The effect of variation in the amount, composition, form, origin and distribution of organic matter on the reaction of soil to compactive loading under both laboratory and field conditions is reviewed. For experimental purposes, soils having different organic matter contents may be obtained using samples from a number of sites or from a single site at which different organic matter contents have developed as a result of differences in crop management practices, or by the incorporation of organic material in laboratory samples or in field soils. Compactibility tests made with uniaxial compression and impact loading equipment may not give rise to similar conclusions concerning the role of organic matter. Living roots, and to a lesser extent dead roots, provide a filamentous network which, like geotextiles, resists compactive loads. Fungal hyphae have a similar action, particularly within aggregates. Highly humified material increases the stability and strength of aggregates, and hence decreases compactibility. Soil management practices influence the form, amount and distribution of organic matter in soils. The incorporation of straw or other bulky organic materials results in local concentrations of undecomposed material. Undecomposed organic litter may accumulate at the soil surface or within the top few centimetres after zero tillage. Under forest crops, a heavy cover of woody material acts as a protective mat during the passage of loaded wheels. Increases in organic matter may reduce compactibility by increasing resistance to deformation and/or by increasing elasticity (rebound effects). Compactibility is sensitive to even quite small changes in the amount of organic matter. These observations have important implications in the improvement of soil management to avoid over-compaction problems in crop production.

The role of organic matter in soil compactibility: a review of some practical aspects.

The soil and wheel parameters which together influence compaction resulting from the passage of agricultural vehicles are reviewed. The need for standardisation in soil testing procedures and in the presentation of results is stressed along with the desirability for soil characterisation to be consistent with the application of suitable theories of soil—wheel interaction. The critical state soil mechanics theory may find particular application in this context. The role of soil strength, water status and organic matter in influencing the compactibility of field soils is considered. The use of mathematical models to predict compactive responses from a knowledge of the relevant soil and wheel parameters is important in furthering understanding of the subject and in meeting the widespread demand for information on the possible techniques for reduction in compaction. However, such models are unlikely to have general application without a sound theoretical understanding of the behaviour of layered soils when subjected to high stresses of short duration.

Compaction by agricultural vehicles: A review I. Soil and wheel characteristics

Effects of organic matter on the physical and the physicochemical properties of an illitic soil

Four estuaries (Necanicum, Nestucca, Siletz, and Yaquina), located on the northern Oregon coast and within the central Cascadia subduction zone, record evidence of peats that were buried during coseismic subsidence and postseismic deposition of tsunami sands and estuarine sediments as a result of great subduction zone earthquakes (magnitudes greater than 8). The identification of great earthquakes in these estuaries complements similar evidence in other estuaries in northern Oregon. The same number of burial events (6) are found within the top 3 m and within approximately the last 2,800 14 C years in at least seven estuaries on the northern Oregon coast. Unequivocal evidence for great earthquakes could not be provided for all buried peats identified in the stratigraphic record. The stratigraphic records of buried peats vary because of differences in morphology, sediment supply, wetland surface elevations, and preservation potential within and among estuaries. However, consistent records of abrupt subsidence and corresponding tsunami deposition are observed in each of the estuaries, confirming the potential for great earthquakes along the northern Oregon coast. Wetland sites that record the best history of earthquakes are those with 1) elevations that are low enough to document rapid environmental changes that are easily recognized in the stratigraphic record, 2) an adequate sediment supply that allows wetlands to redevelop after coseismic subsidence, 3) locations that are in the path of sand-laden tsunami surges, and 4) locations that are away from migrating channels which erode the geologic record.

Stratigraphic Evidence for Great Subduction-zone Earthquakes at Four Estuaries in Northern Oregon, U.S.A.

The engineering-geological characteristics and classification of the major superficial soils of the Niger Delta

The objectives are to examine the variation of these properties with organic content and to compare the behavior of mechanical mixtures with that of natural samples of comparable organic content. The effect of mineralogy is also studied to check its possible influence on mechanical behavior and on the organic content determination. Two methods of organic content determination, ignition and wet digestion, were used and the results compared. It was found that, for the soils studied, ignition at 400° C was appropriate as a test for organic content. While minor organic content had a strong influence on the compaction behavior, its effect on strength is much less than that of minor mineralogical or structural differences. Oven-drying was found to reduce the influence of the organic matter. Mixtures behave in compaction like natural samples of similar organic content.

Luis F. Orozco

Papers

Compaction and strength of slightly organic soils