The Situation And Effect Factor Of Soil Structure In Loess Plateau

The ecological environment of Loess Plateau was very poor. Water erosion and wind erosion were acute in there. Rainfall infiltration, surface runoff, water erosion were influenced by the situation of soil structure. The lower aggregation and higher dispersion were the main reason of dust-storm occurrence. So the soil surface structure has an important effect on soil erosion. The situation and stability of soil structure also were affected by anthropogenic factors and environmental factors. It was means that research of soil structure under different land used conditions can clarify the mechanism and intensity by anthropogenic and environmental factors on soil structure. It was knows that the soil structure can coordinate fertility, so the research can evaluate the situation of soil fertility and indicate the direction of soil quality evolution, offering scientific advice for soil management. The research of soil surface structure under different utility condition, the relationship between soil structure and utility condition was very limited in Loess Plateau. The research of soil structure in Loess Plateau can provide theoretical data for evaluate soil anti-erodibility ,elucidating environmental effect of soil management and utility condition,exploring strategy of environmental construction in Loess Plateau. This paper was in order to discover the influencing factors of soil structure by researching the soil structure situation,physical and chemical properties of aggregate,soil hardness and swelling rate in different farmland and orchard soil of Loess Plateau Guanzhong area. The main conclusions were as fellows: 1. Exploring the dominant factor of forming soil structure and its spatial distribution. Based on the theory of soil structure forming, it was conclude the O.M.,total N.,total P. content was low in research area. CaCO3 content was different in plow layer; the long-term fixed experiment could obvious decrease CaCO3 content. Particle composition of tested soils were uniformly followed the order of Silt> Fine sand> Clay> Coarse sand, and the sum of silt,fine sand and clay was nearly occupied 97% of total soil particle. The clay content was increased from north to south, also from west to east, the coarse sand content was decrease. Eolian deposits of parent material and warm-wet climatic factors in soil-formation process determined the particle composition. The specific soil particle composition,poor O.M. accumulation and the richness of CaCO3 caused the soil in this area with strong dispersion and poor aggregation. 2. The micro-aggregate was the main component in soil of the investigated region and O.M. content was the main determinant of the soil particle aggregate. The micro-aggregate was occupied nearly 97% of total soil, and was the main factor of soil structure in the investigated region. The percentages of different aggregate size were in the order of [0.02~0.002mm]>[0.25~0.05mm]>[<0.002mm]. The initial forming aggregation with poor quality was obvious found. The main factor influence micro-aggregate stability was O.M. content, the clay and CaCO3 content also showed some effects. The structure index (K) increased from north to south, but there were no regularity from west to east. Utility condition has some effect on micro-aggregate forming, micro-aggregate content were higher in orchard soil than farmland, and the long-term experiment could obvious increase aggregation of soil particle. 3. The content of stable aggregate was low, which was main factor influencing soil anti-erodibility and soil fertility. The results of dry sieving was demonstrated a low mechanical stability of aggregate in the investigated region. The aggregate diameter was less than 4 mm unless bad tillage. The content of aggregate (>0.25mm) was higher in south than north, east than west. O.M. content,clay content has some effect on aggregate. However, it was more markedly affected by utility condition, soil management, acute alternation of drying and wetting and environmental factors. The aggregate in research region showed low water stability. The content of aggregate (>5mm) was less than 30%. The water stability of orchard soil aggregate was higher than farmland. As a combined assessment index of aggregate stability, disperse rate was showed lower stability of aggregate in the research region. Utility condition can affect aggregate stability and the disperse rate was higher in farmland than in orchard soil. The long-term experiment could obvious increase particle aggregation. O.M. content and clay content respectively has an obvious correlation with disperse rate. The only way to enhance soil aggregate stability was reinforcing soil management and improving O.M. content. 4. The chemical properties of aggregate reflect the relationship of soil components, and the mechanism of soil structure forming.The O.M. content and total N. content were increased with the decrease of aggregate size. There was a linear correlation between O.M. content and total N. content, the correlation coefficient were increase when the aggregate diameter decrease, it is indicate O.M. has a obvious effect on the aggregation. O.M. type and content were distinct in different aggregate. No obvious variance was found of P in different size aggregates suggesting that the proportion of clay content was also no variance in different size aggregate. When O.M. content was poor, CaCO3 showed some effect on the soil structure forming. But the aggregate ability was limit. Contributing rate of different size aggregate in soil fertility was various, the mean contributing rate was [>10mm]>[5~2mm]>[<0.25mm]>[1~0.5mm]>[10~7mm] >[7~5mm]>[2~1mm]>[0.5~0.25mm]. 5. The physical properties of block cloddy can indicate the effects of tillage on soil structure. Block cloddy was present generally in plow layer of Loess area. It was caused by bad tillage. The structure characteristics of block cloddy was with tight surface but loose inner, the bulk density of crust can be 1.4~2.0g/cm3, which was affect by environment. The tight surface can not be crushed by nature alternation of drying and wetting (ADW). And farming machinery to impact and rotary tillage were necessary. Ca2+ can migration from block cloddy inner to surface during the environment evolution, which was deteriorated the block cloddy surface tight. It can verify the environment factor has great effect on the properties of block cloddy. So it is important to disperse block cloddy after tillage. 6. In poor structure soil, the hardness was rapid increase with the decrease of soil moisture, which velocity was mainly affected by soil texture. In Guanzhong area of Shaanxi province, the plow-pan was in 20~30cm of soil profile, its thickness about 10~15cm. The hardness in the plow-pan was great higher than in the plow layer. Soil hardness and water content has negative correlation. The variation velocity of hardness was difference when water content decrease, the order was Argic horizon > Huangshan soil> Lou soil. When the variation velocity was equal to 0, the soil water content followed the order of Argic horizon > Lou soil> Huangshan soil. Dynamic change of hardness was various with different soil texture, and the more fine the texture, the more higher the hardness. The hardness of Huangshan soil surface was changed slightly. The hardness of subsurface soil changed acutely but affected little by environment variation. The hardness of Lou soil surface changed acutely and significantly affected by water content, the subsurface soil was compact and also was affected by water content. Argic horizon was compact, and only about 15cm surface affect by environment.7. In poor structure soil, the swelling rate was affect by soil texture, O.M. content, soil bulk density and Ca2+ content Soil swelling rate was influenced by soil texture, O.M. content, bulk density, Ca2+ content in different degree. Soil swelling rate was closely correlated to the content of soil clay particle. The soil swelling rate was greatly influenced by the content of soil organic matter with similar soil texture. Accompanying the increase of bulk density; soil swelling rate was increased. The temperate of ADW influenced soil swelling rate, when the temperate changed from 30℃to 40℃, soil swelling rate was increased. The frequency of ADW also affected soil-swelling rate. When the ADW was frequently applied, the soil-swelling rate increased, and pronouncedly increased in the first cycle of ADW treatment.