How to kill your soil

I recently saw an infographic that stated, “There are no life forms in the soil, which is sterilized…” What was it talking about? Soils on the moon?  A toxic chemical spill? Soils around Chernobyl? Nope, this was the description of soils under industrial agriculture. I have heard it before, the epidemic of “dead soils” caused by “chemicals.” This may make good copy for organic food advertisements, but it is not good science.

Soils are very hard to kill. Soil scientists wanting to sterilize the soil expose it to high-pressure steam for 30 minutes or more in an autoclave. Often, because soils are notoriously hard to sterilize, they repeat the process. It is hard to imagine how the equivalent to autoclave could be occurring in fields, regardless of management. Even fumigation, the most drastic of attempts to kill off soilborne pests, does not kill everything. The huge diversity of bacteria and fungi in soils, and the variety of microhabitats available to them means that much life survives.

Soils are also very resilient. After fumigation, farmers know that they have a limited time before the pests and beneficial organisms rebound (Li et al., 2022). Fumigating every year, rather than working better, tends to select for those organisms that use the applied chemicals as an energy source (they eat it). These organisms proliferate and diminish the effects of future applications (Matthiessen et al., 2004). This should not be a surprise, after all, most pesticides are organic chemicals, and bacteria and fungi are experts at organic chemistry.

So, if soils are hard to kill and resilient, why the talk about dead soils? I think that much of it is just marketing hyperbole; “dead soils” stir emotions. However, there are soils that do not function well, that can seem dead. The cause of this problem is generally not “chemicals,” but rather a lack of organic matter, especially fresh organic matter. Organic matter can be categorized by its age (2019-Updated view of soil organic matter) Humus is very old (50-10,000 years) organic matter that is stable because it is highly resistant to further decomposition by soil organisms (the process of organisms extracting energy from, or “eating” organic materials). Material that is 5-30 years old is on its way to becoming humus, but can still be decomposed further by a select group of organisms. These two pools are large, but do not provide much food for the majority of soil microorganisms. It is fresh organic matter, “the recently deceased,” that feeds the soil. When soils do not receive a regular supply of this fresh organic matter, biological activity decreases. The same can occur when soils are tilled often, which stimulates rapid decomposition. With no fresh organic materials to eat, the soil biology slows down and eventually many microorganisms go into a resting state. These soils are not dead, but they are starving.

Starving soils need plant inputs, what is produced by photosynthesis, or stored photosynthesis in the form of manure or compost. Within your operations constraints, MAXIMIZE photosynthesis, minimize tillage is a good strategy to follow for soil life. And unless they are talking about Antarctica (Dragone et al., 2021) or the moon, ignore talk about dead soils.

All Perspectives from Andrew McGuire

References

Dragone, N.B., M.A. Diaz, I.D. Hogg, W.B. Lyons, W.A. Jackson, et al. 2021. Exploring the Boundaries of Microbial Habitability in Soil. Journal of Geophysical Research: Biogeosciences 126(6): e2020JG006052. doi: 10.1029/2020JG006052.

Li, X., V. Skillman, J. Dung, and K. Frost. 2022. Legacy effects of fumigation on soil bacterial and fungal communities and their response to metam sodium application. Environmental Microbiome 17(1): 59. doi: 10.1186/s40793-022-00454-w.

Matthiessen, J.N., B. Warton, and M.A. Shackleton. 2004. Enhanced biodegradation reduces the capacity of metham sodium to control soil pests. Australian Journal of Entomology 43(1): 72–76.