Changing procedures for soil texture analysis

Texture analysis is the determination of the particle size distribution of the soil, where the small particles (clay and silt) are determined by sedimentation, while the larger particles (sand) are determined by wet sieving. The following changes have been implemented: 

• For estimating <20-µm particles (clay+silt), two hydrometer readings are used after 5 and 10 minutes, respectively. Previously only one reading was taken after 6.5 minutes, which may be problematic, since the reading after 6.5 minutes does not necessarily correspond to a particle diameter of 20 µm. Prospectively, the clay+silt content can be interpolated based on the two readings, which will give a more accurate estimate.
• For the estimation of <2-µm particles (clay), readings after 4and 18 hours are now used. Previously readings after 2 and 18 hours were used. Interpolation based on the new readings will provide a more accurate clay estimate.
• In the calculation of clay (<2 µm) and silt content (2-20 µm), the logarithm of the particle diameter (D) is now used. Previously, D was used. In international as well as Danish protocols, the logarithm of D is used.
• When calculating the soil organic matter content in connection with soil texture analysis, a conversion factor from soil organic carbon to soil organic matter of 2.00 is now used. Previously, a conversion factor of 1.70 was used. The change occurs as a result of the findings in Jensen et al. (2018).
• It is now possible to choose to have a dry matter analysis done for a sub-sample as part of the soil texture analysis, i.e. determination of soil water loss from air-dry to oven-dry condition. Internationally, soil properties are expressed in relation to oven-dry soil weight, which is why it is recommended.
• It is still possible to actively choose to have organic matter removed before soil texture analysis. The rationale for this is described in a previous biweekly article  (“Texture Determination: Opportunity to Follow International Practice”) as well as in Jensen et al. (2017).

Jensen, J. L., Christensen, B. T., Schjønning, P., Watts, C. W., Munkholm, L. J. 2018. Converting loss-on-ignition to organic carbon content in arable topsoil: pitfalls and proposed procedure. European Journal of Soil Science, 69, 604-612. 

Jensen, J. L., Schjønning, P., Watts, C. W., Christensen, B. T., Munkholm, L. J. 2017. Soil texture analysis revisited: Removal of organic matter matters more than ever. PLoS One, 12, e0178039.