I’m a firm believer that inputs drive the microbial community. For without regular water, food, shelter and play area, organisms don’t thrive. The best inputs do all of those and more.
Living plants do all these things and more, but sometimes they can use a helping hand to maximise the rate of soil production. What builds soil? Carbon life forms. Without the life forms, soil is called dirt.
The most productive soil to date appears in a protected bay growing sea grass, it has the highest natural soil organic carbon recorded so far. All because when the grass dies it gets buried in the anaerobic sediment and stays there a long time aided by the fact the bay is well protected and nutrients aren’t washed out to sea. The edge effect in action. Other ecosystems that are highly productive are low lying grasslands that receive nutrient run-off from higher elevations. Many of these are in high rainfall areas and actually store more soil carbon than forests. Grasslands tend to create a linear soil carbon profile down to about two foot, whereas forests tend to have a lot up top but less as you go deeper. The key with grasslands is the long, thin and deep rooted perennials that maximise photosynthetic area and can penetrate subsoil. The deeper you go, the less air, the less oxidation, the longer living the soil carbon.
Knowing this, I like to think about what different inputs put into the soil by way of the nutrients, structure, and biology, to change it. I also like to think about what houses and holds onto those nutrients in the soil and builds fertility. Components of soil like the sand, silt, clay, and the succession of these towards structured organic matter. The structure that is formed by carbon-based lifeforms creating the soil mucilages, exudates and porous structures as those lifeforms feed on nutrients and swim and burrow though soils.
How do different farming practises compare at creating these microbial community structures and soil carbon?
Distinct soil microbial diversity under long-term organic and conventional farming
FYM=Farm Yard Manure, CONMIN=Conventional Mineral, BIOORG=Bioorganic, BIODYN=Biodynamic
There is now Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls.
Basically that means microbes fed glucose can build soil structure without any added plant matter aka humus. Glucose is just carbon, oxygen and hydrogen, and these all exist in the air that plants and some microbes then photosynthesise into sugars. The organisms that can’t, like mycorrhizal fungi, rely on photosynthesising hosts.
When I see just how often carbon appears in the below image, it reminds me of its importance. Carbon beats out nitrogen by 4 atoms and it also forms the sugar phosphate backbone.
So along with carbon, nitrogen, and oxygen, if you just add nitrogen from the air and phosphorous from the soil then you’re ready to spin the spiral staircase and begin to churn the primordial soup.
As a result, carbon is responsible for so much diversity.
Soil organic carbon (SOC) appears to be correlated with microbial diversity and abundance.
Bacterial abundance and diversity appears to increase linearly, at least to 4% SOC.
Fungal abundance also increases linearly, however diversity appears to taper off up to 4% SOC, or it could just be that there isn’t enough data on high carbon soils.
I’ve read that “Higher soil pH values were consistently associated with higher bacterial diversity indices.”
I’ve also read the same of fungi and that Soil-available potassium was positively correlated with mycorrhizal colonization and species richness.
There’s also the Bacterial:fungal ratios are of limited utility for understanding soil processes that goes against some people beliefs.
One subject kept popping up in my soil carbon learnings, and that was just how important soil moisture is in keeping microbes active and not dormant or dead. Increasing aridity reduces soil microbial diversity.
Biocrusts are an amazing example at one end of the extreme. They can wake at a drop of rain landing on hydrophilic hairs on their surfaces, yet sleep an age during the dry times. Sleeping because without that rain the protein folding within cells doesn’t happen. The fine hairs on their surfaces even harvest atmospheric water, it’s that important.
A large Australian Soil Carbon study showed that soil moisture was correlated with 76% of SOC. That pH and soil bulk density were also important, but after that, not much else. The better the soil bulk density is, the better the soil/air exchange, while pH makes for happy microbes that breathe that air. It’s not a coincidence that soil carbon tends to buffer pH then.
Higher levels of soil carbon also increase soil moisture holding and reduces soil bulk density.
Another Brazilian forests study found 86% of soil nitrogen correlated with SOC.
I watched a Terra Preta documentary the other day. They were harvesting the high carbon black soils and trucking it out. The land owner claimed that if he left 20cm of the soil, that the deep black soil “grew back” over time. Fact or fiction? Are there microbes in the soil somehow creating long lasting soil carbon or are the soils simply at a tipping point with the 20cm and it’s enough to create or hold shorter-term carbon that may oxidise over time? Beats me. I do however know there are different forms of soil carbon and not all are equal.
Why I really love the notion of a Soil Carbon Continuum, it’s never ending. 🙂
Carbon and nitrogen additions induce distinct priming effects along an organic-matter decay continuum
As for adding biology to soils, there is a lot to consider. I wrote about Bokashi and Natural Farming in this thread recently. The results at the bottom of that post seem to indicate it works, but is that the ingredients or biology or a combination? Work With Nature on YouTube and his friend are performing a trial where they boil a compost tea and compare with a biologically active tea. I’m interested to see their results.