Melilotus species - Melilots

There is a good account of separating out Melliots by David Merrick here.

Melilotus albus - White Melilot

This white flowered plant tends to be looser and less dense that flowers of M. officinalis. The standard (the top petal) is longer then the wings or keel (bottom petal). 

Melilotus officinalis - Ribbed Melilot

This can be differentiated from the similar M.altissimus because the standard (top petal) and wings are slightly longer than the keel (bottom petal). The ripe pods are also brown and hairless.

Melilotus altissimus - Tall Melilot