At night, in the absence of photosynthesis, plant and soil respiration will cause CO2 levels to increase. In a timothy-meadow fescue forage crop, CO2 levels exceeding 430 ppm were observed (Monteith 1962) during the The greater density of carbon dioxide will cause it to remain within the crop canopy, and under conditions of still morning air, CO2 levels within the canopy will be high when photosynthesis begins. As photosynthesis depletes the CO2 supply within the crop canopy, air turbulence and wind currents are essential to renew and maintain carbon dioxide levels. Niciporovic(Black and Watson 1960) suggested that the main source of CO2 for photosynthesis within a crop canopy must come from atmospheric air. He reasoned that if 20% of the total weight of a previous crop remained in the soilas roots, and if a further 20% was returned as organic matter (crop residue at harvest or applied as manure), the return of 40% organic matter from a previous crop(s) would satisfy only a small part of a subsequent crop's CO2 requirements. In a commentary on this paper, Black and Watson (1960) criticized the suggestion that CO2 was replenished by convection currents and suggested that CO2 is replenished by downward eddy-diffusion from air moving horizontally, and except in very calm conditions,atmospheric transportprocesses aresufficiently active to maintain the CO2 concentration near 300 ppm above the crop canopy. High rates of air flow cannot increase it further