The soil grade composition is affecting the soil physics and chemistry nature, this article is uses the hydrometer method becomes the soil to southwest area several kind of crags (purple ochre, calcareous soil, volcanic ash soil) the grade to carry on the determination Draws the following conclusion: ⑴The southwest area crag becomes the earth, purple ochre, the calcareous soil and volcanic ash soil's pellet sand grains account for the proportion to be big, the clay, the silt account for the proportion to be ⑵Between identical soil different soil layer, as a result of function and so on leaching deposition influences, on the soil lower level grade composition existence certain difference, rendzina's sand grains and the clay content increases along with the depth But volcanic ash soil's sand grains and the clay content increase along with the depth ⑶Because the different place identical soil the weathering, the leaching function and human activity's influence is different, soil separates composition existence certain ⑷Different soil identical soil layer, because becomes the earth the rock to be different, sandstone growth soil sandy many, the limestone, the mudstone growth's rock clay are ⑸The identical soil type different subgroup's soil, the granulometric composition is also different, but the general trend is