They are not considered to be part of the fine earth fraction to which the term soil texture properly applies.

which引导定语从句并在从句中作介词的宾语时，介词可以留在原处（非正式语体），也可以置于which 前。本句中定语从句的谓语为applies to sth. 关系代词为介词to的宾语，和to一起引导定语从句了。这是正式语体。to 留在原处为非正式语体，例如：

They are not considered to be part of the fine earth fraction which the term soil texture properly applies to.