Facultad de Ciencias, University of La Coruña, 15071, La Zapateira, La Coruña, Spain
SUMMARY
Dry and wet pore space organization of a set of seventeen grassland and adjacent cultivated soil samples was assessed using different methods. Specific volume dynamics, derived from aggregate shrinkage data and pore size distribution patterns, as measured by water characteristics retention and Hg intrusion porosimetry were studied. On grassland horizons the compactness was less than on its cultivated counterparts. The low shrinkage potential of the studied soils was verified. Mercury intrusion porosimetry showed that lacunar pores prevailed, whose volume increased as organic carbon content increased. The volume of clay-fabric pores was very small and did not appear to depend on the variation in organic matter content. Water content at saturation increased with increasing organic matter content, whereas for potentials of about 1 500 kPa water retention curves tended to converge.
Keywords: aggregates, Hg porosimetry, organic matter, porosity, resilience, shrinkage
INTRODUCTION
Soil structure is highly sensitive to human activity and in many parts of the world there is evidence that it is becoming less favourable for agriculture. Since numerical values and indices almost always depend on the measurement procedure, there is no general agreement on criteria to judge soil structural decline under different soil management practices. Thus, when dealing with the impact of cultivation on soil structure, a large number of methods have been used. The main effect of the diversity of procedures was to minimize the possibility of inter-experiment comparisons.
Some structural characteristics, such as porosity, strength and stability are considered to be most important (Kay, 1990), because they influence a multitude of processes. Central to the assessment of soil structural quality and increasingly used is pore space analysis (porosity, pore size distribution, shrinkage-swelling, resilience, etc). Soil pore space can be partitioned into a structural compartment and a textural one (Monnier et al., 1973). Soil textural porosity represents the pore space due to packing of soil. The structural pore space results from the arrangement of aggregates and depends on factors such as climate, soil organisms and cultivation (Stengel et al., 1984, Horn, 1990).
Textural porosity can be calculated from soil bulk-density data determined on small volume samples. This measurement standardization, allows the testing of the consistency of the above-mentioned pore space model and also increases the possibility of inter-experiment comparisons.
One method commonly used for assessing the effects of cultivation on the soil system is to compare grassland and cropland. In the present study changes in soil structure due to tillage practices on moderately coarse to medium-textured soils were investigated. These were assessed relative to adjacent grassland areas.
MATERIAL AND METHODS
The investigated soils are located in Galicia (Northwest Spain). Clods were collected from the surface horizon (0-20 cm) of grassland and cultivated soils. Site characteristics were described in detail by Fernández Rueda (1977).
Aggregates of 2-3 mm diameter were obtained by breaking and seaving the air-dried clods.
The assessment of volume and pore space change with associated change in water content was made using Archimedes principle. The bulk density was determined on small fragments (2-3 mm) as described by Monnier et al. (1973). Particle density was determined by the water picnometer method. The degree of shrinkage, Sp, potentially achieved by the soil aggregates was estimated as proposed by Stengel et al., 1984.
Mercury intrusion porosimetry was carried out on dried 2-3 mm aggregate samples, with a Micromeritics porosimeter. The textural pore espace was partitioned into lacune pores and pores of the clay fabric (Fiès, 1992).
Water retention curves were also determined on 2-3 mm aggregates in a pressure cell.
RESULTS AND DISCUSSION
The results of the present investigation successfully condense those of previous work (Paz and Guerif, 1993), namely that the soil in the grassland treatment has better structure. The most appropriate index to describe structure quality of the studied soils is dry bulk density.
The term resilience is used to designate the ability of a soil to recover its structural form through natural processes, when external forces are removed. The index of potential shrinkage, Sp, derived from the shrinkage curves, was found to be a useful indicator to estimate the potential recovery of soil structure by regenerative processes, i.e. the soil resilience. Low Sp values can be interpreted as a lack in the properties which promote natural structure regeneration. Some of the investigated horizons demonstrate severe limitations for maintenance of good structure and, in general, a lack of potential for regeneration of soil structure by fragmentation was deduced from the shrinkage curves.
The organic matter content of the investigated soils varies between 20.3 and 130.4 g/kg. Within this range a significant relationship (p<0.05) between Sp index and organic matter content was found (Figure 1). Therefore, the amount of organic matter influences the swelling-shrinking magnitude from water saturation to dry state. This influence appears to be more important at higher water content.
Total porosity measured by Hg intrusion in the range 400-0.006 mm was lower in all cases but lowest in the cultivated soils than in the corresponding uncultivated. Generally, differences were more pronounced where each cultivation practice lasted longer.
Figure 1. Relationship between organic matter content and shrinkage index.
Differences between cultivated and uncultivated counterparts could be better appreciated taking into account the lacunar pore range. Thus, the cultivated soils generally showed a decrease of larger pore classes with respect to the corresponding uncultivated soils.
Water retention near saturation was increased with increasing organic mater content, but for potentials of about 1 500 kPa water retention curves determined in samples from adjacent plots tended to converge. This confirms that the organization of the clay fabric is not modified by mechanical tillage, as previously demonstrated by Hg intrusion porosimetry.
CONCLUSIONS
This study corroborates previous research, indicating that mechanical tillage leads to a marked increase in dry bulk density. A lack of potential for regeneration of good soil structure by fragmentation was deduced from the shrinkage curves. Mercury intrusion porosimetry showed that lacunar pores prevailed, whose volume increased as organic carbon content increased. Water content near saturation increased also with increasing organic matter content. The possibility of quantifying differences between tillage and no-tillage plots by assessing the specific volume dynamics on soil layers with limited amounts of swelling materials was demonstrated
ACKNOWLEDGEMENTS
This work is part of a project funded by the Xunta de Galicia and the University of La Coruña.
REFERENCES
Fernández Rueda, M.J. 1997. Características físicas y mecánicas de suelos de cultivo. Unpublished Thesis. University of La Coruña, La Coruña, Spain. 283 pp.
Fiès, J.C. 1992. Analysis of soil textural porosity relative to skeleton particle size using mercury porosimetry. Soil Sci. Soc. Am. J., 56: 1062-1067.
Horn, R. 1990. Aggregate characterization as compared to soil bulk properties. Soil Technology, 6: 47-75.
Kay, B.D. 1990. Rates of change of soil structure under different cropping systems. In: B. A. Stewart (de.), Advances in Soil Science, Vol 12. Springer Verlag Inc., New York, USA. pp. 1-52.
Monnier, G., Stengel, P. & Fies, J.C. 1973. Une méthode de mesure de la densité apparente de petits agglomérats terreux: application a l'analyse des systèmes de porosité du sol. Ann. Agron., 24: 533-545.
Paz, A. & Guerif, J. 1993. Propidades físicas y mecánicas a escala textural de suelos con cantidades limitadas de materiales hinchables. Invest. Agr.: Prod. Prot. Veg, 8: 387-410.
Stengel, P., Douglas, J.T., Guerif, J., Goss, M.J., Monnier, G. & Cannel, R.Q. 1984. Factors influencing the variation of some properties of soils in relation to their suitability for direct drilling. Soil Tillage Res., 4: 35-53.
