Soil and Tillage Equipment

Transcription

1 Soil and Tillage Equipment C. I. Ijioma, Ph.D (McGill, Canada) Department of Agricultural Engineering, Federal University of Technology P. M. B. 1526, Owerri, Imo State, Nigeria A b s t r a c t This paper has related the behaviour of tilled soil to the considerations in the design parameters of tillage equipment. The objective of this paper has been to review various related engineering studies on the behaviour of tilled soils in terms of stress-strain relations and to describe how these relations has been utilized to design agricultural soil cutting equipment for effective soil and water conservation practices. 1. Introduction Soil is extremely weak in tension, very strong in compression and in practice fails mainly in shear. When soil is strained the shear stress builds up to a peak value which, in certain loose dry soils, remains constant with increasing strain, and in other soils, falls off before leveling out to smaller constant value, the residual stress. The magnitude of the shear stresses developed are frequently a function of the compressive stress normal to the plane of shear failure. If the peak or residual stresses are plotted against the corresponding normal stresses a graphical relationship is obtained. This relationship can be expressed in the form of the coulomb equation. These stress/strain and stress/stress relationships exist for a soil sheared in bulk, the strength being termed the bulk shear strength. They also exist for the shear of individual clods or aggregates clod shear strength. For the disintegration of clods in the soil mass by shear, the bulk shear strength must exceed the clod shear strength. The magnitude of the bulk and clod shear strengths vary for a given soil with soil moisture content. The components of the shear strength are cohesion and internal friction which depend on the nature and condition of the soil. The resistance to sliding at a soil/metal interface, like the soil/soil shear described above is frequently a function of normal stress between the surfaces. In the soil/metal interface, the adhesion component which corresponds to the cohesion component of soil/soil shear is usually very small, except under certain plastic soil conditions when a non-scouring condition frequently develops. Adhesion is a function of the wettability of the implement surface and in related to the soil moisture suction. The angle of soil/metal friction is a function of the roughness of the surfaces and any lubrication between them. The above soil/metal and soil/soil shear processes lead to new behavioural characteristics of the sheared tilled soil. Tillage terms which have been used to define the behaviour of tilled soil or the ease of crushing, crumbling or falling apart of tilled soil include, friability, compressibility, degree of compaction, index of looseness, soil structure stiffness and others (Ijioma, 1991, Scott Blair, 1937; Christensen, 1930; Ijioma and Mckyes, 1995; Spoor,1969; Bodman, 1949) Friability has been defined as the tendency of a mass of unconfined agricultural soil in bulk to crumble and break down under applied stress into smaller fragments, aggregates and individual particles. Under friable soil conditions, adhesion is 55

2 normally zero and the angle of soil/metal friction for a reasonably polished implement is between 15 O 30 O. Ijioma (1991) defined the index of looseness on the basis of soil dilatancy to be a measure of the relative tendency of the soil to change in volume when sheared. The tendency of a tilled soil to increase in volume has been described graphically by Mckyes and Desir (1984) as action and reaction relationship between an agricultural soil and a tillage implement. The action and reaction process can be further quantified by a loosening efficiency of the implement (Ijioma and Mckyes, 1995). A model describing the loosening efficiency of a tillage implement on a soil can be utilized as blade design parameters are altered. However, a lot of changes have occurred in the last couple of years in tillage tool design from earth moving principles (Reece, 1965, Hettiaratchi and Reece 1974). Furthermore, field experiences and experimental results have led to a trend towards less soil manipulation and movement. There have been a number of comparative studies of soil tillage and no tillage effects on crop production in parts of Europe, America and Asia (Hughes and Baker 1977; Rizvi, 1991; Raper,2005, Mc Laughlm et al, 2008; Liu et al, 2008; and Chung et al., 2008 Mckyes and Desir, 1984) In all the studies cited, soil cultivation as defined as any form of soil movement which occurs during tillage for crop production can either be positive or negative. When steps are actively taken to move the soil in such a way as to improve the conditions for crop growth, the soil cultivation is positive. However, when unavoidable and unwanted soil movements occur during tillage, the soil cultivation is negative. Before the optimum cultivation treatment can be decided, the soil physical environment required must be clearly defined. This environment may have to satisfy the needs of the crop being produced, the needs for the mechanization practice/the desire of the tillage implement designer and or the needs for effective soil and water conservation. The above requirements or needs could if all the information were available, be defined in terms of the necessary soil particle and aggregate size distribution and arrangement, and the nature and shape of the soil surface or cover. These requirements could be conflicting and so, priorities must be decided upon for localities and ecological zones in the world. What may likely be desirable in Europe may not be completely desirable in Nigeria. Taking a long term view, soil conservation should have first priority in every locality, followed by crop production and finally the mechanization practice. In each locality, there should be comprehensive experiments to decide on the desirable shape and geometry of the tillage implement for the agricultural soil in that locality. The universally accepted indices for quantifying tilth soil friability should be tested in every agricultural soil locality in a collaboration between the farmer/user of the tillage tool and the designer for effective crop production and soil/water conservation. Though the Nigerian mound making hoe is still a manually operated tool for soil cultivation, local studies should be intensified to build up scientific knowledge on the effect of the different types of hoe on the soil behaviour. The hoe is the cultural cultivation tool used for soil movement and manipulation in Nigerian crop production. There are different types of hoes used by Nigerian farmers. Ijioma et al (1985) categorized Nigerian hoes into three based on their geometric shapes. These hoes have been grouped into Ogu Uku (Abakaliki hoe), Mada (Katsina Ala/Zaki Biam hoe) and Bakatsiniya (Keffi/Akwanga/Lafia hoe). The Ogu Uku is a section of ellipsoid,the mada ia a rectangular section of elliptic cylinder while the Bakatsiniya is a section of elliptic paraboloid. These hoes are used for making different shapes and sizes of mounds. Presently, some studies on the effects of these hoes on soil index of looseness have been progressing 56

3 (Ohanyere, 2005). The use of motorized tillage operations are still relatively low in Nigeria agricultural farming system, yet there have been lots of reports of sheet erosion and soil degradation in the farm soils from the use of these tools (Van der Meijdan 1998). These soil losses could be as a result of limited knowledge of the appropriate selection and management of cultivation tools in Nigerian agricultural soils (Ijioma,1990). This paper therefore discusses from engineering point of view and from agricultural practice, the need to study the reactions of soils to the action of tillage equipment in the Nigerian agricultural soils. 2. Mechanical Behaviour of Soils The mechanical behaviour of soil can be quantified by the changes brought about in the pore space of the soil in response to applied stresses. Pore space can be readily related to the specific volume of the soil (Ijioma 1992; Ijioma 1991). The choice of a parameter to characterize the stresses acting on a soil element is less straight forward. In general, the stresses at a point in the ground can be represented by six stress components. Visualizing a cube being use to illustrate the stresses, there will be corresponding principal stress cube and three principal stress axes. If the principal stress cube is replaced by a regular octahedron constructed on the principal axes, then the stresses acting on the eight identical equilateral triangular surfaces of the octahedron will no longer be principal stresses but will now carry a normal stress and a shear stress αoct and a shear stress бoct. This move may appear to be complicating the issue, because there will now be 16 stress components instead of the original six (Hettiaratchi and O Callaghan, 1980). However, it can be shown that the normal stresses on all the faces are of identical magnitude as are the shear stresses. The octahedral stresses have therefore reduced the three principal stresses to eight identical pairs given by oct 3 (1) 2 ( 1 2 ) ( 2 3 ) ( 3 1 ) oct 3 (2) Where α 1, α 2 and α 3 are the major, minor and intermediate stresses. 3 oct P od ( 1 2 3); q (( 2 1 2) ( 2 3) ( 3 1) ). (3) 2 The P, and q stresses characterize those acting on a soil element and are referred to as the mean normal stress and the deviatoric stress respectively. The precise relationship, q = f(p) to which a soil element in the ground is subjected during a loading cycle depends on the geometry and kinematics of the loading mechanism, the boundary conditions, the soil type and its location in the ground. The loading mechanism can be a tillage equipment (Ijioma and Mckyes, 1995). During tillage operations, the desired intent is to bring the soil to failure and the behaviour of the soil in this phase of loading is of particular interest. It is however, difficult to attribute physical meaning to space and hence discussions on failure are simply carried out in the context of the stress-strain behaviour of soils. It is relatively easy to relate such characteristics with observable behaviour during field tests and simple compression testing. Four types of failures can occur in agricultural soils when loaded by tillage equipment. The four types of failures which are dependent on the soil moisture conditions are plastic flow; 57

4 general shear failure, fragmentation and fracture. Of interest in friable soil is the fracture and crack propagation.. The pore spaces in a soil constitute a large collection of randomly oriented pre-existing cracks. When the external loading system (tillage equipment) provides sufficient energy to grow new crack surfaces and simultaneously overcomes the energy absorbed in plastic deformation, then the cracks will propagate and failure will ensue. Extent of crack propagation is dependent on the soil moisture condition. Ijioma (1986) observed that as a tillage blade penetrated into a firm soil, cracks were initiated. These cracks extended in a near horizontal direction, thus clearing the path for the blade edge. As the tillage blade reached and operated at its required depth in the soil, it penetrated into the cracks like a wedge, so that the cracks developed further. The extent of the crack formation is dependent on the geometry of the blade (the width, the depth of operation and the angle of approach of the blade into the soil) and the soil moisture condition. The plastic deformation in a soil limited the crack propagation. Spoor (1969) attributed the action of a tillage equipment in the soil disturbance to the weakening of the structural bonds between the soil particles and to the rearrangement of the oil structural pattern. The dynamics of the crack propagation show the extent of soil disturbance in the mechanization of agricultural soil cutting. Ijioma and Mckyes (1995) described the relative extent of the soil structural pattern during tillage to soil structural stiffness. This is a concept of soil behavior that aids in selection of the geometry of a tillage tool for soil tillage. The soil structural stiffness H d oct d (4) oct defines the resistance of the oil to deformation and distortion forces. In this dαoct = the change or increment in the octahedral shear strength and dαoct at is the change or the increment in the maximum shear strain. Extensive energy utilization in motorized soil tillage operation destroys the soil structure and the soils ability to further resist deformation. Thus the Nigerian hoe system conserves the soil by limiting the extent of agricultural soil disturbance. The volume changes that occur in a soil is another aspect of the mechanical behaviour of a tilled soil. The volume of soil cut is defined as the volume per unit tool travel distance of soil which is moved appreciably from its original position and changed in density or structure. Mckyes and Ali (1977) have shown how such a volume can be estimated by mechanical models. The assumption in developing the mechanical models was that negligible disturbance was imparted to the soil outside of the range of the soil failure and that the cut volume boundaries were straight lines. Noting that the volume cut per unit travel distance was the same as the cross sectional area of soil cut, the mechanical models predicted the cross section area A, of soil disturbed by a typical implement having width w, and depth d as A = (w + s) d (5) where s represented the side crescent dimension. Though the real cross sectional boundaries of soil cut were curved and not straight, none the less, the models proved useful for predicting the trends of cut volume and loosening efficiency. Ijioma (1991) quantified loosening efficiency of a tillage implement by relating the volume change that occurs when the implement cuts into the soil to the angular distortion thar occurs in the soil during the soil deformation. The relationship between the volume change and the shear strain changes have 58

5 been defined as the soil s index of looseness. The index of looseness as defined by Ijioma dv (1991) is given as Sin V max (6) 3. Agricultural Soil Structural Stability d While the main objective of the uses of tillage equipment is to relatively increase crop production, the structural stability of the soil should always be a priority. It is always important to emphasise the proper agricultural land practice to prevent erosion of the soil so as to assure maximum agricultural productivity and to meet the food and fibre needs of the nation. It is important to plan the soil and water conservation measures on this basis. Sound conservation practices to protect the soil and water resources must be encouraged. One of these practices is proper tillage. Proper tillage is the least tillage necessary to produce the desired crop as efficiently as possible. Too much tillage over pulverizes too much of the soil and speeds up loss of soil moisture and of the soil. It is essential to fit the conservation system to the equipment used. The farm equipment industry has done much in developed economies to improve its machine to fit the concept of minimum tillage. Many experimental machines have been produced by equipment manufacturers and experimental stations. Many of these equipment have proved successful when used under conditions to which they are best adapted. Many fail when conditions are not suitable for their use. The concept of soil stability index is a soil conservation concept developed by Ijioma (1986) to illustrate the stability of a tilled soil in water. The index is determined from the moisture characteristics of the soil when subjected to tension. The ratio of the quantity of water drained during the increase in tension to the total quantity of water at saturation provides an index of stability of the soil structure. In an ideal stable structure, a great quantity of the water should be drained during the increase in tension, but in a deteriorated, unstable structure, a great quantity of the water would be retained by the structure. The retention of large quantity of water for a long time would lead to the destruction on inter-pod voids and inter particle pods and the instability of the soil structure in water. A large stability index indicates a stable soil structure in water. Physically, the tension progressively applied to the soil can be in the form of the rapid drop of a water table and the proximity of the soil surface from the table. In this study, it was observed graphically that the stability index of a loosened clay loam increased with the increase in the width of a tillage blade of a certain width and at a certain depth of tillage. Above that point, the stability index of the loosened clay loam increased curvilinearly with the increase in the rake angle of that tillage blade. The point of the minimum stability index of the loosened clay loam was between 27 O and 35 O rake angle of the tillage blade. In the clay loam, which was loosened with a tillage blade of a certain width and rake angle, the stability index increased with the increase in the depth of tillage. The stability index of the unloosened clay loam was higher than that of the loosened clay loam. The stability index of a loosened sandy loam was observed to increase with the increase in the width of a tillage blade of a certain rake angle and at a certain depth of tillage. The stability index of the loosened sandy loam decreased curvilinearly down to a point with the increase in the rake angle of a tillage blade of a certain width and at a certain depth of tillage. Above that point, the stability index of the loosened sandy loam increased curvilinearly with the increase in the rake angle of that tillage blade. The point of the minimum stability index of the loosened sandy loam was between 27 O and 34 O rake angle of the blade with a tillage blade of a certain width and rake angle, the stability index of the sandy loam decreased with 59

6 the increase in the depth of tillage. The stability index of the unloosened soil was greater than and less than the stability indices of some of the loosened soils. These findings confirm the need for field tests to determine the extent of tillage required and the appropriate tillage equipment. The foregoings illustrate the effects of tillage equipment on agricultural soils structural stability. These findings also confirm the need for extensive research studies on the use of tillage equipment in Nigerian agricultural soils. This is a challenge to researchers in soil tillage in Nigeria. There is a lot of work to be done in Nigerian agricultural soil tillage. 4. Design and Construction of a Machinery for Agricultural Mound making Hoes and a Machinery for incorporation of Plant Residues into the Soil. The drudgery associated with the use of hand hoes to make mounds for tuber crops cultivation in Nigerian farming system has discouraged many Nigerians from farming. This has resulted in the inability of the farmers to relatively increase the production of the tuber crops and the staple food in Nigeria. In order to contribute to the efforts to mechanise the cultural hoe farming system, a machinery for the mechanization of the agricultural mound making hoes was designed and constructed in the Department of Agricultural Engineering, Federal University of Technology, Owerri, Nigeria (Ijioma, 1991). The machinery consisted of three selected modified hand hoes radially mounted in three hydraulically operated shanks. The shanks were pin-jointed to a hydraulically operated central table. The raising of the blade facilitated the shoveling and heaping of the soil by the hoes. This mechanism was further modified by Ohanyere (2005) for a comparative study of the effect of the mechanized hoe system on index of looseness of the cultivated soil. The machinery happened to be the first significant attempt in Nigeria to mechanise the mound making hoe system. Further work on the different hoes being used in Nigeria, showed that from the material hardness test carried out, the Abakaliki hoe (ogu uku) was materially superior to the others in resistances to wear, durability and strength. The results of the tests reflected the quantity of material used in fabrication and the skill of the Abakaliki blacksmiths. In the second machinery developed for the incorporation of plant residues and manure into the agricultural soils, Ijioma (1994) adopted the conventional soil conservation machinery approach to design and construct a towed equipment with a tank for manure, a furrow opener in the form of a chisel plow and the furrow coverer (Negi, et al 1976). The manure tiller is presently an experimental equipment that facilitates incorporation of plant residues into agricultural soils. 5. Conclusion It is evident from studies reported in this paper that for each basic cultivation operation, there is an optimum consistency state in which the operation should, if possible be performed. Whilst it is not always possible to work under the ideal soil conditions, if work is carried out outside them, a greater amount of soil damage or poorer result can be expected. In some cases, rather than carry out an operation under the wrong conditions, it may be better to reach the desired end point in another way. For example, in clod disintegration, if it is necessary to reduce the size of clod when the soil is in a plastic rather than friable condition, efficient soil cutting equipment should be used rather than disintegration implement. In all cases of tillage equipment selection, the behaviour of the tilled soil should be considered paramount for 60

7 soil/water conservation measures. The selection of appropriate tillage equipment should be matched with the objective to be achieved in the tilled soil. Table 1 presents the summary guide for the selection of tillage equipment in agricultural soil cultivation. References Bodman, G. B Methods of Measuring Soil Consistency. Soil Science. 68: Christensen, O An Index of friability of Soil. Soil Science, 28: Chung, S. O., K. A. Suduth, C. Plouffe, N. R. Kitchen Soil Bin and Field tests on an on-the-go soil Strength Profile Sensor. Transactions of American Society of Agricultural and Biological Engineers (ASABE). VOL 51 (1) : Hettiaratchi, D. R. P. and J. R. O. Callaghan Mechanical behavior of Agricultural Soils. Journal of Agric. Engineering Research. Vol. 25: Hughes, K. A. and C. J. Baker The Effects of tillage and zero tillage systems on Soil aggregates in a silt loam. Journal of Agricultural Engineering Research. Vol 22: Ijioma,. C.I Soil Failure Crescent Radii Measurement for Draft in Tillage study. Nigerian Journal of Technology, Vol 10 (1): Ijioma, C. I Structural Stability Index of a tilled clay loam. Nigerian Journal of Engineering, vol ) : Ijioma, C. I Hardness Index and Tensile properties of Nigerian Mound Making hoes. Proceedings of the NSAE. VOL II: Ijioma, C. I Tillage tool for the Incorporation of Manure and Plant Residue into Nigerian Agricultural Soils Proceeds. NSAE. Vol. 14: Ijioma, C. I Effect of tillage tool geometry on soil index of looseness. Journal of Agricultural Science and Technology. Vol.2: Ijioma, C. I Design and Construction of a Machinery for Nigerian Agricultural Mound Making hoes. Proceeds of the International Soil Tillage Research Organization (ISTRO) Vol. 12. Ijioma, C. I Effect of Tillage Tool Geometry on Soil porosity. International Agro Physics. Vol. 6 (1-2) : Ijioma, C. I Manure tiller for Agricultural Soil improvement in the erosion prone south eastern Nigeria Proceeds of ISTRO, Vol. 13: Ijioma, C. I and E. Mckeys The effect of tillage tool Geometry on Soil Structural Stiffness. International Agro Physics, Vol. 9: Liu, J., D. A. Lobb, Y. Chen and R. L. Kushwaha Steady State Models for the Movement of Soil and Straw during tillage with a single sweep. Transactions of American Society of Agricultural and Biological Engineers (ASABE) Vol. 51(3): Mckyes, E. and O. S. Ali The Cutting of Soil by Narrow Blades. Journal of Terramechanics, 14 (2):

8 Mckyes, E. and F. L. Desir Prediction and Field Measurements of Tillage Tool Draft Forces and Efficiency in cohesive soils. Soil Tillage Research Journal Vol. 4 (4): McLaughlin, N. B., C. F. Drury, W. D. Reynolds, X.M. Yang, Y.X. Li, T. W. Welacky and G. Stewart Energy inputs for Conservation and Conventional Primary tillage implements in a Clay Loam Soil. T. ASABE, Vol. 51 (4): Negi, S., E. Mckyes, R. J. Godwin and.j. R. Ogilvie Efficient Injection of liquid Wastes into Agricultural soils. Report to Agricultural Canada, Engineering Research Services. Dept. of Agricultural Engineering (now BioResource Engineering), McGill University, Montreal, Canada. Ohanyere, S. O Effect of Mechanised Nigerian hoe tillage system on Sandy Loam Soil index of Looseness. M.Eng. thesis. Department of Agricultural Engineering, Federal University of Technology, Owerri, Nigeria. Raper, R. L Subsoiler shapes for Site specific tillage. Applied Engineering in Agriculture (ASABE). Vol. 21 (10: Reece, A. R The Fundamental Equation for Earthmoving Mechanics. Symposium on Earth Moving Machinery, Automotive Division. Proceeds of the Institution of Mechanical Engineers, 179. Part 3E. Rizvi, H. A Soil Physical Properties; before and after tillage. Proceeds of the 2 th International Conference of ISTRO. Scott Blair, G. W Compressibility curves as a Quantitative Measure of Soil Tilth. Journal of Agricultural Science. Vol. 27: Spoor, G Design of Soil Engaging Implements1. Farm Machine Design Engineering. December Edition: 1-5. Van der Meijden Motorized Soil Tillage in West Africa. FAO Survey on the Current use and consequences of tillage done with engine-driven machinery. 62

9 Tillage Operation Disintegration Clod formation Rearrangement Puddling Loosening Inversion Mixing Smoothing Table 1. Summary guide for selection of tillage equipment in agricultural soil cultivation Objective Bulk Clod shear Soil Direction of soil strength Consistency shear strength Reduce clod size by breaking along cleavage planes Compact particles into clods Increase soil Unit weight by filling larger pores with smaller aggregates Increase soil unit weight by destroying all structure and compacting Reduce soil unit weight either generally or locally Invert soil completely complete burial Mix up the soil Leave smooth soil surface resultant implement force in the soil Recommended equipment Relative value Relative value High Low Friable Downwards Roll, rotary cultivator Low Low Plastic Downwards or Sideways Moldboard plow Low High Friable Sideways Spike or oscillating harrow High Very low Very low Very low Low High or low High High or low Low High or Low Low High or low Plastic Liquid Friable Cemented Friable Plastic Friable Liquid Friable Downwards Or Sideways. Any direction Upwards Complex Upwards Disk harrow Rotary cultivator 45 degree rake angle tine Moldboard Mole Plow 45 degree tine Leveller 63

10 Moling Smearing Table 1 (ctd). Summary guide for selection of tillage equipment in agricultural soil cultivation Create High Low Plastic below Upwards Mole plow 45 permanent degree tine channel with smeared or sides at depth. Loosening Low High or Friable above above the low channel. Increase High Low Plastic Slipping wheel unit weight of thin layer by destroying soil structure Cutting movement Mounds and Conditions depends upon other requiremen ts Traditional mound seed bed for tuber crops Low High Plastic Friable Complex Abakaliki hoe Ogu uku Mada hoe 64