Comment by jbotz | Hacker News Reader

Comment by jbotz | Hacker News Reader

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As someone who has practiced (or tried to) sustainable agriculture in the tropics, I have to say that I've long ago become dubious about the whole idea of storing carbon in the soil. I think in cold climates, under dense forest growth, maybe more carbon accumulates than the microbial life can consume, up to a point, but in the tropics that sure doesn't seem to be the case. Most of the soils I've seen have essentially no carbon content below the first few centimeters... all the nutrients are in the litter layer above the soil, and most of the native plants will grow their feeder roots right into that litter (mulch). With a lot of effort and a lot of mulch, you can start to accumulate a bit more carbon in the soil, and a lot of crop plants sure appreciate that, but by far most of your mulch is going to disappear amazingly fast if you don't keep applying more, and then that bit of soil carbon quickly disappears, too. You can't build those deep, black, "healthy" soils you see in temperate climates in the tropics. At a guess I would say 99% of the carbon in tropical rain forests is in the living matter.

And the problem is that with global heating, the tropics are on march toward the poles.

This is interesting to me. For the past 15 years I have had a small plot in the tropics on which I have tried to improve the soil by adding copious amounts of compost, manures and growing cover crops that I till under. All that organic material just seems to disappear quickly. Before I owned it the plot was planted with corn and sugar cane each year and lots of chemical fertilizers and pesticides were used. So the soil was ruined. I thought I could bring it back within a few years but it is still rather poor. There was one patch that looked much better after a giant pile of scrap wood left over from house construction was burned and the left over ash and char was plowed in. But that did not last long. It is all back to hard crusty soil now. The pH is rather high due to the high percentage of karst in the region and the high calcium carbonate content of ground water that is used to irrigate. Maybe that has something to do with it?

I think your experience is just normal for the tropics, and it has nothing to do with the specifics of your soil. Certainly I and lots of others have had similar experiences.

In natural tropical environments the nutrients aren't in the soil, they are almost all tied up in the living biomass, and as soon as some of that dies the nutrients it releases are absorbed by other living vegetation. And generally, if we want to do things in a way that's sustainable, we should look to nature and try to imitate it (and once we can manage that, try to improve on it).

What I've found is that the only way of doing tropical agriculture that's remotely sustainable is "slash and drop" agroforestry, which is where you interplant (a lot of) biomass for mulching together with your crop plants and keep cutting/pruning that to produce mulch, so that you can keep your crop plants heavily mulched all the time. This is fairly labor intensive, though, and worse, it requires highly knowledgeable labor to maintain.

That is very interesting. What are the preferred plants to grow to produce the biomass for slashing and dropping?

There are too many to list, and it depends entirely on the crop system you're trying to support. For example, for annual crops it would be different than for multi-year agroforestry systems, and you may want some plants to provide windbreak or shade for others at an early stage, and then remove them when others are more mature, etc. Also, you want a mix including some legumes for their nitrogen fixing ability. And some plants are better at utilizing and accumulating specific minerals (bioaccumulators), so may want some of those.

In short, it's complex and requires a good deal of knowledge or willingness to experiment, which is why it's not more widely practiced, even though when done correctly it undoubtedly is very effective, sustainable, and requires far fewer inputs than any other agricultural practices. At least in theory, a master of this art can design systems that are planted once and the produce different crops for several years while mimicking the natural succession of a grasslands in transition to becoming a forest with a fairly small amount of maintenance (harvesting crops at different stages and chop-and-drop'ing mulch plants). One master of this art here in Brazil, whom I've learned a lot from, is Ernest Götsch[0][1].

[0] https://agendagotsch.com/en/

[1] https://en.wikipedia.org/wiki/Ernst_Gotsch

It seems that the rich black soil in the Amazon was man-made and has lasted thousands of years:

https://en.wikipedia.org/wiki/Terra_preta

Right. That's biochar. See my top-level comment.

But note that this is not all or most of the soil in the Amazon, by a long shot... the "terra preta da Amazonia" exists in isolated patches where humans had been conducting slash-and-burn agriculture for hundreds of years. Most of the soil of the Amazon region is just like any other topical soil, nutrient and carbon poor.

But biochar establishes the fact that carbon can be (semi-?)sequestered by a sustainable human agricultural process, this scheme has been implemented on a large scale in the past, and similar schemes could be implemented in future ag systems. That’s not a trifecta you hit very often in complex systems of any sort.

Upshot: if you are a young scientist/engineer/agronomist/ag economist and want to do real hands-on exploratory science where everybody is in the dumb club, and still make a significant direct middle/long-term difference in the world and its population, soil science could be your ticket.

PS: The content of the article is great. The click-bait title is absurd.

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