An established lawn should not need additional fertilizer. When the Viking settlement in Greenland was rediscovered after four hundreds of abandonment, the Vikings took their life stock, the carefully built and guarded grass fields were still green. The lawns had not spread into the hard, dry, gravelly soil. There being no trees meant that nothing had seeded in to take over the plots. The grass plains of America never needed fertilizing because they had developed deep soils with robustly diverse ecosystem of microbes, fungi and bacteria.