The specification ASTM D1621 will change in the next issue of the Geosynthetics Specifier’s Guide (December 2016/January 2017).
The change is from:
Compressive Strength at yield per ASTM D1621 kPa (psi)
Compressive Strength at yield per ASTM D6364 units kPa (psi)
For a complete explanation, read this column from the Geosynthetic Institute.