The Structure Status And Effect Of Topsoil In Loess Plateau

Because of the weak structure performance and stability of the topsoil in loess plateau, the structure became fragile and the surface layer became compact easily under intensive change of environmental factors, which were the main obstacles of fertility development in this area. Also the hardness of surface layer soil became more and more serious year after year under the chemical fertilizer application.For soil matter composition is the important factor that affects the topsoil structure, the rich CaCO3 in soil was transferred to soil layer, which was the important factor that hardened the soil; also the organic fertilizer application decreased while more and more abio-fertilizer was applied significantly, which caused the organic matter content decreased sharply, furthermore, farming patterns changed, so the topsoil become more compact and crop growth was restrained severely than before. Loess Plateau lies in drought and semiarid climate area where climate change intensively and ADW(Alternation of Drying and Wetting)occur frequently, so soil swelling and shrinking occur alternately, the topsoil structure developed hardly and its stability is usually weak; also the soil easily suffers erosion in rainstorm, which was a serious problem in this area. Before there were many researches focused on the structure characteristics of soil structure, the researches on the effect of soil matter and environment factors on soil structure formed were inadequate, even blank. So the study on the formed factors of topsoil structure became urgent in soil science.This paper was to discover the factors affected soil structure by investigating the soil structure situation,soil matter composition and environment factors; also the function and effect of different topsoil tightness of soil were studied inside in this paper. The main conclusions were as fellows:1. The soil organic matter content was very low and ranged between 2.28～15.34g/kg in this area, and the content of topsoil was >10g/kg and <10g/kg in bottom respectively. CaCO3 content had a certain difference in different topsoil and the scope was 87.21～127.30g/kg , also the content usually was >100g/kg in the bottom. Fine sand and silt was the framework of topsoil particle composition, and the coarse sand content decreased from north to south in the representation that the content of fine sand decreased from 62.96% to 8.6% while the clay content increased from about 22.14% to 52.83%. The performance of soil particle composition, poor O.M. content and the richness of CaCO3 caused the soil with strong dispersion and hardened topsoil in this area.2. Farmland and orchard soil hardened seriously in this area, and the crust thickness and shape had a certain correlation with vegetation cover. The crust was thicker but unitary structure with vegetation cover; if without cover the structure was duality of top compact and incompact below; the thickness of topsoil crust usually between 2cm and 4cm, and the bulk density of crust usually was >1.40 g/cm³, also with the crust thickness increase the bulk density decrease; the farmland had maximal compaction in topsoil and with depth increase the compaction increase , when it approached 20cm the compaction had max value and subsequently decreased until about 3000KPa. Orchard surface layer soil was harder than other using type, and the compaction was uniform in whole section; the vegetable land was incompact in whole section; the compaction of fallow soil was very low in topsoil and higher in bottom but lower than that of farmland.3. The main cause of soil structure instability and harden were frequently swelling and shrinking, soil swelling rate was influenced by soil texture, O.M. content, CaCO3 content, aggregate size, and salt ion in different degree. The results showed that the soil particle fraction were one of the most important influential factors on soil swelling rate, the maximum of soil swelling rate was significantly positive correlated with silt particle content (<0.02mm), and was significantly negative correlated with the content of CaCO3, of which the clay particle had double effects on soil swelling rate. The influence of organic matter on soil swelling rate was not obvious in appearance from the analysis of correlation, but in fact, it restrained the soil swelling rate from removing of soil organic matter significantly. Soil swelling rate decreased with aggregate size increase and the <0.25mm contribution was biggest. Different salt ions had different effect on soil swelling rate that sodium and ammonium salt improved swelling rate but calcium and aluminium salt restrained it. The process of ADW affected the swelling rate remarkably. When the ADW increased frequently, the soil-swelling rate increased, and pronouncedly increased in the first time of ADW treatment. The temperature of ADW influenced soil swelling rate, with the temperature heightened the soil swelling rate increased.4. The continual of ADW was the environment and dynamics factors of soil structure and hardness status, the bulk density was increased after ADW and it commonly was >1.40g/cm³, and even reached 1.50g/cm³; with the ADW intension increase the bulk density enhanced differently, and with the frequency of ADW increased it decreased, but not clearly as the former. The bulk density had some degree decrease by removing CaCO3 or OM compared with soil without treatment, and the effect of removing CaCO₃ was more evident than that of OM.5. The effect of different topsoil tightness on soil biology activity was significant. The results of drying effect experiment indicated that incompact of surface layer was important to keep soil biology activity. The biology activity was intensive when surface layer incompact, by contrary the biology activity was weak.