"The first consideration is to rebuild the humus in our soils to supply carbon. The most important plant to get into your pasture mix is white clover. This is the powerhouse plant for biologically based pastures. Fortunately most farmer pastures do have clover, often in conjunction with ryegrass, but usually not in high enough density being around 20% of the sward. To optimise productivity clover content needs to be a minimum of 30% of the sward, aiming for 50%." http://www.biodynamic.org.nz/guides/ch3env_app.pdf (page 119 in this guide) http://www.biodynamic.org.nz/guides/intro_ch1.pdf (an accompanying guide, very very interesting)
I have a good covering of clover in my lawn. It has developed since I stopped using broad leaf sprays and cut back on nitrogen heavy fertilisers. The clover supplies much of my nitorgen needs now. It has packed quite densely in some areas but seems to complement the grasses nicely giving a decent effect on the lawn. Cut out the sprays & the chemical fertilisers & your lawn can look quite quite nice I have discovered.