| Rice-wheat rotation is a major cropping system for South China agriculture.However,a drawback of this system is a dense root-zone soil layer resulted from flooded paddy season,soil consolidation and compaction.Thus the demand of a fragmented,loosened and aerated seedbed by the wheat season could not be satisfied.Soil tillage is an effective management system leading to soil fragmentation and improved soil physical properties,which is proved as an effective practice for remediating paddy soil compaction.Whereas a mechanical fragmentation process of agricultural is governed by a number of influential factors,including soil friability and both the designing and working parameters of tillage tools.The objective of soil tillage mechanics research is therefore to illustrate how,starting from compacted dense state,an externally induced mechanical disturbance fragments soil and leads to desirable soil structures.Therefore,mechanism of an applied disturbing stress for soil failure and the induced soil structural state is the key for soil tillage research.The aim of this research is,by combining indoor soil fragmentation test and field mechanical disturbance measurement,to systematically investigate soil fragmentation behavior and related soil structures.The main tasks and achievements include:Fragmentation properties of paddy soil and yellow brown soil were compared with indoor fragmentation test.Specific fragmentation energy,soil fragment fractal dimension,mean weight diameter and specific surface area of the two soils were compared,revealing that both compacting density and moisture content significantly affect soil fragmentation.Soil fragment fractal dimension under the influence from the 2 factors varied in a range of 1.5-2.5,and increased with improved soil moisture contents.Whereas,specific surface area,another index for soil fragmentation,decreased with improved water content.As a higher fractal dimension and lower fragmentation energy is attainable under lower water content,a higher fragmentation energy efficiency could be achieved with lower water content for remolded soils.Due to the unavoidable disturbance experienced by a tractor-mounted measurement system,high precision control on the tillage implements could not easily be achieved in the field state.An independent mechanical soil disturbing and testing platform was developed,which is composed of a traction system,tillage tool adjustment mechanism and computerized measurement system.The developed testing platform was adaptable to different tillage implements tests and was capable of providing precision control on a number of working parameters of tillage tools,including forwarding speed,laminated and luffed adjustments for mouldboard plough.These adjustments guarantee that boundary condition and controlling parameters for soil fragmentation can be achieved and respective soil structures can be acquired,facilitating a guaranteed methodology for field-level soil tillage research.Both the constructed ploughing test platform and soil physical and mechanical measurements were implemented in the field to illustrate paddy soil workability and post-tillage soil structures.It was found that cone penetration revealed an increased penetration resistance with soil depth,while the direct shear explained a decreased trend of soil strength followed with an increased trend,as soil depth increased.Compared with cone penetration and direct shear,laminated ploughing was able to illustrate that paddy soil was stratified and soil structure deteriorated along the depth.It was thus recommended that,in performing soil tillage,existing soil layers should not be mixed and deep loosening tillage tools are more preferable than those turning the soils.Specific traction of mouldboard plough varied in a range of 19.8-52 kN/m.But post-tillage soil structure was poor.The mass content of soil structures>32 mm amounts to more than 60%.Luffed ploughing showed that increased tilling width led to increased mean weight diameter and higher fragment fractal dimension,while specific surface area decreased.Thus narrow tilling width is the best strategy for paddy soil tillage.The method of digital image processing was applied to structure analysis on ploughing-induced soil structures paddy soil.Soil structures collected in different crop were compared.The angularity,shape factor and rectangularity were statistically evaluated,revealing the fact that angularity,shape factor and rectangularity were 3 parameters suitable for soil structure analyzing.Comprehensive analysis was made on both indoor experiment and field test and 3 different models for soil fragment mass-size distribution were used to test the results from the 2 experiments.Gaudin-Schuhmann model was found to provide the most precise quantification on soil structures from the 2 tests.Model parameter λ of remolded soil was 32mm and co increased with mean weight diameter(MWD).Fitted equation between λ andω was ω= 0.1344 MWD-1.1153,with R2 of 0.91.In the field state,however,model parameter λ changed with different cropping seasons,which was 256mm in winter and 128mm in summer,while ω increased with MWD,the fitted equation between the two isco＝0.0036 MWD + 0.3432,and R2 is 0.68.Surface index(S)decreased with the increase of compacting work.Compared with specific surface area and fragment fractal dimension,S has a higher fit precision with MWD,which was S =-0.0028 MWD + 0.8837 and S =-0.0056 MWD + 0.9216 for the rice crop and the wheat crop,respectively. |
