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| Author:
Grant Tranter |
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Understanding the physical characteristics of soil is of particular importance to farm managers. Issues such as tillage, soil compaction, root growth, gas exchange and hydraulic properties are all functional of a soils strength (Baver et al., 1972). In the past soil strength assessment has been limited due to instrumentation cost, repeatability and data interpretation. The advent of the dynamic cone penetrometer has allowed cheap repeatable and comparable soil strength assessments. An advantage of the penetrometer is that the force applied can be quantified so that comparisons can be made between brands and models. The dynamic cone penetrometer is a cheap instrument that measures both a soils shear and compressive strength. As soil strength is functional of soil moisture the penetrometer can also be an indicator of changes in soil moisture across a study site as well as down a profile (Gribb et al., 1997). |
Source http://myplace.westnet.com.au/images/potw/photos/Bogged-Tractor-101201.jpg
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Agricultural applications of soil strength versus soil moisture include erosion management, stock management and machinery scheduling to avoid bogging. In this study the penetrometer will be used to test the soil strength of soils under different management regimes. The test encompasses an area of pasture and a cultivated area will assess whether there is a significant difference between the two regimes.
Description of apparatus/method:
The Dynamic Cone Penetrometer measures the soil strength attributes of penetrability and compaction.The instrument is comprised of a metal rod (approx. 1.6m) with a hardened steel cone afixed as the striking tip. The cone has an inclination of 30° according to the Americal Society of Agricultural Engineers (ASAE) standards and has a basal diameter of either 20.27 mm or 12.83 mm depending on the condition of the material to be tested. This study utilises the 20.27mm cone diameter. 60° cones can be used in very soft soils where the 30° would penetrate to easily. To reduce the effect of the friction between the steel rod and the soil, the basal width is larger than the the rods width. The exact dimensions of the cone are very important as this allows the force and friction to be calculated.
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The cone is first pressed into the soil surface until buried. Held vertically, checked by referring to bubble float, the slide hammer is lifted to a specified height and dropped onto the anvil.Assuming the slide hammer is dropped from a vertical position, friction from the metal rod is neglible and the force in which the slide hammer places on the soil can be calculated.This action is recorded until a certain depth is achieved. The hardness of the soil should be considered when deciding on what depth the cone should be driven. Given time constraints the depth was decided at 15cm. So that the two sites are directly relatable the 15cm depth was used on both regimes. |
Two methods of recording the data were used in the study. For the Survey study, the number of hammer hits to penetrate 15cm was recorded. This method will be used to show how soil strength changes over study site, in particular the difference between pasture and cultivated areas. This method will be assessed in Field Survey, Penetrometer.
The second method was actually recording the depth of penetration for each hammer hit. This method is designed to evaluate changes in soil strength down the soil profile. As soil strength decreases with increasing soil moisture in cracking clays (Friend and Chan, 2001) this method also indicates changes in soil moisture.
The advantage of the dynamic cone penetrometer is that applied energy can be quantied. This is the initial step of any analysis, converting "hits" into joules. The advantage of this is that a real and quantifiable state can be attained. To do this the actual energy input per slide hammer hit needs to be calculated. This is simply calculated by applying the formula:- Mxgxh=Energy(J), where M is the mass of the applied hammer (kg), g is the gravitational constant (9.8m/s/s) and h is the height at which the weight is dropped. Total energy applied is simply M x g x h x Hits= Total energy.
By graphing the relationship between Cumulative depth and cumulative energy, physical structures can discerned. Lower gradients represent soils that possess greater compressive strength. This method can also be used to show hard pans or duplex profiles where changes in physical structure occur.
By differentiating the Cumulative energy vs depth equation, the amount of energy required per unit length at any depth can be attained. This is achieved by fitting a cubic polynomial equation to the Cumulative energy vs depth graph and then differentiating. This allows the energy required per cm penetration can be derived.
The above figures show the difference in physical structure between the pasture and cultivated regimes. It is obvious that the pasture requires far more energy to penetrate the 15cm required. One feature which is notable on both figures but more applicable to the cultivated regime is the lack of any hard pans or impedence boundaries. If there was the presence of a harder boundary then the curves would flatten considerably. This is applicable to cultivate regimes as they are vulnerable to hard plough pans, which can restrict root growth and water movement.
The above figure shows how compressive strength changes down the profile. Although there appears to be now clear boundary where strength changes it can be deduced that strength increases with depth. It is clear that the cultivated soil possesses far less strength than the pasture soil, in particular the top 10cm of the profile. This may be a function of different moisture contents. Another explanation may be that due to intensive cultivation the pedality of the soil has been reduced, leaving a dusty complexion as opposed to the solid almost massive structure of the pasture.
Due to the simplicity of the methodology there is little that can go wrong at an experimental level. It is the analysis and subsequent interpretation that problems arise. One misconseption is that the differences can be accounted for merely by differences in water content. This is flawed, as through cultivation the physical struture of the soil is altered. Understanding how the cone penetrometer readings interrelate with other physical properties allows a more representative account to be formulated.
http://www.sdec-france.com/us/doubleanneaux.html
McKenzie N, Coughlan K and Cresswell H. (2002) Soil Physical Measurement and Interpretation for Land Evaluation. (CSIRO Publishing: Melbourne).
Baver, L, D., Gardner, W, H., Gardner, W, R., (1972). Soil physics. 4th ed. John Wiley & Sons, New York.