The particle size of silt is 0.002 mm - 0.02 mm. This means that silt particles are smaller than sand particles (which are 0.02 mm - 2 mm) but larger than clay particles (which are smaller than 0.002 mm). Silt particles are often considered ideal for soil fertility because they are small enough to increase soil porosity and water-holding capacity, but not so small that they easily compact and reduce soil aeration.