| Loess surface crust is a natural phenomenon commonly occurrencing on the ploughland with is widely distributed on the Loess Plateau of china, it occupies on important place in water erosion process of slope land. As slope ploughland in southern loess plateau for objective, Using the principles and methods of soil science, eroded science, agriculture science, adopting technical line of combining with indoor simulated rainfall method and out door location observe and survey, macro-experiments and micro-analysis, systematic analysis and research the formation and function of soil crust on slope ploughland. The major results contain four aspects as follows:1. The formation of soil crust: The effect of directly raindrop impacting on the ground and the pressure of runoff to the ground and the two mutual effect lead to the development of crust. The main effect shows the crust hardness and bulk density and thickness of crust layer. On the whole, the comprehensive effect on development of crust about rainfall and runoff is higher than the effect of rainfall and runoff factor individually, and the effect of rainfall is higher than the effect of runoff.2.Characteristic of Crust: (1) The main composition of soil crust is fine sand and coarse mealy sand, while their contents are lower than comparison soil and the contents of the crust with physically clay (<0.01mm) are higher than comparison soil. (2) The contents of the organic mater and CaCO3 about soil crust is lower than non-crust slightly. (3) Crust could improve soil hardness and has negative relation with the water content. The thicker and the harder, but average bulk density decrease with it. For the particular soil, the corresponding values can be obtained through some values of observation. 3. Erosive effect of soil crust: (1) On the condition of splash about single raindrop, the result shows that soil splash may be decreased to the soil with crust through the splash research of soil crust, The vaviation of the earlier stage water content and raindrop diameter is the same. The effect of crust decreases and splash has a increasing trend as the increasing of the earlier stage water content and raindrop diameter. The effect of the thickness of crust is obvious, the splash decrease as the increasing of thickness gradually. To field rainfall, On the condition of weak rainfall density, the anti-erosion of crust is comparatively obvious where on the condition of stronge rainfall density, the development of splash can be decreased. (2) Loess crust may reduce infiltration of rainfall and increase the topsoil runoff. When the rainfall is comparatively weak, the effect of crust is obvious, the effect is not obvious when the rainfall is comparatively stronge. On the condition of weak rainfall density, the process of infiltration may be divided into three stages, that is, the stage of original infiltration, the sharp decline stage of infiltration and the stage of stable infiltration. On the condition of stronge rainfall density, the process has only two stages that is, the stage sharp decline of infiltration, the stable stage of infiltration. (3) The effect of soil crust about single time rainfall is obvious. The time of runoff is early on the slope ploughland with crust, and the amounts of runoff is increasing and sediment is decreasing. On the whole, soil erosion is decreased. The more obvious is the effect, the higher is rainfall density. The results showed runoff has increasing trend and soil erosion has decreasing trend as the increasing of rainfall density and slope degree through many fields rainfall. In addition, rainfall-erosion with crust soil is related with the earlier stage water content (C), sloping degree (S) and rainfall power (PI).4. Effect crust on the crop growing: The effect crust on rate of emergence rate of wheat is relatively obvious but it hasn't nearly effect on maize. The wheat with crust soil grows slowly and its average height is low. The effect crust soil on the depth of soil water content is limitation. Water content of topsoil wi... |
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