4. 1. INTRODUCTION
During tillage operations, soil aggregates and other bulk solids such as seeds, fertilizers, stones, tubers (left by the potato harvester) and roots, are often displaced from one layer to another by a sorting process causing the larger particles to be displaced into higher layers and smaller particles into lower layers. This process is influenced by particle properties and handling factors, and is counteracted by mixing.
The influence of particle properties and handling factors on mixing and sorting was studied in the laboratory both qualitatively and quantitatively.
Most of the experiments were done in a model soil bin filled with glass spheres and worked with normal and model tines. Localization of the processes and flow of the particles around the (model) tines were determined by filming and measuring the displacement of coloured markers, whereas mixing and sorting intensity and the sorting degree were determined by sieving or by counting the markers from different layers after working the bulk solid with normal harrow tines.
Tines were used in three positions: Forward facing, straight and backward facing; travelling speeds were: 0.01, 0.10, 0.45 and 0.80 m/s. Four fractions of bulk solid were used, monosized and polysized. The influence of particle properties on angularity and fractional solids content were determined. Dilatancy and relief were measured by means of a micro-reliefmeter; positions were determined using a position indicator.
4.3. RESULTS4.3.1. Rearrangement of soil aggregates by tines (I)
With respect to seedbed and seedbed preparation, experiments were started using dry soil aggregates as the bulk solid, that was accompanied by interparticle percolation. Sorting was characterized by sorting degree and sorting intensity and mixing by mixing intensity. It became apparent that mixing mainly occurs behind the tine and is influenced by the type and inclination of the tine and by tine travelling speed. The direct connection between particle size and mixing intensity was probably caused in front of the tine. Wider and more backward facing tines increased mixing intensity. Sorting mainly occurred in front of the tine. The greater the amount of material moved and the greater the difference in size of the particles involved, the higher was the sorting intensity.
The final effect of the rearrangement of bulk solids by tines as a result of sorting and mixing actions could only be understood if the various processes were studied separately.4.3.2. Rearrangement of glass spheres by tines (II; IV)
Because of the necessity for high demands on uniformity of the bulk solid used, especially on the narrow limits of particle size (at mixing), dry soil aggregates were replaced by glass spheres. The sizes of glass spheres were chosen so that interparticle percolation did not occur, allowing the possibility of studying the flow of particles around the tine.
The experiments revealed that mixing occurs by a strong downward displacement of a small number of spheres behind the tine and by a small sideways and upward displacement of a large number of spheres alongside of the tine. Mixing intensity increases with a decreasing travelling speed, more backward facing inclination of the tines and larger particle diameters.
Both backward facing tine inclination and low travelling speeds had a similar effect on the mixing intensity. Differences in mixing intensity of monosized bulk solids mainly originated from differences in dispersion in front of the tine; dispersion was stronger with increasing particle size.
Even without interparticle percolation, sorting mainly occurred in front of the tine, strongest with low travelling speeds and backward facing tines. Sorting intensity decreased with an increase in travelling speed and a more backward facing inclination of the tines.
In the equilibrium state, the bulk solid was found to be sorted to a higher degree after working with a high travelling speed and forward facing tines. A high sorting degree was characterized by small vertical displacements at a pass of the tine, by a strong stratification and so by a relatively pronounced dilatancy.
When interparticle percolation was absent, sorting intensity was determined by the smallest fraction. Forward facing tines were accompanied by a relatively high sorting intensity and a high sorting degree.4.3.3. Particle properties and handling factors (III; V)
Particle properties like size, shape, surface roughness and apparent particle density and handling factors such as size ratios, moisture content and travelling speed, influenced bed properties and sorting results: relatively large, oblong, rough and light particles were displaced into higher layers and vice versa. The proces is accelerated, when the bulk solid is dry and worked at a low travelling speed.
For glass spheres, the diameter was the predominating particle property determining the results of the experiments. With an increasing sphere diameter, bed properties such as fractional solids content and angularity increased. In order to obtain reproducible beds for mixing experiments, the depth of the inserted layers was related to the diameter of the spheres involved. Tapping increased fractional solids content by an average of about 6 per cent.
Bed properties can only be determined significantly if determination methods are standardized. For standardized methodology large containers with sufficiently roughened bottoms and very smooth walls, low speeds and tapping to improve reproducibility, are advised. The 2-dimensional angle of repose, α d2
, is the one that can be determined easily and objectively. The angles of repose increased and so flowability decreased, with an increasing particle size, surface roughness and apparent particle density. The fractional solids content of a cultivated layer is inversely related to the size ratio of the particles (and to the sorting degree).4.3.4. Rearrangement of bulk solids in the field(V)
Particle properties are more varied in the field than in the model experiments; handling factors such as moisture content and travelling speed are higher.
Throwing and sorting by interparticle percolation normally occur in the field. Throwing offers the possibility of killing weed seedlings by covering them with soil and of incorporating granulated chemicals and broadcasted seeds superficially._
Sorting in the field is of paramount importance: fine soil aggregates, seeds and granulated chemicals are placed into or near the seed placement zone, clods and stones are transported upward with respect to erosion control and rhizomes are brought to the surface as a measure of mechanical weed control. When draught animals are used, sorting is predominant; when tractors are applicated, throwing becomes more important as the travelling speed increases.
Sorting intensity in the field is highest for forward facing and wide or widened tines and tine-like tools, when moisture content is low, with a great variation of flowability of the particles involved and with low ( < 1 m/s) travelling speeds.
The effects of tined tillage tools in the field could largely be explained from model experiments using dry soil aggregates and glass spheres.