The Penetration Resistance - Case Study Example

Discussion The objective of this study was to develop the relationship between the soil penetration resistance and bulk density with respect to field texture and moisture content. The interplay of Penetration resistance, bulk density and moisture retention curve enables to find the moisture content which is at high risk of causing compaction and the duration of that moisture content in the field. This also allows for the increase of water use efficiency without compensating for pasture growth in terms of available water capacity. Penetrometer Penetration resistance is widely used to measure soil compaction. In general soil with penetration resistance values of more than 2000 kPa (2 MPa) are considered to limit plant growth. Penetration resistance is highly sensitive to moisture content. It is also influenced by soil texture and bulk density. Field penetration resistance The penetration values are high in top soil which also have shown that compaction is occurring in the field and limited to top soils only. The Penetration resistance values results ranges from 1850 to 2050 kPa at 3 cm (30 mm) as presented in Figure 4 which is considered to be plant growth limiting values. However, these Penetration resistance values are not restricting the root elongation as described by Van Quang and Jansson (2012) and Gao et al. (2016). They also explain that the roots exert the pressure not straight as the cone penetrate as well as root hairs penetrate the natural cracks. But the cone penetration mimic the root pressure which is helpful in studying the root penetration. The penetration resistance is highly influential by moisture content. The dry soils decreases the mobility of soil particles as rearrangement of soil particles is hindered due to friction coefficient (Bennie, 1988). The penetration resistance of Span 6 was higher than Span 7 and Span 8 as this block have lowest volumetric moisture content than Span 7 and Span 8 for top ~ 5 cm soil. Penetration resistance also varies with soil texture as Kumar et al. (2012) suggested that sand particles have higher friction than the silt and clay. So therefore sandier soils have more penetration resistance than other soils. Similarly Span 6 have sandier soil than Span 7 and Span 8. However, clay soils have low penetration resistance as these soils smoothen the cone to pass through because of low friction coefficient. He also supported friction coefficient specified with water holding capacity. As clay have more water holding capacity which facilitate the penetration thus reduces the penetration resistance. The penetration resistance decrease with fineness of texture; sandy loam, sandy clay loam and Sandy clay loam (+) in Span 6, Span 7 and Span 8 respectively. In terms of constant moisture percentage, penetration resistance changes with the change in bulk density or compaction (Bennie, 1988). Though, Span 6, Span 7 and Span 8 are same in terms of bulk density. The field bulk density is not effecting the Penetration resistance values. Laboratory penetration resistance; Proctor column density Figure 1: Bulk density vs. Penetration resistance The penetration resistance was lowest when bulk density was about 1.72 g/cm3 to 1.78 g/cm3. However the penetration resistance 1870 kPa when bulk density was above 1.85 g/cm3. The main soil factors that influence penetration resistance are soil moisture, bulk density, type of soil and soil structure, and the cone design. Important soil factors that influence the bulk density of the soil are soil texture, chemical properties, mineralogy, total pores and organic matter, which also subsequently affect the penetration resistance of the soil. Assuming the effect of cone design such as cone base, penetration rate and cone roughness, the soil resistance values highly vary with the bulk density of the soil and the moisture content if we consider a one soil type with a uniform soil structure (Vaz, et al., 2013). As stated by JuniorI, et al. (2014), soil moisture and soil bulk density are highly related to the penetration resistance in the sense that increasing bulk density and reducing moisture content causes a linear increase in penetration resistance. However, as evidenced in figure 4, the relation between bulk density and penetration resistance is not always linear. As the bulk density approaches the maximum, smaller variations in bulk density causes a larger variation in penetration resistance, with the effect of moisture content being higher in more clayey soils. Increase in bulk density causes soil particles to be compacted tightly together, which slows down the penetration rate of the cone throught the soil (Reichert, et al., 2004). The bulk density of a soil is inversely related to the porosity, which gives us an idea of the amount of porous space left in the soil after compaction. Different types of soils have different optimal bulk densities needed for plant growth. In general, bulk density below the optimal value leads to poor water and air movement. On the other hand, high bulk density reduces movement of air and water through the soil, and increases penetration reistance which limits plant root growth (Costantini, 1996). Thus, for the soil to support the growth of pasture in the fields, the bulke density must be within optimal values. Figure 5: Penetration rsistance vs.volumetric moisture content The soil penetration resistance is influenced by the moisture content i.e. with increasing in volumetric moisture content causing a decrease in the penetration resistance. The penetration resistance decreases from as high as 1500 kPa at a volumetric moisture content of about 38% to nearly zero as the volumetric moisture content is increased to above 65%. This depicts a strong negative relationship between volumetric moisture content and penetration resistance. This observation is attributed to the fact that increasing the soil water content reduces the frictional resistance between the soil particles, enabling easy penetration of the con penetrometer through the soil. The sandy clay loam and sandy clay loam (+) field soils maintain higher water content due to increased fine texture than sandy loam, which explains why these soils have a lower penetration resistance (Vaz, et al., 2001). Apart from bulk density and moisture content, the resistance offered by the soil to the propagation of the cone penetrometer is also influenced by other factors such as the type and nature of the soil, cone base and the speed of cone penetration. To maintain water infiltration and pasture growth, the field penetration resistance range should be limited to the range 0 - 2 MPa (Vaz, et al., 2013). Bulk density Bulk density of soil is affected by the compaction. These condition on agriculture lands are more challenging than laboratory assessment. However, the core sampling method is used to assess by the field bulk density whereas laboratory proctor density column method is applied to estimate maximum compaction susceptibility with respect to moisture percentage. Proctor column density test The maximum dry bulk density of Span 6, Span 7 and Span 8 were increased and then decreased with increase in the volumetric moisture percentage. In case of Span 6 soil, initially the compaction for Span 6 was higher and then lower than the Span 7 and Span 8 at almost same moisture content. This is because of Span 6 have sandy loam whereas Span 7 and Span 8 sandy clay loams. Kumar et al. (2009) found the same results who suggested that the sand had highest bulk density than other soil texture at critical moisture content. In case of Span 7, the soils are sandy clay loams which decreased the maximum dry density at critical moisture content. This is because the water increases the clay due to swelling there by decreases the bulk density. These swelled soils cannot be compressed artificially (Kumar et al., 2009). However Span 8 soils are in-between Span 6 and Span 7 in terms of soil texture that is exactly observed curve shape for compaction test. Kumar et al. (2009) also suggested that maximum dry density and critical moisture content increase with increase in the fineness of the texture of soil. The linear plot of bulk density against volumetric moisture content in these soils described their strong relationship (r2=.90 to .96). In terms of susceptibility these soils are risk of compaction under the wide range of volumetric moisture percentage when the compactive effort of 600 kPa was applied. This pressure mimic the hove pressure in the field as data from Greenwood and McKenzie (2001) and Herbin et al. (2011) , cattle hove exerts the static pressure 170 kPa. The static pressure folded up to double or thrice on movement that is dynamic pressure which would range from 250 to 500 kPa. When the Volumetric moisture content is more than 50 percent it can cause severe damage to soil which are often termed as pugging. This also led cattle grazing to cause subsurface compaction along with runoff. Soil pugging and poaching conditions are caused by cattle grazing on fields that are too wet – with cattle hooves sinking as deep as 15 cm into the wet soil, causing subsurface compaction along with runoff. This has a significant effect on the growth of pasture as the compacted soil restricts the growth of roots through the soil, and the free movement of water and air. The soil structure in the compacted layer also gets destroyed, and it is well known, soil structure is basic to the health and productivity of soil. When moisture is added to a dry soil, the soil particles are drawn close together by coating them with very thin films of water. When the volumetric water percentage is high, the soil becomes stiff and becomes difficult to pack together. As more water is added to the soil in span 6, 7 and 8, the volumetric water percentage increases. The added water act as a lubricant, causing the particles to come closer to each other due to the increased workability of the particles and the percentage of air voids is reduced. In span 7 and 8, the percentage of clay is higher, and these soils tend to absorb more water than sandy and loamy soils. Initially, the bulk density of the soils increase with increase in moisture content. But the addition of water beyond the optimum moisture content reduces the bulk density of the soil because the excess water occupies the space that was initially occupied by soil particles and forms larger films around the soil particles (Archer & Smith, 1972). A part from the volumetric water content, other factors that affect the bulk density of a soil include: soil characteristics such as soil plasticity, texture, mineral composition, water capacity etc., and the intensity of competiveness resulting from grazing cattle. Field bulk density The mean bulk density of natural soils from natural soil ranges from 1.46 to 1.48 g/cm3. Span 6 bulk density was slightly higher than Span 7 and Span 8. Figure 6: Field bulk density for all spans The bulk density recommended for arable land is 1.2 g/cm3. The maximum dry density of soils above 1.8 g/cm3 is more than sufficient that it can restrict the plant roots elongation which reduces the plant growth and finally effect the productivity. Though, in situ, the roots as well as water and air movement is possible by the development of cracks. As a consequence, high bulk density do not restrict the root penetration but this have adverse effect on crop yields (Van Quang and Jansson, 2012, Gao et al., 2016). The proctor column density values are slightly higher than natural bulk density values. The field soil is naturally compacted and may not match the proctor column bulk density due to different compactive forces involved. Vane shear Soil strength, is assessed in the vane test for surface soil. The vane shear results have shown that the soil strength is almost the same in all Spans. The soil strength of Span 8 is slightly higher than Span 7 almost by 1 kPa. Span 6 has a lower soil strength than Span 7 and Span 8 at approximately by 13 kPa. This implies that span 7 and 8 can develop higher cohesion and internal angle of friction. A very high value of soil strength can limit root penetration through the soil and affect plant growth. On the other hand, soils with low strength are susceptible to compaction by cattle and can easily be eroded by both wind and rain water runoff. According to Gill, et al. (2004), clay soils have a greater shear strength when they are dry. However, they are generally susceptible to waterlogging which leads to restricted plant root growth. In clay loam soils and sandy clay loam, there is improved soil hydrological properties, lower shear strength, and better plant growth than in clay soil alone. Thus, the sandy loam in span 6 would have better growth of pasture than span 7 and 8. The clay content in the field soils influence soil strength through their cohesion effect. This explains why the soil from span 8 has a slightly higher strength than span 6 and 7. Water content also modifies cohesion significantly, and therefore, influence soil strength. This is attributed to the fact that increasing water content leads to separation of soil particles and softening of soil cements, thus easier slippage and lower strength. The presence of clay in the soil samples can have a significant impact on the cohesion of the soil as well as on the angle of internal friction. If the moisture content is slightly above the optimum level, the presence of clay in the soil improves the soil’s cohesion. However, if the moisture content is high above the optimum level, this improvement in soil cohesion may not be attained (Dafalla, 2013). Thus, clay content has a significant influence on shear strength parameters (cohesion and angle of internal friction). In general, an increase in the soil moisture content is associated with a reduction in the cohesion. This causes soil particles to have little internal friction and subsequently, exhibit lower shear strength. Span 7 and 8 have higher clay content which has smaller particles with larger surface area that build a larger frictional force, hence, a higher shear strength. Other factors that can influence soil shear strength include mineralogy, size and shape of soil particles, density of the soil, and other physical properties (Reichert, et al., 2004). Mineralogy can influence the shear strength capacity of sandy clay soils. Shrinkage and swelling in expansive soils are the major extreme effects on the soil shear strength. For fully expanded clay loam soils, the shear strength is generally low, while dry shrinking clay can develop high angle of friction and cohesion (Costantini, 1996). The shape of the sand in the sandy clay loam, whether angular, rounded, or sub-rounded also affects the shear strength of the tested samples. Angular sand grains provide a greater interlock and shearing resistance. The size and gradation of the sand particles also affect the shear resistance of the soil. Materials that are well graded provide a greater grain to grain area contact than materials that are poorly graded. Porosity and the space occupied by sand and clay in the sandy clay loam soils is of great influence on the shear strength of the soils (Dafalla, 2013). It is also known as denser soil samples is expected to have a greater shear resistance. The bulk density of sandy clay soils is therefore crucial in determining the influence of soil unit weight on the shearing resistance. 8.1. Soil field texture Soil texture refers to the soil composition in terms of the percentage of small medium and larger particles of clay, silt and sand respectively. Hence, clay particles form small particle composition of a soil. The percentage of clay present in a soil has a significant effect on the texture of the soil. In this case, span 8 has more clay content, followed by span 7 and span 6. The presence of clay in the soils make the soil to have a fine soil texture, while the loam sand in span 6 lies between course texture and fine texture. Table 1 Field Texture grade of soils Span Soil texture grade Approximate clay content 6 Loam sand (+) 5-10% 7 Sandy clay loam (+) 25-30% 8 Sandy clay loam 20-25% The soil texture has a great influence on permeability, water infiltration, and water holding capacity of the soil. It is also related to soil bulk density; finer soils compact more than soils with medium and course texture. However, sandy soils have a relatively high bulk density due to the fact that sandy soils have less total pore space than clay soils (Archer & Smith, 1972). Therefore, loam sandy have high bulk density compared to sandy clay loam. High bulk density reflect poor soil structure and high compaction, which reduce or restrict plant root growth (Dafalla, 2013). 8.2. Water retention curve The soil water retention capacity or soil water holding capacity is controlled mainly by the soil texture and the organic matter content in the soil. The soil texture is a reflection of the soil particle size distribution. As we have seen, the presence of sand particles in the soils contribute to the rough texture of the soil, which make more space for water to penetrate through the soil. 8.2.1. Ku/pF From the ku/pF results, the volumetric moisture percentage of is high for span 7 (sandy clay loam (+)) than in span 6 and span 8 at a given water matric potential. According to Archer & Smith (1972), clay has a particle size of less than 0.002 mm. Clay particles have platelets that are stacked together, providing a larger surface area for storage of large amounts of water. Span 7 has the highest percentage of clay (25-30%), which is why the soil has high volumetric moisture percentage at a given water matric potential, followed by span 8 with a clay content of 20-25%. Hence, the higher the content of clay, the higher the retention capacity of the soil. However, much of the water is held up in pores that are too small to allow plant roots to extract water from the soil, thus, hindering plant growth. Clay is easily compacted due to its low small particle size (Vaz, et al., 2001). On the other hand, the lower values of CWC compared to the values of moisture content at field capacity in the loamy (sandy loam, loam and clay loam) soils suggested that loamy soils are more prone to compaction as compared to sandy soils (Howard et al. 1981; McQueen and Shepherd 2002). Figure 7: The water retention curve of different Spans soil. Table 2: Saturation, Field capacity, Plant available water and Permanent wilting point from the soils of Span 6, Span 7 and Span 8. Span 8_1 Span 8_2 Span 7_1 Span 7_2 Span 6_1 Span 6_2 Saturation (0 Kpa) 46 37 40 37 41 41 Field Capacity (33 Kpa) 31 29 28 29 21 30 Paw (Fc-33) 21 21 17 19 15 23 Permanent Wilting Point (1500 Kpa) 10 9 11 10 6 7 The Water retention is also dependent on the volumetric moisture percentage available in the soil. Soils that have reached their saturation point will retain little water and vice versa soils (Dafalla, 2013). Span 7 and 8 are hence expected to have a higher water retention capacity than span 6 due to the presence of clay particles (Archer & Smith, 1972). The relatively high values of saturation and field capacity observed in span 7 and 8 is attributed to the water retention capacity of sandy clay loam. Sandy loam has a high plant water available (PAW), which makes it favourable for plant growth. Hydraulic conductivity is a function of soil capillary radii, porosity and tortuosity (Shevnin, et al., 2006). The presence of clay lowers the hydraulic conductivity of the soil. Hence, the sandy clay loam soils collected in span 7 and 8 are expected to have low hydraulic conductivity and high water tension than the sandy loam in span 6. 8.2.2. WP4C Figure 8: Permanent wilting point of Span 6, Span 7 and Span 8 by WP4C The permanent wilting points of the soils was assessed using the WP4C equipment. Clay and loamy soils generally have a greater amount of moisture within a certain range of soil water potential compared to sandy soils. This is why the soil from span 7 recorded the highest volumetric moisture percentage at a given soil water matric pressure. The sandy loam from span 6 contain less than 10% clay, and has a lower volumetric moisture percentage at the same water potential. Table 3: Permanent Wilting Point estimated from trend line at 1500 kPa. Span 6 7 8 PWP 5.8 11 8.5 8.3. Plastic limit 8.3.1. Drop cone penetrometer; liquid limit (LL) and plastic limit (PL) From the cone penetration results, it is observed that the cone penetration distance increases with increase in the volumetric moisture percentage. As the volumetric moisture content of the soil increases, the soil’s consistency changes from semi hard to reach a liquid consistency state. At the liquid limit state, the soil has no cohesive strength to maintain its shape under its own weight, and finally deforms. This explains the trend of the cone penetration curve of span 7. Higher liquid limit is typically associated with soils that have a high percentage of clay (Benson, et al., 1994). The soils collected from span 7 and 8 contain clay and therefore, have a higher liquid limit than the sandy loam in span 6. Figure 9: The dropped cone penetration distance and volumetric moisture content of soil from Span 7. Figure 10: The dropped cone penetration distance and volumetric moisture content of soil from Span 8. The plastic limit of span 7 and span 8 occurred at a volumetric moisture percentage of 56%. Clay soils typically. In most clay soils, the liquid limit is in the order of 50-90% (Gill, et al., 2004). Both liquid limit and plastic limit of a soil depend on the amount of clay present in the soil. Thus, span 7 has a high LL than span 8 due to the high amount of clay in span 7. The PL and LL are important soil mechanical properties for tillage and land preparation (Dafalla, 2013). 8.3.2 Field Moisture Data Figure 11: Field volumetric moisture Figure 12: Field moisture by soil sample The digital moisture data obtained from span 6 show variation in the amount of moisture contained in the field span. This may be attributed to temporary wetting and drying of the fields due to rain and heat from the sun. The moisture data is an indicator of the amount of water present in the soil, and whether it can support plant growth. Variations in the field moisture is also influenced by the soil-plant interactions. Plant growth is supported by adequate moisture availability in the soil. Figure 13: Field moisture content and water retention curve for span 6. For the three spans, the variation in the volumetric moisture percentage is due to factors such as different soil properties, physical conditions of the fields, and other factors such as vegetation cover (Dafalla, 2013). The minimum compactive force required to produce a near-maximum soil compaction is determined by the amount of field moisture present in the soil (Archer & Smith, 1972). The field moisture of span 6 also show variations over a period of 2 months. Over this period, the volumetric moisture availability is above 26%, and is within the plant available water for utilization in root growth. Most of the time the volumetric moisture availability reaches the field capacity. Span 6 is made typically sandy loam soil with good drainage. The soil’s drainage capacity enables it to avoid reaching the saturation point, but instead remain within the field capacity range, which favors growth. Conclusion Soil compaction of agricultural land by cattle remains a great concern that has resulted in the formation of dense soil layers. This has a significant effect on the growth of pasture as the compacted soil restricts the growth of roots through the soil, and the free movement of water and air. The soil structure in the compacted layer also gets destroyed, and it is well known, soil structure is basic to the health and productivity of soil. The relationship between penetration resistance and volumetric moisture percentage is influenced by the bulk density of soil. All the soils achieve a maximum bulk density at a volumetric moisture content between 40% and 50%. Soil moisture content below this level will allow the growth and grazing of pastures without risking the fields for compaction. In terms of soil shear strength, sandy clay loam soils sampled in span 8 have the highest shear strength because of the clay particles in these soils. However, soils with clay have low hydraulic conductivity and high water tension and water potential, contrary to their sandy and loam counterparts. High bulk density indicates soil compaction and reduced porosity. References ARCHER, J. R. & SMITH, P. D., 1972. The Relation between Bulk Density, Available Water Capacity, and Air Capacity of Soils. Journal of Soil Science, 23(4), pp. 475-480. BALL, B. & O'SULLIVAN, M. 1982. Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments. Journal of soil science, 33, 609-622. BENNIE, R. D. T. 1988. Penetration resistance of fine sandy apedal soils as affected by relative bulk density, water content and texture. South African Journal of Plant and Soil, 5, 5-10. BENSON, C. H., ZHAI, H. & WANG, X., 1994. Estimating Hydraulic Conductivity of Compacted Clay Liners. Journal of Geotechnical Engineering, 120(2), pp. 366-387. COSTANTINI, A., 1996. Relationships Between Cone Penetration Resistance, Bulk Density, And Moisture Content In Uncultivated, Repacked, And Cultivated Hardsetting And Non-Hardsetting Soils From The Coastal Lowlands Of South-East Queensland. New Zealand Journal of Forestry Science, 26(3), pp. 395-412. DAFALLA, M. A., 2013. Effects of Clay and Moisture Content on Direct Shear Tests for Clay-Sand Mixtures. Advances in Materials Science and Engineering, Volume Volume 2013, pp. 1-8. GAO, W., HODGKINSON, L., JIN, K., WATTS, C. W., ASHTON, R. W., SHEN, J., REN, T., DODD, I. C., BINLEY, A. & PHILLIPS, A. L. 2016. Deep roots and soil structure. Plant, cell & environment. GILL, J. S. et al., 2004. Physical properties of a clay loam soil mixed with sand. University of Sidney, Australia, SuperSoil. GREENWOOD, K. & MCKENZIE, B. 2001. Grazing effects on soil physical properties and the consequences for pastures: a review. Animal Production Science, 41, 1231-1250. HERBIN, T., HENNESSY, D., RICHARDS, K., PIWOWARCZYK, A., MURPHY, J. & HOLDEN, N. 2011. The effects of dairy cow weight on selected soil physical properties indicative of compaction. Soil Use and Management, 27, 36-44. JUNIORI, D. D. V., BIACHINIII, A., VALADÃOI, F. C. A. & ROSAII, R. P., 2014. Penetration resistance according to penetration rate, cone base size and different soil conditions. Bragantia, Campinas, 73(2), pp. 171-177. KUMAR, A., CHEN, Y., SADEK, M. A.-A. & RAHMAN, S. 2012. Soil cone index in relation to soil texture, moisture content, and bulk density for no-tillage and conventional tillage. Agricultural Engineering International: CIGR Journal, 14, 26-37. KUMAR, D., BANSAL, M. & PHOGAT, V. 2009. Compactability in relation to texture and organic matter content of alluvial soils. Indian J. Agric. Res, 43, 180-186. REICHERT, J., SILVA, V. D. & REINERT, D., 2004. SOIL MOISTURE, PENETRATION RESISTANCE, AND LEAST LIMITING WATER RANGE FOR THREE SOIL MANAGEMENT SYSTEMS AND BLACK BEANS YIELD. Brisbane, International Soil Conservation Organisation. SHEVNIN, V., DELGADO–RODRÍGUEZ, O., MOUSATOV, A. & RYJOV, A., 2006. Estimation of hydraulic conductivity on clay content in soil determined from resistivity data. Geofísica internacional, 45(3), pp. 195-207. VAN QUANG, P. & JANSSON, P.-E. 2012. Soil penetration resistance and its dependence on soil moisture and age of the raised-beds in the Mekong Delta, Vietnam. VAZ, C. M., BASSOI, L. H. & HOPMANS, J. W., 2001. Contribution of water content and bulk density to field soil penetration resistance as measured by a combined cone penetrator - TDR probe. Soil Tillage & Research, Volume 60, pp. 35-42. VAZ, C. M. P., MANIERI, J. M., MARIA, I. C. D. & GENUCHTEN, M. T. V., 2013. Scaling the Dependency of Soil Penetration Resistance on Water Content and Bulk Density of Different Soils. Soil Science Society of America Journal, Volume 77, p. 1488–1495. Read More

As stated by JuniorI, et al. (2014), soil moisture and soil bulk density are highly related to the penetration resistance in the sense that increasing bulk density and reducing moisture content causes a linear increase in penetration resistance. However, as evidenced in figure 4, the relation between bulk density and penetration resistance is not always linear. As the bulk density approaches the maximum, smaller variations in bulk density causes a larger variation in penetration resistance, with the effect of moisture content being higher in more clayey soils.

Increase in bulk density causes soil particles to be compacted tightly together, which slows down the penetration rate of the cone throught the soil (Reichert, et al., 2004). The bulk density of a soil is inversely related to the porosity, which gives us an idea of the amount of porous space left in the soil after compaction. Different types of soils have different optimal bulk densities needed for plant growth. In general, bulk density below the optimal value leads to poor water and air movement.

On the other hand, high bulk density reduces movement of air and water through the soil, and increases penetration reistance which limits plant root growth (Costantini, 1996). Thus, for the soil to support the growth of pasture in the fields, the bulke density must be within optimal values. Figure 5: Penetration rsistance vs.volumetric moisture content The soil penetration resistance is influenced by the moisture content i.e. with increasing in volumetric moisture content causing a decrease in the penetration resistance.

The penetration resistance decreases from as high as 1500 kPa at a volumetric moisture content of about 38% to nearly zero as the volumetric moisture content is increased to above 65%. This depicts a strong negative relationship between volumetric moisture content and penetration resistance. This observation is attributed to the fact that increasing the soil water content reduces the frictional resistance between the soil particles, enabling easy penetration of the con penetrometer through the soil.

The sandy clay loam and sandy clay loam (+) field soils maintain higher water content due to increased fine texture than sandy loam, which explains why these soils have a lower penetration resistance (Vaz, et al., 2001). Apart from bulk density and moisture content, the resistance offered by the soil to the propagation of the cone penetrometer is also influenced by other factors such as the type and nature of the soil, cone base and the speed of cone penetration. To maintain water infiltration and pasture growth, the field penetration resistance range should be limited to the range 0 - 2 MPa (Vaz, et al., 2013). Bulk density Bulk density of soil is affected by the compaction.

These condition on agriculture lands are more challenging than laboratory assessment. However, the core sampling method is used to assess by the field bulk density whereas laboratory proctor density column method is applied to estimate maximum compaction susceptibility with respect to moisture percentage. Proctor column density test The maximum dry bulk density of Span 6, Span 7 and Span 8 were increased and then decreased with increase in the volumetric moisture percentage. In case of Span 6 soil, initially the compaction for Span 6 was higher and then lower than the Span 7 and Span 8 at almost same moisture content.

This is because of Span 6 have sandy loam whereas Span 7 and Span 8 sandy clay loams. Kumar et al. (2009) found the same results who suggested that the sand had highest bulk density than other soil texture at critical moisture content. In case of Span 7, the soils are sandy clay loams which decreased the maximum dry density at critical moisture content. This is because the water increases the clay due to swelling there by decreases the bulk density. These swelled soils cannot be compressed artificially (Kumar et al., 2009). However Span 8 soils are in-between Span 6 and Span 7 in terms of soil texture that is exactly observed curve shape for compaction test.

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