Particle size distribution (PSD) aka soil texture is a major feature to understand soils.
Sand, silt and clay classes of mineral particles are so commonly used that they become part of the everyday landscape of agronomists, farmers and others.
However,
Why such size limits have been placed between these categories?
Is there a reason?
In addition, different countries may have different PSD classes (eg., 50 µm or 63 µm between silts and sands).
On which scientific basis has this been made?
To follow up on that,
Don't you think that a more modern approach to soil texture characterisation would be more helpful by measuring soil PSd on a continuous scale? rather than splitting between sand, silt and clays.
Indeed, two soil with the same PSD (lets say 30% sand, 30% silt and 40% clay) could be very different.
Within sands, particles could all be towards th coarse side, or conversely towards the fine side.
Same reasoning for the other classes.
Without solid reasons (physical perhaps?) to set the boundaries between sands, silts and clays, that system seems a bit arbitrary and old-fashioned, isn't it?
Shouldn't it be more useful to represent the frequency distribution of particles on a continuous scale to give a more precise picture of the actual texture of a soil.
New techniques such as laser diffraction seem to be useful to this end, and could give a more representative image of the distribution of the size of soil's mineral particles.
Futhermore, current method to determine soil PSD using sieving and sedimentation is extremely long and prone to errors in measurements.
Any thoughts on this?
Interesting question. I am not a soil scientist, but used some of the tools in soils and channel substrates. I did not know the size boundaries could vary some by country. Nonetheless, I know for those wanting or needing a higher degree of PSD, there is a whole series of standard soil grates that one might purchase and use. Soil series classification to me anyway is detailed enough, without trying to interject more detail and potential separation classes, descriptions, etc. The amount of extra work to upgrade soils data through mapping, field delineation, attribute adjustment, GIS coverages, etc. may be staggering. Would it be worth it? Some practices or uses of soil materials might benefit from more detail. Some activities seldom understand and apply what is available now. Perhaps some activities or actions would have enough benefit from measuring and applying more PSD detail, such as making concrete, which PSDs are best for certain applications. What PSD mix(s) is/are unexceptable for septic fields. How much more or less water could be used for irrigation to still maintain crop vigor and health? More PSD detail is sometimes beneficial in defining channel substrates and aquatic habitats. For those with the laser or other equipment and interest for finer PSD testing and application, the research field seems pretty open, but at some point it comes down to funding, and is this where to best spend valuable time and resources. I would think there might be a few opportunities to justify, but for general soil work and considering available data, there might need to be some driving reasons to change.
The classification of various soils based on grain size depends on the discipline and the respective applications of the soils. For example soils in geotechnical engineering and soil mechanics in general are classified based on their mechanical behaviour as foundations, embankment, improvement materials, aggregates in pavements subgrades etc. In this case the mechanical behavior of coarse grained soils ie sands, gravels, cobbles etc is mainly affected by their gradation whereas fine grained ie silt and clay by the effect of water (plasticity, cohesion, consolidation etc). The respective ranges in diameter between the soil classes vary depending on the classification system ie USCS, BS, DIN etc but the basic criterion is their mechanical properties (shear strength) as mentioned above.
you may refer the ASTM standards in order to have better understanding.
There are two major methods to assist particle size distribution in soil by hydrometer and by pipit, the two methods depend on the time of particles sedimentation in water
Soil texture has been defined, differently (but not too much), according to the behavior of the respective particles in what regards physical and chemical processes.
Texture analysis would be easy. However x-ray crystallography would clearly discriminate the boundaries
To rephrase a little bit my question:
In soil science, for agriculture purposes, (not civil engineering),
Why have we (perhaps historically) chosen to define these groups?
Why have we chosen sand, silt and clay, with these specific size boundaries?
And not four classes of particles with other size limits between them.
There are not necessarily 3 size boundaries. In some analyses you can separate several boundaries (coarse sand, fine sand, silt I, silt II, the so-called physical clay..). There are different boundaries in different systems, and also differences between soil / geology studies. I cannot give you a certain source, but from a study of Canarache Andrei in 1964 correlating these different systems, and from general soil science books, it appears that the limits have been chosen according to the behavior of those particles (for example clay has colloidal properties).
1. The boundaries when defining soil particle sizes depend on purpose; is the definition of size fractions meant for Engineering or Soil Survey purposes? This will be a guide to better responses.
2. Again, I want to believe the question does not imply the interchangeable use of particle size distribution and soil texture. PSD is a necessary path way to the determination of Soil texture and not same as soil texture.
The % of sand, silt and clay in the soil established the nature or texture of soil. Hence, first we have to determine the % of these components of soil.
Grain size analysis is done by analyzing percentage of sand, silt and clay by dry sieving and wet sieving methods. Texture is assessed on the basis of these components.
Modern method of particle size analysis is to laser diffraction in order to know the proportion of all particles. Then they should be classified according to uses.
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