Alfalfa/Lucerne mulch The primary benefit of this pleasant smelling mulch is that it increases organic matter, although it is also a valuable plant-derived fertiliser. Mulch will also retain a more even water content in your potting soil allowing the roots to fully penetrate the full volume of your potting soil. Alfalfa has a minimum of 15% protein, which is the equivalent of 2.4% Nitrogen, contains trace minerals and triaconatol (a natural growth hormone) and is an excellent addition to the compost pile for nitrogen content. Spread a layer on top of your soil (about 2cm thick) in spring and in the middle of summer.