What element is most difficult for living organisms to obtain?

The element most difficult for living organisms to obtain is often considered to be phosphorus. While it is essential for life, particularly for DNA, RNA, and ATP, phosphorus does not exist in a gaseous form and is primarily found in soil and rocks. Its availability can be limited by soil chemistry and biological uptake processes, making it less accessible compared to other elements like carbon or nitrogen. Additionally, phosphorus is often bound in forms that are not readily usable by organisms, complicating its acquisition.