Fertilize this!

Running around the blogosphere, you'll find all kinds of people dancing to apocalypso music by various artists.  One favorite song among the fans of the Energy Depletion Band is that agriculture is bound to collapse when supplies of fuel used for production of nitrogen fertilizer become tight.  Here's a typical example (including typos):

Modern food production is depends on oil. There is no way to get fertilizer from solar, wind or nuclear.

I'm no agro-scientist, but I know that this claim is both factually and historically false.

History:  Nitrogen depletion in soil was historically dealt with using legume crops or green manures in crop rotation.  Just where do you think a bean plant gets the energy to fix its nitrogen?  It comes from the sun, turned into carbohydrates which go to symbiotic bacteria which perform the actual work of fixation.  (External nitrogen supplies from guano deposits and then Haber-process ammonia later supplanted green manures for most farming.)

Legume crops don't fix nitrogen fast enough for modern intensive techniques, but it proves that it is indeed possible to make nitrogen fertilizer with solar energy.  The big question is, how hard is it?

Suppose for a moment that we keep using the Haber process, but decide to power it with solar inputs.  We get the hydrogen from green algae trick operating reliably at 1% efficiency.  An area getting as much sun as mid-Kansas would receive about 1550 kWh/m²/year, so a square meter of this algal hydrogen factory would yield 15.5 kWh worth of hydrogen; at 70600 cal/mole, that's about 380 grams of hydrogen per square meter per year.  It takes 3 grams of hydrogen to fix 14 grams of nitrogen, so each square meter of algae farm could make the hydrogen to fix roughly 1.75 kg of nitrogen, or 17.5 tons nitrogen/ha/year.

US farmers use roughly 12 million tons of nitrogen fertilizer per year over 140 million ha of cropland, or about 86 kg/ha/year.  At that application rate, a hectare of algae farm could produce the nitrate to fertilize about 20 hectares of crops; in other words, a 1% efficient sunlight-to-hydrogen process can make all agricultural nitrate from the sun with about a 5% land-use penalty.  The penalty goes down to 1% if the efficiency goes up to 5%, and 0.5% if it hits the 10% target that researchers believe is possible.

If 10% efficiency can be achieved, the hydrogen production goes up to 38 tons/ha/year (1.55 MWh/ha/yr) and it can become the basis of a general energy business.  If crop wastes such as corn stover and wheat/rice straw are used as carbon inputs and have a general chemical formula of (CH₂O)_n, addition of H₂ is all that is necessary to produce methanol (CH₃OH).  If the process can use the inputs with 100% conversion efficiency, 2 grams of hydrogen plus 30 grams of carbohydrate yields 32 grams methanol; 38 tons of hydrogen becomes 608 tons of methanol (about 203,000 gallons, holding the energy equivalent of 122,000 gallons of gasoline).  At this level of production, inputs of crop waste are probably the limiting factor; long before this level was reached, the fuel production would satisfy all needs for cultivation.

Conclusion:  it is not only possible to generate all required nitrogen fertilizer from solar energy using known processes or slight improvements, at the limit they could lead to large-scale production of biofuels from crop wastes.  All it requires is hydrogen.

UPDATE:  Advances sometimes come too fast to comprehend.  Researchers at UW-Madison have a process to convert plant carbohydrates to alkanes.  This doesn't address the fertilizer issue directly, but turning grain farmers into net producers of motor fuel eliminates that mode of system failure.

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