Which organisms transform nitrogen to a form that is useful to plants

Bacteria

In the process of the nitrogen cycle.                 
The nitrogen cycle is a biogeochemical succession process of nitrogen that involves: fixation, ammonification, nitrification, and denitrification. Like any other biogeochemical cycles. This process undergoes and affects the biological, geometrical and chemical aspects in the ecosystem and the abiotic and biotic community. Hence, the nitrogen cycle leads the abiotic component –nitrogen- to contribute to the biotic community, decomposition and primal production. Further, it becomes an essential part of the environment because some life components are contains it, similarly, amino acids, nucleic acids in RNA and DNA.