Darwinian Agriculture at IRRI 3: A solution to the phosphorus problem?

"Crops that take up nutrients faster may reduce some wasteful nutrient losses (for example, leaching [see glossary] of nutrients in water percolating down through the soil), but every atom of nitrogen or phosphorus that is sold off-farm in grain, milk, or other farm products still needs to be replaced, for long-term sustainability... " -- Darwinian Agriculture, p. 67

This conservation-of-matter argument was the basis of my argument, in my fifth lecture at the International Rice Research Institute (IRRI), that rice with improved phosphorus uptake might offer mostly short-term benefits. (A slide with the only field data I've seen on this rice mysteriously disappeared, about 14 minutes into the talk.)

But the abiotic-stress group at IRRI called my attention to another approach to "the phosphorus problem" that seems really promising. Actually, there are several phosphorus problems:
* Phosphorus fertilizer is expensive, and it will get more expensive as high-phosphorus ore reserves are depleted.
* Some soils bind phosphorus, limiting its availability to plants.
* Phosphorus, mostly from livestock manure, is a major contributor to water pollution.

In the book, I mention crops with "proteoid" roots or increased symbiosis with mycorrhizal fungi as possible ways to increase crop uptake of less-available forms of phosphorus. But that doesn't solve the conservation-of-matter problem, i.e., the need to replace phosphorus in grain sent to distant cities or feedlots. The eventual depletion of phosphorus reserves seems such a severe (though perhaps distant) problem, that I briefly mention the "back to the land" option, to facilitate recycling of phosphorus in our waste.

But what if 1000 kg of grain contained only 1 kg of phosphorus, instead of 4 kg? We could then reduce phosphorus fertilization by 75%, making phosphorus reserves last four times as long. (OK, this isn't a permanent solution, but it could give us many more decades to find a permanent solution.) I didn't consider this option in the book, because I assumed that low-phosphorus grain would be less nutritious.

The abiotic-stress group at IRRI corrected my misinformation. It turns out that much of the phosphorus in grain is in the form of phytate, which neither we nor our animals can digest. So it ends up in manure. I'd heard about attempts to reduce phytate levels in seeds as a partial solution to the phosphorus-pollution problem. But low-phytate seeds would also reduce amount of phosphorus exported from a farm in each ton of grain, which would reduce the need for phosphorus inputs to replace it.

This seems like a win-win solution. Why didn't natural selection think of this? The phytate isn't there for our benefit; it's there to supply the phosphorus needs of the germinating seed and seedling, until it can grow enough roots to get phosphorus from the soil. We might therefore expect low-phosphorus seeds to grow poorly, although this isn't necessarily true for high-phosphorus seeds that have less of their phosphorus as phytate.

Low-phytate seeds with high total phosphorus would be more digestible, increasing the fraction of their phosphorus that ends up in meat or milk rather than manure. So they could reduce pollution. They wouldn't reduce the need for phosphorus inputs, however.

But what if we could supply the phosphorus needs of growing seedlings externally? If 99% of seeds get eaten, and only 1% get planted, could we give the 1% some extra phosphorus, perhaps as a seed coating? I don't see any fundamental (e.g., conservation-of-matter) reason why this wouldn't work, though it would probably require a clever combination of plant breeding and agronomy.


Hi Ford

An interesting post, as always. But I’d like to know how it’s shaped by your larger view of farming futurology. It’s implicit in your post that a permanent solution to the phosphorus problems can be found, but if that doesn’t prove to be the case then low phytate seeds will only delay the ‘back to the land’ moment by a century or two – which could of course be very useful, but not that long in the grand scheme of things (we could maybe project that it would extend the career of urban-industrial agriculture from about 1% to 3% of agrarian history, or from 0.05% to 0.15% of human history, if I’m not being too provocative...) I guess I’m wondering if you’re simply applying a discount rate into the future as economists, ‘ecopragmatists’ and others with a strong belief in perfectibility and human ingenuity tend to do, or whether you think there are good grounds for supposing that the phosphorus problems can be solved? Or to put it another way, does the possibility of developing low phytate seeds cause you to reject the ‘back to the land’ option as a plausible scenario?



I'm not sure. If the P still comes from nonrenewable sources, we're
just gaining time. But could lower P needs make returning manure to
farms more practical, forever? A 75% reduction in seed %P would reduce P input needs 75% forever. I was thinking that would reduce the cost of transporting manure back to the farm by 75%, also forever. But if low-phytate seed means low-phytate manure, you might need to transport more manure to get the same amount of P, for less energy savings.

I still think back-to-the-land options need exploring. There's much more P (tons, not concentration) in livestock manure than in human waste. So making livestock operations smaller and closer to where the feed is grown would have much more effect than moving people back to the land.

But phosphorus is only a small part of the story. Spreading people
more widely could reduce the spread of human disease, but bury farm
land under houses. With current transport options, energy rural
people use driving probably outweighs potential savings from nutrient transport. But what if rural housing was concentrated along light-rail corridors?

Also, a lot can happen in 100 years. Will we invent cheap,
sustainable energy before we run low on phosphorus?

Thanks for those points - lots of interesting questions to think about...the phosphorus is only one term in a multivariate model! But agreed, if rural people are living essentially urban lifestyles in the countryside, then rural life doesn't stack up too well.

Maybe we could throw the inverse farm relationship into these deliberations - more land built over on more small farms, but more productivity per unit area?

Yes, maybe someone will invent cheap sustainable energy and make all my back to the land stuff seem foolish (though it has other things going for it...) I was reading one of Vaclav Smil's books recently in which he sought to deflate the peak oil argument by saying that the Victorians had worried about peak coal...until they found oil. Somehow I don't find that too reassuring.

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