The state of compactness is an important soil structure and quality attribute, and there is a need to find a parameter for its characterization that gives directly comparable values for all soils The use of some relative bulk density value for this purpose, particularly the degree of compactness (Hakansson, 1990), is discussed in this review The degree of compactness has been defined as the dry bulk density of a soil as a percent of a reference bulk density obtained by a standardized uniaxial compression test on large samples at a stress of 200 kPa The bulk density should be determined at standardized moisture conditions, to prevent problems caused by water content variations in swelling/shrinking soils The degree of compactness (D) makes results of soil compaction experiments more generally applicable Whereas the bulk density or porosity optimal for crop growth vary greatly between soils, the optimal D-value is virtually independent of soil composition Critical limits of penetration resistance (3 MPa) and air-filled porosity (10%, v/v) are similarly related to the D-value and matric water tension in most soils As the D-value increases above the optimal, the tension range offering non-limiting conditions becomes increasingly limited The D-value of the plough layer induced by a given number of passes by a certain vehicle is similar in all soils, provided the moisture conditions are comparable The degree of compactness facilitates modelling of soil and crop responses to machinery traffic Although this parameter was primarily introduced for use in annually disturbed soil layers, its use may be extended to undisturbed soil layers

A review of the usefulness of relative bulk density values in studies of soil structure and compaction

Agricultural traffic impacts on soil

Soil compaction due to vehicular traffic causes severe yield reductions on poorly-drained heavy-textured soils in northwest Ohio. Therefore, effects of harvest traffic and tillage methods were studied on soybean [ Glycine max (L.) Merc.] growth and yield and soil physical properties of a clayey soil during the 1996 growing season. Three axle load treatments were imposed with a single axle grain cart loaded with dry corn ( Zea mays L.) grain to achieve a control (0 Mg), a half full cart (10 Mg) and a full cart (20 Mg). Three tillage methods imposed as sub-plot treatments, consisted of no-till, chisel plow, and moldboard plow. Temporal changes in soil water content, soil bulk density, aeration porosity, and oxygen diffusion rate were measured for axle load and tillage treatments during the growing season and related to soybean yield. There were significant temporal changes in soil bulk density. Throughout the growing season, the soil bulk density values ranged from 1.2–1.6 Mg/m 3 measured by the radiation method, and 1.6–1.75 Mg/m 3 by the clod method. Soybean grain yield for the control treatment was 2.5 Mg/ha and was significantly reduced by 9 and 19% for axle loads of 10 and 20 Mg, respectively. The highest yield of 2.5 Mg/ha was obtained for the no-till treatment, and chisel and moldboard plowing significantly reduced soybean grain yield by 9 and 14%, respectively. Soybean grain yield was significantly correlated with soil bulk density, aeration porosity, the interaction of oxygen diffusion rate (ODR) with aeration porosity and soil bulk density.

Axle load and tillage effects on soil physical properties and soybean grain yield on a mollic ochraqualf in northwest Ohio

Studies addressing the role of large herbivores on nitrogen cycling in grasslands have suggested that the direction of effects depends on soil fertility. Via selection for high quality plant species and input of dung and urine, large herbivores have been shown to speed up nitrogen cycling in fertile grassland soils while slowing down nitrogen cycling in unfertile soils. However, recent studies show that large herbivores can reduce nitrogen mineralization in some temperate fertile soils, but not in others. To explain this, we hypothesize that large herbivores can reduce nitrogen mineralization in loamy or clay soils through soil compaction, but not in sandy soils. Especially under wet conditions, strong compaction in clay soils can lead to periods of soil anoxia, which reduces decomposition of soil organic matter and, hence, N mineralization. In this study, we use a long-term (37-year) field experiment on a salt marsh to investigate the hypothesis that the effect of large herbivores on nitrogen mineralization depends on soil texture. Our results confirm that the presence of large herbivores decreased nitrogen mineralization rate in a clay soil, but not in a sandy soil. By comparing a hand-mown treatment with a herbivore-grazed treatment, we show that these differences can be attributed to herbivore-induced changes in soil physical properties rather than to above-ground biomass removal. On clay soil, we find that large herbivores increase the soil water-filled porosity, induce more negative soil redox potentials, reduce soil macrofauna abundance, and reduce decomposition activity. On sandy soil, we observe no changes in these variables in response to grazing. We conclude that effects of large herbivores on nitrogen mineralization cannot be understood without taking soil texture, soil moisture, and feedbacks through soil macrofauna into account.

/pdf/herbivore-trampling-as-an-alternative-pathway-for-explaining-2qxmnsduvs.pdf

Herbivore trampling as an alternative pathway for explaining differences in nitrogen mineralization in moist grasslands

Summary Compactive processes affect soil hydraulic properties and associated soil water flow. Soil water retention and transport properties are altered in response to changes in pore space geometry. Soil water flow is affected not only by soil hydraulic properties but additionally by the distribution of sources and sinks of water in the soil system. Compaction can alter soil pore geometry, and can also affect sources and sinks of water by changing surface configuration, and crop rooting distribution. This paper reviews the literature and presents data and relationships showing the effects of compaction on soil hydraulic properties and water flow, presents numerically modeled water flow for some management systems, and identifies future directions for research.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1312&context=agron_pubs

Effects of Compaction on Soil Hydraulic Properties

Effect of Preplant Wheel Traffic on Soil Compaction, Water Use, and Growth of Spring Wheat

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Effect of Preplant Wheel Traffic on Soil Compaction, Water Use, and Growth of Spring Wheat