restrained and reduced by 50% (Bengough et al., 2011).
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This increase in penetration resistance was lower in the bed planting system as the soil was less compacted than conventional soil. Thus, penetration resistance increased with an increase in bulk density and decrease in soil water content. Furthermore, tillage techniques have a significant effect on soil moisture content due to the effect of soil tillage on soil structure (Jiuhao et al., 2007). Figure 2.11 illustrates the effect of three different tillage systems, as deep loosening in two vertical directions to the depth of 450 mm (ADL), shallow tillage after deep loosening to the depth of 450 mm (SDL) and conventional tillage to the depth of 300 mm (DT) on the volumetric moisture content of tropical soil (latosol). During the experimental period of about 400 days, Jiuhao et al. (2007) highlighted that due to the changes in soil structure caused by tillage systems, volumetric moisture content at different depth was significantly affected. ADL and SDL tillage systems compared to DT system, increased the water holding capacity and porosity of the soil while reducing soil bulk density and penetration resistance.Figure 2.11. Effect of different tillage systems on the volumetric water content at a depth of 0 - 100 mm. ADL = deep loosening in two vertical directions, SDL = shallow tillage after
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Benjamin & Mikha (2010) studied the prediction of winter wheat yield loss from soil compaction on a weld loam soil, which was artificially compacted using an automatic soil compacter. A range of compaction levels (1.4 - 1.54, 1.5 - 1.7 g cm-3 bulk density) were achieved by applying different pressure to the soil under different water content. A correlation was established between soil bulk density and water content to calculate the Least Limiting Water Range (LLWR; method which is the upper limit of water content at field capacity that provide the proper aeration of plant roots growth, minimum air filled porosity of 10%) so the loss yield of winter wheat could be predicted. Although there was a huge variation in soil bulk density after each applied load, a correlation was established between the pressure applied to the soil and soil water content, so bulk density could be determined. For instance, applying 174 kPa at 0.10 - 0.20 g g-1 water content resulted in 1.4 - 1.54 g cm-3 bulk density, and increasing the applied pressure to 614 kPa resulted in 1.5 - 1.7 g cm-3 bulk density. Nevertheless, the applied method which minimises the variations between the treatments should be considered to create different bulk density levels. In conclusion, the increase in the compacting pressure is accompanied by a reduction in the water content. Thus a 500 kg ha-1 loss of winter wheat yield would be expected with each 0.05 reduction in least limiting water range LLWR (Benjamin & Mikha, 2010).Kadziene et al. (2011) reported an increase in soil penetration resistance above 1.5 MPa associated with direct drilling and harrowing from the soil surface to the depth of 120 mm; which was an indicator of studying the effect of tillage intensity on root growth condition in the top soil. For all the treatments, the decrease in water potential increased penetration resistance, thus LLWR was halved to 0.11 m3 m-3 for direct drilling comparing to 0.21 m3 m-3 for ploughing system to 200 mm depth.