ON PARTICLE-SIZE DISTRIBUTIONS IN SOILS

Measurements of soil Particle-size distributions have been performed down to 20-nm radius using, beside classical methods such as sieving and sedimentation, mainly static and dynamic light scattering. The number of particles N per unit volume with a radius larger than r was found to follow a power law N is-proportional-to r(-nu) with the exponent nu = 2.8 +/- 0.1. This exponent was observed in all soils investigated and can be interpreted as the fractal dimension of an underlying structure. The power law usually held over two to five decades of length scales. Below the lower cut-off, typically located around 50 to 100 nm, the number of particles (almost-equal-to 10(16)-10(19) cm-3) remained roughly constant. Above the upper cut-off, typically between 10 and 5000 mum, the distribution fell off much more steeply, possibly also like a power law with an exponent nu = 5.4 +/- 0.2.