Using Dried Topsoil and Subsoil to Kickstart Nutrient Cycling

Nature’s Voice

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.

Adrianna MarchandIn a lot of ways it also reminds me of the Soilkee Renovator. Here they only disturb strips of the topsoil and bring organic matter to the soil surface to dry and oxidise it, thereby┬ákilling microbes and releasing their nutrients and mucilages to feed the soil and surviving plants. And from results I’ve seen it reduces initial organic matter and trades it for soluble nutrients that feeds the microbes that produces plant available nitrogen and other nutrients. Thereby generating a burst of fresh growth and photosynthesis. It also appears to recover that lost organic matter over about a year. So not something you’d want to do regularly but it certainly could help kickstart nutrient cycling, especially where they trialled as they had existing high carbon levels. The effect is likely similar to that seen when soils dry and rewet and produce CO2-bursts.There’s hope these farmers will eventually use any additional fertility to transition to a more regenerative farming approach. That this could be a tool in the regenerative farmers toolbox when they have little initial nutrient cycling to help get started, but it’s also open to abuse when misunderstood.
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.



Dig, then No Dig! – Supercharge Your Soil.

All soils are different. However No dig and diverse cover crops that keep fungi fed and alive are definitely the preferred method as the less the soil disturbance, the more the plants photosynthesise and feed the undisturbed soil microbes like fungi that then build soil carbon and structure.

However for soils with poor structure, low soil carbon, and hard clay pan compaction layers, then subsoil manuring can be a faster option as it gets the food to microbes where it’s most needed. Ideally with minimal inter-layer disturbance, and optionally then with a layer of weed suppressing material, mulching and planting through. You have to always remember to plant out bare mulch and soil however, otherwise mycorrhizal fungi that require plant hosts will die off. About half of mycorrhizae also die within the first month of tilling and a further half every subsequent month soil is left fallow.

When subsoil manuring you have to make sure there is enough soil moisture for the microbes to break down that subsoil manure and integrate it too. Studies suggest materials with a C:N of less than 25:1 appear to be the best to feed those subsoil microbes. Basically anything green, eg. grass, fresh leaves that aren’t allelopathic, or alternatively compost or animal manures if you want a higher bacterial to fungi ratio that the high phosphorus, particularly in animal manures encourages (normally you don’t). Mulch helps maintain moisture, and the mulch should ideally be below a C:N of 100:1 as this speeds the building of soil carbon. Those are mulch materials like leaf mold, straw, shredded newspaper, and ramial (branch) chipped wood, ideally all mixed together for diversity. Those materials that are high in potassium and low in phosphorus are also likely to encourage fungi.

Digging without amending however is generally a bad idea as it destroys soil structure, and can lead to soils drying out where those cracks form, which then causes the soil mucilage (glues) and microbial exudate to dry out and leach. Leaving sand, silt, and clay ready to combine and compact with one another.