Basically, food production requires farm crops, and crops require solar energy that they can convert by photosynthesis into food energy during a warm season. Thus, the quantity of solar energy forms the elementary limit on food production. The global quantity depends upon the units of energy received per area of land, the area of land, and the length of warm season for integration of the energy into yield per plot. By per plot I mean per area, per hectare, or per acre.
Beginning with the rates of photosynthesis measured in laboratories for single leaves, deWit (1967,[deW67] 315-320) calculated the potential productivity of the earth by 10 degree rings of latitude, considering clouds, length of season, and the expansion of layers of leaves. For a single ha of land, he obtained integrated photosynthesis, for example, of 60 t carbohydrate/season/ha at 50 North latitude and 90 t at 40. Reducing the integrated photosynthesis by half for the respiration of the plant, he estimated production of 30 and 45 t/ha biomass. Assuming that half the biomass was edible, he reached edible yields of 15 and 22 t/ha at 50 and 40.
By another route, deWit (1968[deW68]) arrived at similar maximum yields. He noted that scientists had measured plants from algae and grass to potatoes and maize growing about 200 kg of dry matter/ha/day. These rates agreed with Alberda's conclusion that grass in the Netherlands could produce fully 20 t/ha in a season (Alberda and Sibma, 1968[AS68]). The 20 t/ha resembles deWit's other calculation of 30 t/ha biomass at 50 latitude.
The reader can compare the limits of 15 to 22 t/ha of edible yield with some world average yields in 1990: wheat, 2.6; rice, 3.5; and maize, 3.7 t/ha. Some notably high national averages in 1990 were 8 to 9 t/ha Irish wheat, Australian rice, and Chilean maize (Food and Agriculture Organization of the United Nations, 1992[FotUN]).
When deWit (1967[deW67]) multiplied integrated photo- synthesis by the land area in each 10 belt of latitude, ``The staggering conclusion ...is that 1,000 billion people could live from the earth if photosynthesis is the limiting factor!" The sunlight received on land during warm days would energize the photosynthesis of far more than present yields. The high yields limited solely by solar energy would feed a population 100-fold the ten billion envisioned in my question about sparing land for Nature. Solar energy, warm days, and land to stand on will not soon limit food supply.