Methodology Of Soil Suitability Evaluation

Soil suitability evaluation aims to optimally allocate crop planting through measuring the coupling of designating crop to given land, and considers simultaneously soil physical features and current/future land use patterns. Nowadays, with the combination of mathematical theory and computer technique, multi-objective method based assessment for soil suitability has been increasingly observed. However, recognizing the excessive complexity of the interactions among factors influencing soil quality, some papers often confine these processes of soil quality to either a single process, or a single discipline. Certainly, assessment methods also are only limited to one or several in terms of the study objective, even individual experience and preference. Under such circumstances, the characteristics of different assessment methods can't be presented; furthermore, the acourate and reliable assessment results for soil suitability can't be obtained. In hilly and mountainous areas, complex topography features reduce the greater variability of soil properties at spatial scale. In these areas, currently, the greatest challenge has been what assessment methods, at some degree, can improve estimate accuracy of soil suitability thus being implemented to practices? Moreover, some researchers confine given soil to homogeneous units, and often the results of soil suitability from sampling units to the whole area. Seemingly, the results of these studies carry out spatial scaling, but fail to present soil being a continuous body at spatial-temporal scale, due to be short of theory and application. Undoubtedly, the pronounced differences between research results and soil practical value at some certain can occur.In this study, a GIS-based decision support system was built for assessing soil suitability in tobacco-planted areas, Pengshui of Chongqing, China. This was done by integrating an exponential test, geiostatistics, fuzzy mathematics and artificial neural network into geographical information system software to assess soil suitability. In this system, complex problems are solved within program: this paper, firstly, compared the results and their accuracy obtained by different methods, and then planned land use patterns in tobacco-planted areas through combining climate factors and farmers' household behaviors. These results indicated:1. Assessment of soil suitability based on exponential testGIS-based assessment indicators of soil suitability were selected through frequent statistic and system analysis methods. Designing weight for every soil quality indicators and evaluating experiential index of every soil sampling were obtained by integrating AHP and expert decision support system. And assessment of soil suitability was carried out applying subareas of an administrative village and Kriging spatial interpolation, in terms of experiential index of every soil sampling, and tested by coupling classification index of tobacco sampling and these results from different methods. The difference between different treatments was significant. At slope scale, assessment results from dryland and regional treatments increased following research scale upscale. The variation proportions of suitable areas (e.g., moderate/highly suitability) to limited areas (marginal/non suitability) were enhanced. Based on subareas of an administrative village, at dryland scale, the proportions of suitable areas to limited areas was 1.24:1, while at regional scale, this number was 2:1. Applying Kriging spatial interpolation, they were 2:1 and 3:1, at dryland or regional scale, respectively. The coincidence rate of assessment results from subareas of an administrative village and Kriging spatial interpolation was 20.37% and 46.30%, respectively. They both were excessive 50%. But, the assessment results from Kriging spatial interpolation were more accurate.2. Assessment of soil suitability based on geostatisticsSpatial model of soil properties was built to predict the regional suitability of different soil types, analyzing the characteristics of spatial distributions of soil properties in tobacco-planted areas, and the accuracy of geostatistics method used in regional scale and mountainous conditions. The characteristics of spatial variation of soil properties were measured by integrating classical statistics and geostatistics, and considering anisotropic and trend effect. Thus soil suitability in tobacco-planted areas was assessed, and accuracy test was presented using the same methods described in first context. Sampling data of assessment indicators of soil suitability, associating with effects of factors, presented different distributions and trend effects. Assessment indicators of soil suitability reached geometric anisotropy. Spherical model, exponential model and Gaussian model were the best fitting model of semivariance function of soil suitable indicators. Most of raw data were non-normal distribution, due to the effects of terrain, soil sample size and analytical error. Indices data were trend effects of the first/second order. Every indicator was normal distribution, due to soil sample size and simulation equation, and trend effects were not pronounced. Similarly, indices of mixture samples were normal distribution, and presented trend effects of the second order as well. At different scale, significant differences between different data treatments occurred. The proportions of suitable areas to limited areas were 4.34:1, 2.86:1 and 2.44:1, when the raw data treated at sampling, dryland and regional scale, respectively. Under different soil types, the proportions of corresponding scale were 6.70:1, 7.22:1 and 4.97:1, respectively. However, mixture data treatments resulted in the proportions accounting to 6.70:1, 6.62:1 and 5.40:1, under above-mentioned corresponding scale, respectively. Increasing samplings of different soil types can effectively improve accuracy by adopting spatial simulation. This method integrated geographical environmental factors and spatial distributions of regional soil properties, and can obtained more closed results to the reality.3. Assessment of soil suitability based on fuzzy mathematicsUnder Matlab7.0, fuzzy ISODATA algorithm and comprehensive evaluation were adopted to assess soil suitability at different scales, and the same method of accuracy test was used. The proportions of suitable areas, including moderate/highly suitability, obtained by fuzzy comprehensive evaluation, were obviously greater than the results of clustered by fuzzy ISODATA. The reasons for these results were that fuzzy comprehensive evaluation emphasized excessively the effects of higher value indicators, when multifactor summation with different weight done. Significant differences between ranges of soil suitability can observed at different scales. The proportions of suitable areas to limited areas were (1.78:1 and 1.16:1), (1.86:1 and 1.10:1) and (1.66:1 and 0.67:1), at sampling, dryland and regional scale, respectively. The accuracy of fuzzy ISODATA algorithm was higher at regional scale than that of fuzzy comprehensive evaluation.4. Assessment of soil suitability based on artificial neural networkArtificial neural network and GIS were used to assess soil suitability in tobacco-planted areas, associating with Matlab7.0. In this part, the method of accuracy test was similar to that described in above context. Soil suitability of tobacco-planted areas was divided into four classifications, using the learning algorithm of RBF and BP neural network. This result indicated that artificial neural network is possible to assess soil suitability. Obviously differences between these results from RBF and BP soil suitability assessment can be observed at different scales. The proportions of suitable areas to limited areas were (3.35:1 and 2.35:1), (1.01:1 and 2.09:1) and (1.10:1 and 1.74:1), at sampling, dryland and regional scale, respectively. The accuracy of RBF neural network was greater than that of BP neural network, compared the results of soil suitability assessment.5. GIS-based comparison of soil suitability assessmentThe theory foundation of different evaluation methods was various, and the evaluation processes were clearly different. Each one had its own advantages and disadvantages. Exponential test method, fuzzy clustering and neural network were mainly for the evaluation and gradation of soil sampling at small scales. Then did upscale according to evaluation units, furthermore soil suitability assessment transformed from geological variable space to comment space. But, this method had the higher accuracy at the point scale and was lack of theory foundation, when transformed from point to space. The geostatistics spatially simulated and evaluated the continuous variables at regional scale, overlaying the variable layers to get the comprehensive research objectives. There was some mathematics knowledge, and the whole evaluation processes were executed through computers. So the manmade errors were reduced and the evaluation result was very objective. (2) Comparing the evaluation results obtained by above-mentioned methods and indicators through double variables and partial correlation analysis, showed: the simple correlation analysis of the double variables and partial correlation analysis were similar. Significant positive relation among elevation, pH, available K, etc. grained by different methods occurred. Thus those indicators strongly affected the evaluation results, and also were the major factors that affect the tobacco growth. The relations of organic matter, available N and P, water-soluble Chloride, slope and physical clay obtained by different methods were different. Availability of soil nutrients measured by different methods was clearly different, while the other indicators were lower different. Certainly, different methods applied, for the same indictor, could present different results. CEC was not obviously relation among different methods thus CEC possessing lowly effects on the result. (3) Geostatistics was suitable for the soil suitability evaluation in study area. Comparing the accuracy of different evaluation methods, and showed: the evaluation results through geostatistics and the actual quality of tobacco had a higher corresponding ratio accounting to 74.07%. Whilst, the corresponding ratio of RBF-ANN, fuzzy ISODATA and exponential test were 61.11%, 57.40% and 46.30%, respectively. The research believed that the corresponding ratio of geostatistics was the highest and the most accuracy from the point of corresponding views. This method was suitable for the soil suitability evaluation.6. Comprehensive suitability evaluation of tobacco-planted areas in Pengshui CountyIntegrating GIS, Geostatistics and mathematics, spatial distribution model was built. The geostatistics accuracy at regional scale and mountain simulation evaluation, and the meteorological data (1975-2005) of Pengshui County were applied to do the single factor suitability regionalization. The major factors and behaviors of tobacco farmer investment through participant rural appraisal were analyzed. According to meteorological observation distributions, factual tobacco planting situations, physical geography conditions and district regionalization of Pengshui County, this paper divided tobacco-planted soils into the region by along the river valley zone, Northern middle mountainous and hilly zone, Southeastern hilly zone. Applying comprehensive indices calculated the suitability of every subarea. (1) Soil spatial variability of Pengshui County was pronounced significant. Climatic and topographic conditions were suitable to tobacco growth. Physical geographical conditions could reach the tobacco growth demands. (2) Tobacco fanner part-farm behavior, land size and labors were the main factors that affected the tobacco planting investment. The higher tobacco farmer part-farm percentage and the tobacco non-agricultural income ratio were, and the smaller the possibility of tobacco farmer investment behavior and planting size. Land fragment leaded to investment benefit decreasing and make the farmers unwilling to addition input. Labor conditions of tobacco farmer often became a restrict factor for the tobacco planting. (3) The suitable indices of tobacco-planted soils along river valley zone were 0.634 thus belonging to the lowest suitable gradation, and don't fit planting tobacco. In Northern middle mountainous and hilly zone, where was suitable to plant higher productive and quality tobacco, with the more suitable gradation 0.864. Moderate gradation located in Southeastern hilly zone with 0.723 was good to large-scale planting.This paper compared spatial variability of soil properties under different geographical conditions and methods of soil suitability assessment at different scales. Based on continuous distribution function and mathematics, spatial simulation model and scaling assessment methods of different soil properties. This research solved effectively these complex issues, which produced by spatial non-uniform and variability of soil properties due to sparse sampling, and at some certain, declined the effects of uniform soil viewpoint on study results. Moreover, GIS-based regional soil evaluation methods favored to soil quality measuring under complex environments, and improved evaluation accuracy. However, soil is a continuous body at spatial-temporal scale, and its spatial variability and scaling have been paid more attention to by multidisciplinary.