Soil bulk density estimation methods: a review

Measurement of soil bulk density is important for understanding the physical, chemical, and biological properties of soil. Accurate
and rapid soil bulk density measurement techniques play a significant role in agricultural experimental research. This review is a
comprehensive summary of existing measurement methods and evaluates their advantages, disadvantages, potential sources of error,
and directions for future development. These techniques can be broadly categorised as direct and indirect methods. Direct methods
include core, clod, and excavation sampling, whereas indirect methods include the radiation and regression approaches. The core
method is most widely used, but it is time consuming and difficult to use for sampling multiple soil depths. The size of the coring
cylinder used, operator experience, sampling depth, and in-situ soil moisture content significantly affect its accuracy. The clod method is
suitable for use with heavy clay soils, and its accuracy is dependent on equipment calibration, drying time, and operator experience, but
the process is complicated and time consuming. Excavation techniques are most commonly used to evaluate the bulk density of forest
soils, but have major limitations as they cannot be used in soils with large pores and their measurement accuracy is strongly influenced
by soil texture and the type of analysis selected. The indirect methods appear to have greater accuracy than direct approaches, but
have higher costs, are more complex, and require greater operator experience. One such approach uses gamma radiation, and its
accuracy is strongly influenced by soil depth. Regression methods are economical as they can make indirect measurements, but these
depend on good, quality data of soil texture and organic matter content and geographical and climatic properties. Also, like most of
the other approaches, its accuracy decreases with sampling depth.