Based on field sampling

indoor simulation experiment and laboratory analysis

roots density parameters (i.e.

root length density

root volume density

root surface area volume) in different roots diameter classes of four typical plants (Sophora viciifolia

Bothriochloa ischaemum

Artemisia gmelinii and Lespedeza davurica) in the hilly area of the Loess Plateau

and their relationships with soil physicochemical properties and soil separation rate were studied in this paper. The results showed that: (1) Root diameter of the four plants were mainly concentrated in the range of 0～1.0 mm

and root with these diameters accounted for 86%～93% of root length and 46%～61% of root surface area. And the root diameter was larger

the root density parameters were smaller. (2) The effect of root density parameters on soil physicochemical properties (bulk density

organic matter

and aggregates) were different among the four plants. Roots density parameters of S. viciifolia were significantly correlated with the soil bulk density and organic matter when the root diameter was 0～1.0 mm. The root density parameters of A. gmelinii were significantly and negatively correlated with soil bulk density

while they were significantly positively correlated with soil organic matter in 0～1.0 mm diameter

there were significantly positive correlations between soil aggregates and roots density parameters in all diameter classes. The root with 0～5.0 mm diameter of B. ischaemum could effectively improve the soil bulk density

and the density parameters of the root with diameter of 0～2.0 mm were significantly positively correlated with soil organic matter and aggregate. The roots with diameter of 3.0～5.0 mm of L. davurica had significantly effect on soil bulk density

and the roots with diameter larger than 1.0 mm had significantly effect on soil water stable aggregate. (3) There was a negative correlation between soil separation rate and the root density parameters. Besides L. davurica

the other plants’ roots density parameters were significantly negatively correlated with soil separation rate when the roots were 0～1.0 mm in diameter.