Urban soils at the scene of construction where subsoil has been brought to the surface, mixed or that have had the topsoil removed and construction materials like sand, stone, brick etc. embedded in them can pose a challenge to regenerate. Stabilising these soils such that rain and flood doesn’t cause erosion is also an important task.
To help understand what contributes to soil aggregate formation and the stability of these soils researchers studied the “Interactive effects of compost, plants and earthworms on the aggregations of constructed Technosols” and found increasing amounts of compost needed increasing amounts of plants or earthworms to make a difference.
Aggregation is an important physical process to study during the early formation of Technosols. It is known to be influenced both by the organic matter content and soil biota. Constructed Technosols represent good models to test the importance of these factors since their composition can be easily manipulated by mixing different proportions of parent materials and introducing soil organisms. In this study, we performed a 5 month mesocosm experiment, using excavated deep horizons of soils (EDH) as mineral material mixed with green waste compost (GWC) at six different proportions (from 0 to 50%) in the presence or absence of plants and/or earthworms. After 21 weeks of incubation, aggregation was characterized by: 1) determining the size fraction and morphology, 2) measuring the distribution of organic carbon (OC) in each fraction and 3) testing the aggregate stability. Results showed that organisms accounted for 50% of soil aggregation variance while green waste compost (GWC) was responsible for only 5% of the variance. The percentage of total variance of OC distribution in aggregates explained by organisms, GWC, and the interaction of the two was similar (28%, 22% and 26%, respectively). The effect of GWC on structural stability was negligible (2%) compared to that of organisms (70%). The effect of earthworms combination with plants was complex: plants had a dominant effect on the distribution of the size of aggregates by disrupting earthworm casts, but earthworms had a dominant effect over plants for aggregate stability under fast wetting only when the percentage of compost was low. This study underlines the importance of considering the interaction of the organic matter and soil biota: in this case, increasing compost proportion in a Technosol has significant effects on aggregation only in the presence of plants or earthworms.
This picture is an example of what can be achieved by managing existing trees that have been over harvested by man that still have large roots hidden below the surface. These trees are managed for regrowth. And the picture is from desert to regrowth in four years. The tree then created microclimates under the trees that increased yields.
When the microbes aren’t doing the work because they’re not being watered, housed and fed well, some farmers do that work for them.
In the video from India they explain how they use dried topsoil and subsoil for fertigating their crops via foliar spray. This has multiple effects, the first is providing soluble and insoluble nutrients to the plant surfaces for plants, microbes and sunlight to break them down, and second is adding to existing topsoil where more active microbes may utilise them.
However care should be taken as many clays from subsoils are known to have antibiotic effects, even on superbugs, and the application of foliar sprays with these clays has been shown to eliminate some plant pests and diseases. Many subsoils also have low pH that make kill some microbes.
So on one hand applying subsoil may be supplying nutrients and could increase productivity, and this appears to be the case in India. On the other and depending on the soil it could initially be killing the plant and soil microbes that produce them. This can potentially break the natural cycle and make this a system that requires continuous human intervention.
In the video they recommend 3:1 dried topsoil to dried subsoil in their foliar spray, with that increasing in subsoil content to 1:3 for disease eradication.
Every 10 days or even weekly…
They are effectively mining the soil to liquid feed the plants for continuous cropping.
Whether this is sustainable or even regenerative is a good question.
Does this practice build soil over time? Could it? Is that building as much as they excavate and does it compensate for the energy used to distribute those nutrients? They do mention increased plant nutrients, but I’m not sure if they also tested the soils.
On one hand the drying of soils is effectively hunting and killing microbes and their mucilages for their nutrients, on the other you get increased productivity. It’s like robbing Peter to pay Paul, which is the best investment? The same applies to killing off plant predators with foliar spraying, effectively feeding the plants with dead microbes and dead soil.
But perhaps this produces more plant exudates that produce more symbiotic root microbes to kickstart nutrient cycling above the level in the root zone needed to build soil rather than consume it?
If done in combination with diverse cover cropping and chop and drop to provide a cover and food for the soil I can see it being a useful tool to help get back to letting nature do the work, instead of the farmer.I think of this in the same way as I think of tillage. Initial minimal tillage can kickstart a system faster towards a regenerative approach by decompacting soils and releasing nutrients for plants to establish and grow and photosynthesise thereby feeding more microbes that build soil and reduce soil density.
It’s important to keep in mind too that tilling kills off fungi and earthworms, and so using any technique that disturbs soil should be minimised.In situations when access to organic matter is limited I can see these approaches helping get an initial crop in the ground to then be regeneratively managed. On the other hand where there is plenty of organic matter and soil moisture a no dig approach may be more appropriate.
“According to a recent article at Grist”
While I can’t tell if David is being sarcastic about the recent part of the Grist article since it’s from 2010, I totally agree with what he says and quotes.
Surface application of synthetic nitrogen alone has been shown to deplete the surface layer of organic carbon and increase the amount of dissolved organic carbon. This then leaches into the lower layer and initially increases C mineralization there. As a result there is then less carbon in the surface layer for microbes to use and to bind nitrogen from air and so more fertilizer is then needed, leading to even more leaching over time and the creation of the more-on farmer.
Now there are studies showing that if synthetic nitrogen is added in moderation with organic matter that it can actually increase carbon mineralization, I just can’t find where I filed them. 🙂 What really matters is the form of organic matter added.
Recent studies[2,3] indicate a C:N of 100:1 or less is good for carbon mineralization depending on the clay in your soil, with between 11:1 and 50:1 being near ideal for priming in lab conditions. Split the difference and you get 30:1 which is often recommended for compost piles for microbes to break them down, not a coincidence!
Responses of priming of organic matter (OM) decomposition to OM C:N ratios (horizontal axis) and labile C:N ratios (vertical axis).
This contour figure was made based on all priming results of four OM forms from Fig. 2, using C:N ratios in OMs as x-axis, C:N ratios in the labile inputs as y-axis, and all priming data as z (color) axis. Priming effects vary strongly among substrates along the white dashed line, where labile carbon inputs are high and nitrogen is low. Priming effects do not vary strongly among substrates along the black dashed line where labile carbon is low and nitrogen is high. The dashed pink line indicates the substrate C:N threshold ratio where priming changes from negative to positive.
Ramial(small branch) chipped wood tends to be at the upper end of the region between 30:1 and 170:1 and can be a good choice long-term. However getting the C:N down should build soil faster. Chopping and dropping is one approach. Growing and harvesting cover crops before grazing and trampling them in with animals like Gabe Brown does would be another.
There are also mechanical versions like those used in biodynamics and regenerative practice. Hand operated versions of the latter are also possible.
Growing lawn grass and leaving any mower clippings can even build soil carbon. However it may take 30 years to raise Total SOC levels by 1% doing it this way…
However in cases where you’re adding heartwood chips like many Back to Eden peeps, then nitrogen addition may help, I believe this is why many like Mr. Back to Eden himself Paul Gautschi has found that high nitrogen chicken manure helps.
Another example would be after forest fire or biochar creation where you have a lot of carbon but are nitrogen, phosphorus and sulfur limited as these tend to boil off at fire temperatures.
In the comments on David’s post Bob also mentions the N impact on ectomycorrhizal fungi.
I found a carbon-13 labelled study that seems to suggest that by adding nitrate in the form of calcium nitrate, that trees significantly reduce below-ground C allocation, probably because the trees are getting their nitrates from the fertilizer and don’t need the microbes to fix the nitrates for them. As a knock on effect the fungi also reduce their C allocation to soil biota by 60%, likely because fungi need continuous carbon input to grow and fruit and can only afford to give up so much. This all suggests that nitrate addition short-circuits the carbon-nitrogen cycle when it doesn’t increase below-ground plant C allocation. Those papers Bob quotes suggests the effect is not limited to nitrate but also ammonium further up the nitrogen cycle.
I’ve also read that the symbiosis between legumes and their rhizobia breaks down with use of nitrogen fertilizer. And that there’s a consistent change in soil microbial communities with additions of N and P.
If N doesn’t do it, I’m wondering what will actually increase below-ground C allocation to build soils fast… any suggestions? [Other than legumes or perennials with long thin roots]
Off to Google Scholar I go…
 Carbon mineralization in response to nitrogen and litter addition in surface and subsoils in an agroecosystem http://www.tandfonline.com/doi/abs/10.1080/03650340.2016.1145792
 Is the fate of glucose-derived carbon more strongly driven by nutrient availability, soil texture, or microbial biomass size? http://www.sciencedirect.com/science/article/pii/S0038071716302103
 Carbon and nitrogen additions induce distinct priming effects along an organic-matter decay continuum, Figure 5 doi: 10.1038/srep19865 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4726261/figure/f5/
 Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest – Högberg – 2010 – New Phytologist – Wiley Online Library http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2010.03274.x/full
 Long-term nitrogen fertilizer use disrupts plant-microbe mutualisms http://www.rdmag.com/news/2015/02/long-term-nitrogen-fertilizer-use-disrupts-plant-microbe-mutualisms
 Consistent responses of soil microbial communities to elevated N & P nutrient inputs in grasslands across the globe PNAS | Mobile http://m.pnas.org/content/112/35/10967.abstract
 Turfgrass Selection and Grass Clippings Management Influence Soil Carbon and Nitrogen Dynamics https://dl.sciencesocieties.org/publications/aj/abstracts/0/0/agronj2016.05.0307