Crops need several elements to grow, and high yields to spare land require fertilizer, notably nitrogen, phosphate, and potash. In 1990, farmers around the world applied these million tons: nitrogen, 79; phosphoric acid, 37; and potash, 27 (United Nations, 1992[UN:92]; U.S. Department of Interior, various years[USDb]).
The preeminent source of nitrogen fertilizer is synthesis of ammonia from the hydrogen of natural gas and the nitrogen of the atmosphere, a source without obvious limit.
Phosphate fertilizer comes mainly from phosphate rock in widely distributed marine deposits. Treated with sulfuric acid, it becomes superphosphate. Recently, about 150 million t of phosphate rock has been mined annually. ``There are surely billions more tons in reserve that have not yet been discovered. ... Oceans are rich in phosphate, a virtually limitless supply which needs only extraction technology and economics to make it a viable source." (Darst, 1993[Dar93]) The reserve base of rock is 34 billion t. More important in the short run, the reserve that would cost less than about $40/t freight-on-board to mine is 13 billion. So the reserve is greater than 200 and the inexpensive supply is greater than 80 times the annual mining.
Potash fertilizer comes from mines and other sources. The reserve base is 17 billion t, hundreds of times the annual use of potash fertilizer.
Given the synthesis of nitrogen fertilizer and the large reserves of phosphorus and potash, it is not surprising to read a corporate statement that might as well apply to all three nutrients: ``Disruptions in world trade patterns, due to lower demand, resulted in the lowest phosphate fertilizer prices in more than 17 years; international sulphur prices fell to their lowest level in almost 15 years." (Freeport-McMoRan, 1993[FM93]) Indices of prices paid by farmers reinforce the corporate statement about prices. In 1990, the price of fertilizer was three times, whereas the price of all production commodities 10 times, the price in 1910-1914 (U.S. Department of Agriculture, 1991,[USDa] 385).
In some climates and where fertilizer already is
applied copiously, applying more fertilizer to the
present varieties raises yields little. There, tailoring
fertilizer application on a site specific basis will increase
efficiency of its use.
Nevertheless, in countless
places, as Figure 9.1.1 will later illustrate, fertilizer lifts yields.
Can one calculate--as was done from the annual receipt of sunlight and evaporation of water--how many people the global supply of fertilizer might support? Because more nitrogen than phosphorus or potash is applied and taken up by crops, I turn to nitrogen. Grain contains between 1 and 2% nitrogen; allowing for leaching, one can reasonably assume 30 kg crop requires 1 kg nitrogen. The 10 quadrillion calories of food, feed, and other crops in Figure 5.3.1, which is equivalent to 2,500 million t of grain, was grown with 79 million t of nitrogen fertilizer. The preeminent source of nitrogen fertilizer is ammonia synthesized from nitrogen in the atmosphere and a source of hydrogen, such as natural gas. With tens of thousands of tons of nitrogen over each ha, one turns to natural gas in search of a limit. But technology and exploration are expanding the known reserves of natural gas faster than the annual use of about 70 trillion cubic feet consumes them (Burnett and Ban, 1989[BB89]; Economist Books, 1990[Boo90]). Nevertheless, the 70 provides an annual global quantity from which I can calculate a limit.
If three cubic feet (83 liters) can furnish three gram moles of hydrogen to combine with nitrogen to form two gram moles of ammonia, the annual production of natural gas could become 800 million t of ammonia. This, at the rate of 30 kg yield per kg nitrogen, could in turn become more than 20 billion t of yield that would feed 80 billion people. This calculation has ample shortcomings. On the one hand, it appropriates all natural gas for fertilizer. On the other, it neglects other sources of hydrogen and of nitrogen, too. And unlike the limits of sunlight and evaporation, which are natural ones, the limit of natural gas production is an industrial one that changes. Nevertheless, producing a limit well above the ten billion of the question, the calculation reinforces the evidence of ample supplies and falling prices.
The global, physical supply of fertilizer--and of
sun, 
, and water, too--likely will limit neither
yields nor the sparing of land for Nature.