Fertilizers and biomass

Whenever I see harvest pictures like this, I always think about mass redistribution.


That is, this corn is transported to my plate from, let’s say, Iowa. And more corn like it keeps coming every year, harvest after harvest. Every year Iowa gets shaved a little thinner, its sweet abundance getting hacked off and carted away to the four corners of the hungry corn-eating globe. Won’t Iowa eventually be scraped down to a barren parking lot? That corn was originally little seeds in the ground. What did the corn eat so that I can eat it? Is there enough corn food to keep the party going?

There’s plenty of Iowa to go around, but still, something is getting sent away. What is it? The good news is that water and carbon, two of the heaviest parts of the crop, come straight from the sky in the form of rain and carbon dioxide. But there are some other things that are sucked out of the soil and not replaced. What are they?

Fertilizer bags tell the story. Each bag is labeled with three numbers, say 20-5-10. These three numbers correspond to the amount of nitrogen, phosphate, and potash (potassium) in the mix. Fertilizer is, and is carefully designed to be, Gatorade for soil. It replaces what the soil sweats away every harvest. Stop fertilizing and your farm productivity would crash. You might think that nitrogen, also being abundant in the atmosphere, would be easy enough for plants to suck from the sky like carbon dioxide. But sadly no plants (without some bacterial help) have ever figured out this trick, so humans have to spend fantastic amounts of energy putting atmospheric nitrogen into a “fixed” form that the plants can eat. Potassium is easy enough to come by, but it turns out that our phosphate supplies are petering out.

I know, it’s yet another alarmist peak-this or peak-that story. But we shouldn’t really be surprised by the Peak Phosphorus story. The earth’s population is so large that we have to keep a very big food-making machine running at top speed all the time. It can’t break down, and every year we have to make it run faster still. A lot of things have to work just right; there’s always going to be a weakest link. As long as we have adequate energy, we’ll have fixed nitrogen for our plants. But phosphates need energy and phosphate rocks, and if we’re not careful we can run right through our supplies. Science News has a recent article on this: Salvage Job.

Sensing an opportunity for profit, farmers sought more fertilizer to nourish their fields. But high oil prices had increased the cost of processing phosphate rock, which provides a key ingredient in fertilizer. With rising demand and tight supply, phosphate rock prices leaped from about $45 per metric ton to $80, then $135, then $367 — a roughly 700 percent spike in just one year.

All those bags of fertilizer have to come from somewhere…

2 thoughts on “Fertilizers and biomass”

  1. the legume family (along with Rhizobium) is nitrogen fixing so planting and then tilling in cover crops such as clover can increase nitrogen in the soil

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