What is grain size distribution and how is this determined?

Grain size and grain size distribution have substantial influence on the structure and the quality of a substrate. A highly fine structure means less air capacity and the material tends to compress. The methodology for regulating the grain size distribution for peat (and substrates) is still based on the DIN standard 11540, item 5.9. The required parameters for sieving are described as well as the calculation and representation of the data.