Grading of germanium contents and establishment of standard for germanium-enrichment in cultivated soils in Guizhou Province

Germanium is a rare and dispersed element in nature. Positioned between metals and non-metals on the periodic table, germanium possesses properties of both groups. In soils, germanium mainly originates from rock weathering and mineral formation. Monitoring and statistical data show that the background content of germanium in soil in China is 1.7 mg·kg⁻¹, while the latest research suggests that the background or benchmark value for germanium in cultivated soils in China ranges from 1.3 mg·kg⁻¹ to 1.4 mg·kg⁻¹. At present, studies on grading germanium contents in soils mostly adopt the cumulative frequency analysis method. This approach tests whether the data follow a normal distribution or log-normal distribution. Once the data conform to one of these distributions, a critical percentile value is established to determine the grading standard, which is typically divided into five levels. This germanium content in soils is then classified according to these levels. In recent years, Chinese government’s emphasis on functional agriculture has led to a surge in research and exploitation of germanium-enriched soil and crops. However, no unified national or industry standards currently exist; only Heilongjiang Province has issued the local standard for evaluating germanium-enriched soil. Therefore, the timely establishment of such standard is of great significance for the industrialization of germanium-enriched products and for the sustainable and healthy development of the relevant market.The geochemical survey and evaluation of cultivated land quality in Guizhou Province is a high-precision study encompassing all cultivated land in the province at a scale of 1:50,000. The sampling areas mainly include cultivated lands, such as paddy fields, dry land, irrigated land, and a small proportion of tea gardens, orchards, etc. A total of 454,453 surface soil samples from cultivated lands were collected. Based on the analysis and testing results of these samples, this study adopted three grading methods to classify soil germanium contents and calculated the critical values for grading soil germanium contents according to different influencing factors. The threshold values for five grades of all samples were calculated as 2.00 mg·kg⁻¹, 1.70 mg·kg⁻¹, 1.35 mg·kg⁻¹ and 1.07 mg·kg⁻¹, respectively. These values were divided into five levels to represent the abundance or deficiency of germanium contents in soils. The critical value of the first level is generally used as the standard for germanium enrichment. Considering the current germanium contents in cultivated soils of Guizhou Province, the proportion of germanium-enriched soils, and the development needs of the germanium enrichment industry, it is advisable to use the second-level critical value of 1.70 mg·kg⁻¹-corrected by the arithmetic mean method across all samples-as the reference standard for germanium enrichment in cultivated soils in the province. Additionally, reference values for germanium enrichment in soils can also be established based on variations in soil pH ranges, parent rocks, or land use types. This study uses 1.70 mg·kg⁻¹ as the reference value. The number of samples meeting the standard for germanium enrichment in soils is 126,395, accounting for 27.8%. The cultivated area characterized by soils enriched with germanium in the province is about 16.08 million mu, representing 22.3% of the total surveyed area. The sample scope of this study covers agricultural soils throughout Guizhou Province, ensuring broad representativeness and diversity. Analysis and testing were carried out in accordance with the established standard, employing quality control measures such as inclusion of the substances meeting the national first-class standard, monitoring samples, repeated checks, and spot checks for anomalies. Therefore, the results are authentic, scientific, and valuable for reference. In addition, the contents of organic matter, aluminum/iron oxides, clay minerals, and redox potential in soils have a certain impact on the available contents of germanium in soils. Future research should focus on a more detailed classification of standards for germanium enrichment in soils.