Spatial variability of soil properties in a qualitative suitability map unit (A case study: soybean cultivation in Astaneh area, Guilan province)

Abstract
Background and objectives: Soil maps are a common source of soil information for land suitability evaluation. Thus, soil mapping is one of the most important steps in soil studies. Indeed, soil mapping has a significant effect in land management because that used as a source of soil data for decision making in land suitability for different purposes. Therefore, the accuracy of the source maps used for crop recommendations is due to the accuracy of soil maps. Soil is a complex system and effective factors on its changes are not identified properly. So preparation of soil maps can show the soils variability, is one of the main apprehension of soil scientists. The objective of this study was to qualitative land suitability evaluation for soybean in Astaneh area, Guilan province and investigate the soil properties’ spatial variability in one of the qualitative land suitability map in study area.
Materials and methods: In order to qualitative land suitability evaluation 24 pedons were excavated. Then, representative pedons were chosen and soil samples were taken from the genetic horizons to determine soil classification. Qualitative land suitability maps were obtained according to representative pedon analysis using parametric method. Next to know the spatial variability of soil properties, 76 surface soil samples (0-30 cm) were taken from a regular grid of 20 × 20 m in a 80 × 380m plot. Ordinary kriged maps were achieved for studied soil attributes such as percent of clay, silt, sand, organic matter, equivalent calcium carbonate, pH and EC after physico-chemical analyses.
Results: The results of qualitative land suitability evaluation showed that the study area is moderately and marginally suitable (S2, S3) and soil drainage is the main constraint for soybean production. Statistical results in land unit with marginal suitability class (S3) showed that the highest and lowest CV was related to EC (24.35%) and pH (1.03%) respectively. Variography showed that a good spatial structure for studied variables. The ranges of spatial dependence showed a variation from 22.1 m for pH up to 120 m for EC. Kriged maps demonstrated that soil properties did not have a random pattern but had a spatial distribution. Considering the kriged maps were generated for land unit with marginal suitability class (S3w), it can be stated that expansion of representative pedon suitability class to all unit area and assumption of uniformity soil attributes in suitability map units may lead to not credible results.
Conclusion: Soil properties’ spatial variability pattern can be different in a farm managed by a farmer. Generalization of representative pedon analyses to all unit area, the use of soil map units as land suitability units may lead to unsatisfactory results. This indicates that these land suitability maps have not enough credibility for land use planning. Using information of all pedons as well as representative pedons in land units and combining the information of suitability maps with geostatistical data can be a choice way to improve the accuracy and quality of land suitability maps.
Keywords: Qualitative land suitability evaluation, Variogram, Kriging, Soil mapping