Open access publications in the SLU publication database

Bulk density is an important soil physical property. However, the soil bulk density is not so often determined by taken soil samples, as the sampling can be quite complicated, especially in stony forest soils. The bulk density may however be estimated, for instance by pedotransfer functions (PTF). In these PTF’s soil bulk density is estimated from other measured soil properties, such as carbon content and soil depth.

In this report previous published PTF’s from several countries, as well as special elaborated PTF’s, were tested for estimation of bulk density by using a dataset of forest soils (833 samples) from Sweden. These samples were analyzed on i.a. the organic carbon content (OC) and bulk density (BD). Sample depth (cm below ground or below the humus layer) was also included in the database. The sample depth varied from 0.5 cm to 185 cm below soil surface. The different PTF’s were tested on the entire dataset (n=833), and for samples from mineral soil horizons (n=678) or the humus layer (n=155). Generally, there was a stronger relationship between BD and OC than between BD and depth.

The tests of the different PTF’s, showed that the special elaborated PTF’s usually fitted better to the Swedish data than earlier published PTF’s. If only the organic carbon content (% DW) is used (or accessible) as an independent variable, the following functions are recommended for calculation of bulk density in Swedish forest soils:

Samples from mineral soil horizons:
BD = 1.7262*EXP(-0.3632*OC0.5)

Samples from the humus layer:
BD=OC/(-2.1278 + 0.1528*OC + 0.2105*OC2)

A general function for samples from both mineral soil and humus layer:
BD = 1.7587*EXP(-0.3865*OC0.5)

If information on sample depth is also available the following functions can be used:

Samples from mineral soil horizons:
BD = 1.5463*EXP(-0.3130*OC0.5) + 0.00207*depth
(depth = sample depth, in cm, measured from the top of the mineral soil)

A general function for samples from both mineral soil and humus layer:
BD = 1.6190*EXP(-0.3615*OC0.5) + 0.00168*depth
(depth = sample depth, in cm, measured from the soil surface; including the humus layer)

The above functions where the depth is included had significant higher coefficient of determination (R2) than the functions with only organic carbon content as independent variable. However, the increased prediction is quite marginal and the mean error is somewhat higher using the functions with depth included.