Mechanism of nutrient preservation and supply by soil and its regulation I. P-preservation and supply by brown earth type vegetable garden soil Study on Fractal Features of Soil Microaggregates during Different Restoration Stages in the Loess Hilly Region Mechanism of nutrient preservation and supply by soil and its regulation IV. Fertility regulation and improvement of brown earth type vegetable garden soil and their essence Mechanism of nutrient preservation and supply by soil and its regulation Ⅲ N preservation and supply by brown earth type vegetable garden soil Mechanism of nutrient preservation and supply by soil and its regulation Ⅱ Binding status of humus in brown earth type vegetable garden soils and its role in fertility