This process also builds up humus, keeps soils microbiologically active and, over time, improves soils physically and chemically. Grass is the most effective plant available for reconditioning the soil.  An extremely important function of turfgrasses is soil improvement through organic matter additions derived from the turnover of roots and other plant tissues that are synthesized in part from atmospheric CO2 via photosynthesis. A high proportion of the world's most fertile soils have been developed under a vegetative cover of grass (Gould, 1968*) Gould, F.W. 1968. Grass Systematics. McGraw-Hill, New York.