The soil organic matter (SOM) pool encompasses plant, animal, and microbial residues in all stages of decay and a diversity of heterogeneous organic substances intimately associated with inorganic soil components. The soil microbiota and fine roots may also be considered part of the SOM pool. The turnover of the different SOM components varies continuously due to the complex interaction of biological, chemical, and physical processes in soil. The complexity of SOM and its importance to soil fertility have challenged generations of soil scientists, and numerous studies, of which some date back more than two centuries (see historical review in Kononova, 1961), have covered a vast array of aspects of SOM.

Physical Fractionation of Soil and Organic Matter in Primary Particle Size and Density Separates

X-Ray Diffraction Techniques

Fractionation of organo-mineral complexes by sedimentation and density techniques

Analyses of 16 soils selected to obtain a wide range in properties showed that the results obtained by a buffer method of assaying urease activity were markedly higher than those obtained by a non-buffer method. Both methods gave reproducible results, but the non-buffer method was the more precise, and studies reported showed that it provides a much better index than the buffer method of the ability of soils to hydrolyze urea under natural conditions. The buffer method detects urease activity that does not occur when soils are treated with urea in the absence of buffer. This is a disadvantage if the purpose of the assay is to obtain an index of the ability of soils to hydrolyze fertilizer urea, but is an advantage if the purpose is to detect urease in soils or soil fractions. Use of toluene to inhibit microbial activity in assay of urease activity by the non-buffer method described did not significantly affect the results obtained by this method, but it is decreased the precision of the method.

Comparison of methods of assaying urease activity in soils

Summary
Calibrations enabled controlled application of a range of known dispersive energies to a selected soil sample. The corresponding levels of dispersion were measured in terms of the weight fraction of particles <2 μm equivalent spherical diameter produced, and are used to define a dispersion characteristic for the test soil.

Interpretation of this characteristic in terms of a microaggregate theory provides the energy value associated with complete ‘primary dispersion’ of microaggregates, which under normal field conditions is a direct practical measure of soil stability. This leads to a definition of the specific stability index (σ) of a soil which for the gleyed brown earth used in this work has the value (25±3) J g−1.

D. A. Genrich

Papers

A Reevaluation of the Ultrasonic-Vibration Method of Dispersing Soils1