Objective In recent years, the problem of soil compaction in mollisols has become increasingly prominent and is considered one of the main characteristics of mollisols degradation. The relationship between soil penetration resistance and soil moisture status is closely related, but the unclear relationship has increased the difficulties of investigating and diagnosing soil compaction in mollisols.

Methods In this study, typical black soil was selected as the research object, with Baijiang soil as the control. The relationship between soil penetration resistance and soil moisture was studied under different bulk densities, and a transfer function for soil penetration resistance with respect to soil moisture and bulk density was constructed to facilitate the conversion of penetration resistance under different soil moisture conditions. For this purpose, six levels of bulk density ranging from 1.0 to 1.5 g cm⁻³ were set for black soil, and five levels ranging from 1.2 to 1.6 g cm-3 were set for albic soil. Soil moisture was set at seven levels ranging from 40% to 100% of field capacity. Penetration resistance was measured under different combinations of these factors.

Results The results found that soil penetration resistance linearly increased with increasing bulk density, and exponentially increased with decreasing soil moisture (P < 0.01). The rate of increase in baijiang soil was higher than that of black soil. There was a significant interaction between soil bulk density and moisture in relation to penetration resistance. Low soil moisture intensified the impact of bulk density on penetration resistance, while high bulk density increased the effect of soil moisture on penetration resistance. Penetration resistance was more sensitive to soil moisture than bulk density, and baijiang soil exhibited higher sensitivity to both factors compared to black soil. The peak sensitivity values for bulk density were 4.5 for black soil and 7.0 for baijiang soil, while for soil moisture, the values were 11.1 for black soil and 15.1 for baijiang soil. The relationship between soil penetration resistance and bulk density/moisture could be expressed using a binary power function, with determination coefficients above 0.95 for both baijiang soil and black soil.

Conclusion By using the constructed transfer function, the conversion of soil penetration resistance under different soil moisture conditions was achieved. Furthermore, for the same soil type, the conversion coefficients at moderate bulk densities could be applied to other bulk densities. Similarly, the conversion coefficients for black soil could be used for baijiang soil under the same bulk density.