The mass per unit volume of soil (including porosity) is called soil bulk density, which is 3 centimeters per gram. Soil quality refers to soil drying at 105~110 C. The magnitude of soil bulk density is affected by texture, structure and tightness. Generally speaking, the pore size of sandy soil is large, but the number is small, the total pore volume is smaller and the bulk density is larger. On the contrary, the pore volume of clay is larger and the bulk density is smaller, and the condition of loam is between the two. However, if the aggregate of loam and clay is well formed, the porosity will increase significantly and the bulk density will decrease correspondingly. The looser soil (especially after tillage), or a large number of root holes, small animal holes or fissures in the soil, has large pore size and small bulk density, and conversely, the greater the soil density, the greater the bulk density. Therefore, the bulk density of the topsoil (or plough layer) is often smaller than that of the core and bottom soil.
Generally speaking, the bulk density of sandy soil varies from 1.2~1.8 gram / cubic centimeter, the bulk density of clay soil varies from 1.0~1.5 gram to cubic centimeter, and the soil bulk density of cultivated soil varies from 1.05~1.35 gram / cubic centimeter, and the soil bottom soil and tight plough soil bulk density change to 1.55~1.80 g / cubic centimeter.
Soil bulk density is a very important basic data affecting crop growth. The bulk density is small, which indicates that the soil is porous and porous and has good structure and is suitable for the growth of crops. On the contrary, the capacity of the soil is important, which indicates that the soil is hard and the soil structure is short of the aggregate, which has a bad effect on the growth of the crops. Generally speaking, the soil bulk density of dry land is within the range of 1.1~1.3 grams / cubic centimeter, which can meet the requirements of the growth and development of many crops. For sandy soil, the suitable bulk density should be higher and the humus rich soil should be lower.