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Geostatistical Characteristics of Soil Data from National Soil Survey Works in China
Scientia Agricultura Sinica 2022, 55(13): 2572-2583
Published: 01 July 2022
Abstract PDF (881.1 KB) Collect
Downloads:9
【Objective】

China carried out the second state soil survey from 1979 to 1987 and the soil nutrient investigation of farmland from 2005 to 2017. Both surveys covered the whole country with a huge amount of ground soil samplings. The data generated from the two surveys have become the most detailed spatial-temporal data for soil types and quality in China. The purpose of the study was to test and to evaluate the geostatistical characteristics of the data by geostatistical testing approach, so as to provide the reference for the use of these data to characterize the temporal and spatial distribution of soil features in different disciplines.

【Method】

7 testing areas were selected to represent different regions in China. Soil organic matter (SOM) contents of 0-20 cm soil layer from soil profile sampled in 1979-1987 and from plough layer sampled in 2005-2017 were extracted from the corresponding data bases. The ground sampling for soil profiles in 1979-1987 was to give priority to typical soil types firstly and secondly to keep an evenly distributed sampling as possible. 100 000 soil profiles with about 1m soil deep were finally sampled. After integrated data processing and coordinate matching, 60 000 profiles obtained coordinates. Ground sampling for soil plough layer in 2005-2017 was in grid distribution. 10 000 000 plough layer soil samples with GPS positioning coordinates have been completed. For each testing area, the data set contained two groups, about 500-1 300 SOM values from soil profile data and 50 000-250 000 values from plough layer data. The data from two time groups of each testing data set were analyzed by ordinary Kriging approach separately. 80% of the data were randomly selected as the training sample set for modeling and 20% as the verification sample set. The linear regression between the predicted value and the measured value of the validation sample was carried out. R2 (coefficient of determination) and RMSE (root mean square error) were calculated to evaluate the reliability and uncertainty of the data sets in expressing the spatial distribution of the soil feature.

【Result】

It was showed that the reliability of mapping SOM content by profile data of all of the 7 testing areas reached significant levels. However, the deviation between predicted values and measured values of the test data set was relatively great. The values of R2 were low, between 0.223-0.380 and RMSE were relatively high. Testing results by soil plough layer data sampled in 2005-2017 showed that through large sample size and grid sampling, the reliability and prediction accuracy of mapping SOM content were improved greatly, for R2 increased and RMSE decreased. The geostatistical test results of two periods with a time interval of 30 years showed that although there were some changes in the contents of soil organic matter, the overall spatial distribution of SOM content in each testing area expressed by the two data groups was similar.

【Conclusion】

The reliability and accuracy of soil maps were much better in terms of characterizing the spatial distribution of soil features, when the soil investigation was by means of a large sample size with grid sampling. It meant that the reliability and accuracy of the original large-scale soil thematic maps, such as maps of soil types, organic matter, pH value, soil nitrogen, phosphorus and potassium nutrient contents from second state soil survey, were better than maps generated by profile data, as these original large-scale soil thematic maps were derived from the large sample size with grid sampling. However, the data of 60 000 soil profiles from second state soil survey, which contained many soil features and could supply reliable soil thematic maps, were also of great importance for understanding spatial characteristics of these soil features. It has been showed that a large sample size was essential for a precise and accurate mapping of soil feature of the whole country. For mapping long-term changing or stable soil features such as soil types, texture and morphological features, it would be difficult to obtain reliable maps by a soil sample size much less than the second state soil survey. Considering the current requirements and the available data resources in China, the soil investigation in the future could be mainly focused in investigating data missing areas as well as some missing soil features for soil functions.

Issue
Overview of Soil Survey Works in Main Countries of World
Scientia Agricultura Sinica 2022, 55(18): 3565-3583
Published: 16 September 2022
Abstract PDF (548.9 KB) Collect
Downloads:27

The reviewing soil surveys and soil investigations in different countries shows that in order to understand the soil resources and soil quality, the soil surveys and investigations carried out by different countries since the end of the 19th century can be divided into four categories, namely: survey for soil classification, investigation and evaluation of farmland fertility, testing of soil chemical properties for fertilizer recommendation and soil investigations related to environmental issues. Among the four categories, soil classification is the earliest and the most widely implemented survey works carried out in the world. Main purpose of the classification survey is to clarify the differences in soil types and their spatial distributions caused by various soil formation processes. Since soil formation processes under natural conditions can reach tens of thousands of years, the main outputs of the classification survey—the soil maps and the records of soil profiles, expressing the physicochemical properties of various soil types, have a long timeliness and are widely used in various research areas. After the completion of nation-wide soil classification surveys in the last century in developed countries, the updated survey was not conducted in these countries. In the second national soil survey in China, large-scale soil maps covering the whole country were completed by relatively higher profile sampling density. The physicochemical properties of 100 000 soil profiles, representing different soil types, were also recorded. Both the map scale and the richness of soil profile data from the survey surpassed the outputs from classification surveys in many developed country. Soil classification data obtained from this survey are valid for a long time to different disciplines and sectors. In order to meet needs for easy-to-understand farmland fertility grades by land management sectors and farmers, investigation and evaluation of farmland fertility have been carried out in Central European countries, where per capita arable land resources have been in shortage. Through high-density profile samplings, soil fertility evaluation for each farmland plot with an independent cadastral code was carried out. An officially certified fertility grades in percentile index and the records of soil profiles were established and archived for each plot. The investigation results have been widely used in farmland management, taxation, agricultural subsidies, farmland leasing, trading, lending, insurance and other sectors, and have become the indispensable and centennial basic information for farmland quality. China's per capita arable land is much less than that of Central European countries, but so far there has been still in shortage of soil fertility data for farmland management. For preservation of arable land resources with high fertility and ensure food safety, it is essential and also urgent to establish precise and reliable fertility archives for each farmland plot in China. Also more efficient investigation and evaluation approaches, which are compatible with China's economic and social condition should be developed. The review of testing soil chemical properties to make fertilizer recommendation shows that such testing has been incorporated into the technical supporting system for best farming management practices in the developed countries. To improve farmers' fertilization techniques, it is more important to establish an efficient technical supporting system and maintain its running, in comparison to nation-wide soil nutrient census of farmland. In the past, there was continuous weakening of applied researches and extension works for fertilization technological chain from basic research to farmers' application. Due to inadequate research works, the technical indexes for fertilization, that should be differentiated to and compatible with the various regional soil and climate conditions and hence easy to be followed by farmers of different regions, have not been issued yet in China. Also the intelligent tools that can reach and provide targeted guidance to farmers were not available. Unbalanced and excessive fertilization was common in vegetable, fruit and other cash crop growing area, which accounted for 23.6% of the total cropping area in China. This resulted in reduction of crop yield and quality as well as benefit of farmers and also leaded to agricultural non-point source pollution. Making up for the shortcomings in the technical supporting system has become the key to improve both crop yield and soil fertility. Since the end of the last century, soil investigations related with environment issues have been carried out in different countries. Main purposes of these investigations are to clarify the status of environmental pollution and changing, to develop control strategy and to check effects of countermeasures. With the quick progresses of GIS-, GPD-, RS- and big data techniques, the traditional mapping approaches have be replaced by digital soil mapping techniques. An exact defining of investigation objectives and a comprehensive reviewing of relevant research progress as well as available auxiliary data are essential for drafting sampling design and achieving investigation tasks finally.

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