Studies On Soil Geochemistry And High-Quality Agriculture In Hainan Island

With the area of33920km2, Hainan Island is the second largest island of China, divided from Leizhou Peninsula by Qiongzhou Strait. Located in18°10′-20°10′north latitude, it belongs to tropic zone. Abundant in resources such as lighting, heat and water with splendid natural landscape, this area is the backyard garden for the whole nation, as well as an important tourist site in China. With favorable ecological environment, it remains in the original ecology, as one of places that are less destroyed by the human pollution. With preferential conditions for crop growing, this area is the important growing area for tropical crops, at the same time, as the important base for producing winter vegetable in China.Based on the major project Ecological Geochemistry Survey of Hainan Island cooperatively carried out by China Geological Survey and Hainan People’s Government, this thesis took the surface soil, deep soil and the typical crop production in the whole island of Hainan as the research objects. Through sample collection and analysis, the theoretical method on environmental evaluation of geochemistry is dynamically integrated with the researches in regional natural geography, economic development, agricultural production, geological background, ecological protection and resources development. The geochemical characteristics and crop adaptability are summarized, on which basis, distribution of agricultural production can be conducted, and high-quality and high-efficient development of regional agricultural resources will be achieved.Ⅰ. Geochemical characteristics of soil1. The content distribution of soil elements in Hainan Island reflects the decisive function of soil parent material. For instance, for soil in the quaternary system area, SiO2is rich. For soil in basic volcanic rock area, the iron group element (Fe2O3, Co, Cr, Cu, Mn, Nb, Ni, Ti, V, Sc), macronutrient (N, C, S) and Br, I, AI2O3, Ge, Ga and Ag, Hg, Mo, P and Zn are rich. For the soil in alkaline intrusive rock area, the radioactive element (K2O, Pb, Th, U, Tl, rare earth and its associated element (La, Ce, Y, Be) and Cd, Rb, W, Cl etc. are rich.2. The content distribution of soil elements in Hainan Island reflects the strong interaction of soil weathering and leaching in hot and humid weather condition. For instance, the soil abundance of Al2O3、Sn、T1、U、Th、Ge、Zr、Rb、Pb、Ga and SiO2in deep soil, and Al2O3、 Sn、F、K2O and Zr in topsoil is higher than the one of the nation, alkali metals in both the two soil is lower. About potential of hydrogen (the ph),37percent of the sample in deep soil is less than or equal to5.5and90.7percent is less than or equal to7, while56.2percent in topsoil is less than or equal to5.5and96percent is less than or equal to7.3. The content distribution of soil elements in Hainan Island reflects the supergene geochemical behavior of element. For instance, the content distribution of difficultly migrated and strongly dispersed elements like SiO2、Zr、Ge、 Al2O3、Nb、Ga etc. is relatively well-distributed, while strongly migrated alkali metals like MgO、Sr、Ca、K2O etc. is mostly asymmetrical.4. The content distribution of soil elements in Hainan Island reflects the constraint of soil type. For instance, for coastal saline soil, the background value of B and SiO2is the highest of the whole island, except alkali metals, the background value of most elements is the lowest. For volcanic ash soil, the background value of SiO2is the lowest of the island, except alkaline intrusive rock and granite, the background value of other elements is the highest of the island. Difficultly migrated Zr of paddy soil is rich in topsoil, while the content of alkaline metals is more and more lower with farmland drainage, which is the lowest of the island.5. The content distribution of soil elements in Hainan Island reflects the constraint of topography. For instance, the biology nutrient of yellow soil (in submountainous area) is much richer than latosolic red soil (in middle-low mountainous area).6. The content distribution of soil elements in Hainan Island reflects the influence of ecological frangibility area. For instance, nutrient element like C、N、P etc. in the soil between cultivated land and town housing land got relative depletion, while in the soil of garden plot and forest land, it got relative enrichment. For that matter, human modification and application of land intensified loss of alkali metals, and the content of nutrient element in soil is getting lower for frequent farming and harvesting.7. The content difference of surface and deep soil elements in Hainan Island is the result of internal and external causes of soil genesis. From the enrichment coefficient of topsoil element, we could know that C、Corg and N are of the high enrichment, while S is medium and P、Hg、 Mn、Cd、Sb、Ag、B、Se、Au etc, is low, which is caused by human and biological enrichment. The enrichment of halogens Br、C1and I may be related to continuous reception of topsoil to ocean vapor. Alkali metals gets depletion in topsoil. Both the median and mode of ph in topsoil are higher than that in deep soil, and the area of acid soil (the ph is less than6.5) is1.18times bigger than that if deep soil, which indicates topsoil is more acidic than deep soil.The above geochemical characteristics of soil element indicates that the soil in Hainan Island truly reflects combined influence of soil parent material, climate, landform, vegetation and ecological frangibility area in the soil developing process. Since the geochemical characteristics of soil element perfectly reflects the natural quality in the soil developing process, we could say that the soil in Hainan Island, basically in the ecosystem and not badly polluted by human, is high-quality resource of agricultural industry in Hainan. Besides, the performance quality of material and method in the study is pretty good, sample collection and test data truly reflects geochemical actuality and process of soil. What’s more, the information of the study could be an important frame of reference to studying the soil in Hainan Island changed with natural and social development.Ⅱ. Quality and compartment of soil8、The soil environmental quality of Hainan Island is pretty good. According to the National soil environment quality standard (GB15618-95), this study assesses the environmental quality of surface soil in Hainan Island. The result shows environmental quality of soil is good in that nearly90percent of soil is between Class1and Class2, and only0.91percent exceeds Class3. Major excessive elements caused by geology units are As, Cr, Cu and Ni, which aggregate in the western region. The clean soil and advantageous geographical conditions make Hainan suitable for developing high-quality agriculture.9、Soil crops in Hainan Island lack nutritive element content except potassium. Through the assessment of nutrient elements in the soil, the organic carbon and N contents are low, particularly in the farm land, because of the little organic fertilizer use and much plant absorption. About P, most of the soil is deleted except area in the north Hainan, which is basic igneous rocks area. K2O content is relatively high in the granite area. So the content of nutrient elements in the nature soil is related to geological condition and in the soil of farmland is related to land use and fertilizer.10. Land agricultural regionalization on the basis of heavy metal and beneficial element content. Green land with4parts in Level1covers about7000km2, which is nutritional in soil and good in environmental quality. High-quality land with2parts in Lever2covers about15500km2， in which soil nutrient and environmental quality are quite normal. Safe land with7parts in Level3covers about9250km2, which is poor in soil nutrient and normal in environmental quality. Safe land with6parts covers about1300km2, in which one heavy metal exceeds. Poor land with2parts covers about900km2, in which multiple heavy metal exceeds and badly polluted, and the land to be investigated and controlled distributes in Penglai and Zuntan region of North Hainan. Accordingly, the area of green, good-quality and safe land accounts for93.5percent of the whole island, which is perfect resources for special, High-quality and sustainable agriculture in Hainan Island.Ⅲ. Resources assessment of selenium-rich soil and regionalization of agricultural products11. It is first time to find that there are resources of selenium-rich soil in Hainan Island. The area is9545km2, which covering28.14percent of the whole island, mainly distribute in the mid-eastern region of Hainan Island. Major soil type is laterite (partly volcanic ash soil, paddy soil, latosolic red soil and yellow soil), major soil parent material are metamorphite, basic volcanic rock and granite, and major geomorphic type are tectonic erosive hilly area, piedmont erosive cumulate undulate plains area and volcanic area (partly erosive tectonic middle-low mountainous area and littoral cumulate plains area). There is a selenium area covering8km2of southeastern Chengmai Country in the north of the island.12. Study on influencing factors of soil selenium element content. Soil selenium element content mainly comes from the soil in which metamorphic rock, partly basic volcanic rock and partly granite develop. Physicochemical index like soil organic carbon, CEC, physical clay and acidity is positively promoting the content of total Se and effective Se, which guarantees the subsistence of soil Se and the high content of effective Se, and leads the improvement measures for high content of effective Se and absorption of soil Se by crops.13. Investigation of Se-rich agricultural products. There are14Se-rich agricultural products and4potential Se-rich agricultural products selected from38agricultural products. Se-rich agricultural products could be classified as Crop (early rice, late rice and peanut), potato (winged yam rhizome, pachyrhizus, cassava and dioscorea fordii), fruit (papaya, sozamnmuiiatnm and watermelon) and vegetables (Chinese cabbage, cowpea and brassica chinensis), and potential Se-rich agricultural products are Corn, brassica pekinensis and banana.14. Planting division of Se-rich agricultural products. Basing on the investigation and evaluation of regional soil and Se-rich agricultural products, there is a production regionalization of selenium-enriched rice, potato, fruit and vegetables of the whole island. It is extracted31regionalization area from selenium-enriched rice with988.04km2,16regionalization area from selenium-enriched potato with1279.89km2,12regionalization area from selenium-enriched vegetables with718.92km2and16regionalization area from selenium-enriched fruit with1176.31km2, and4measures and suggestions of planting regional geological survey for se-rich agricultural product are proposed.