Application and comparison of soil compaction indexes in the evaluation of urban soils

Compaction is ubiquitous in urban soil. This study evaluated urban soil compaction degree by comparing three compaction indexes, i.e., direct compaction measurement (DCM), bulk density (Bd) and porosity (Pr) of 52 soil samples in different land uses in Nanjing City. The results showed that most of the soils in the Nanjing City were compacted, and a high portion of them was severely compacted, which might restrict plant growth. DCM, Bd and Pr were consistent in expressing compaction degree and they had significant relationships, so they could substitute one another. It should be noted that soil water content must be considered when using DCM as a compaction index because water content affects DCM value significantly, which limits the application of DCM. Bd and Pr are affected by texture and organic matter content, but they are good indexes of soil compaction degree when soil texture is similar. Aeration porosity is more sensitive than total and capillary porosity to reflect soil compaction degree. Therefore, different compaction indexes can be used in estimating soil compaction, but Bd is often easier and more reliable than DCM and Pr.