BLOG SERIES: Increasing carbon in soil – a holistic conversation. Part 2: Pools of Carbon – Mineralisation Pathways

Philip Mulvey

April 22, 2022

In part 1 of this blog series on increasing carbon in soil, we discussed the different pools of carbon in soil and how carbon moves from one pool to another.

These carbon pools in soil are very much related and dependent on each other. What’s important to understand is that you need to increase the carbon levels in all your carbon pools to effectively sequester carbon.

The movement of carbon between the pools is controlled by the interplay and ultimately long–term dominance of two key processes: humification and mineralisation.  

When humification dominates, all carbon pools increase in size and the movement of organic matter, as shown in Figure 1 below, is from left to right; removing carbon dioxide from the atmosphere. 

Unfortunately however, mineralisation; the opposite process, has mostly been taking place. Agricultural practice has, for the last 10,000 years since its inception, mostly mineralised organic matter; decreasing our soil’s carbon pools and adding carbon dioxide to the atmosphere (as depicted by the arrow moving from right to left in Figure 1).

Consequently the following blog revolves around exactly this; the topic of soil carbon mining, what causes it and how to recognise it by understanding the rate of change between the three pools of carbon in soil.

Figure 1 – The Three Pools of Carbon in Soil

The mining of carbon

For over 10 000 years, agriculture’s mining of soil carbon has led to the rise and fall of civilisation. This is because the soil upon which they grew and were dependent on was destroyed by soil carbon mining. These civilisations destroyed the basis of their own existence; Natural Capital, which is founded on soil organic matter – soil carbon.  

They collapsed for want of soil carbon. It is not as if we weren’t warned from plato 2500 years ago, via Jacks and Whyte in 1939 with the book the Rape of the Earth with a soil scientist producing a book a decade since on the same subject.  Let's explain why soil scientists are concerned that our civilisation could go the same way.

Soil carbon mining is essentially caused by both tillage agriculture, mechanised agriculture and set pasture management.  The development in the last 15 years of conservation agriculture (so called because this system conserves soil carbon), as well as regenerative agriculture whose focus is to stop soil erosion. However No-Till, of itself, does not restore carbon. This will be addressed in coming blogs. The focus of this blog is why and how soil carbon mining occurs.

How carbon pools in soil deplete

When the amount of grass litter, which usually renews the organic matter in soil, is reduced and/or the soil is exposed excessively to air and/or sunlight, the regeneration of the pools of stored carbon in soil ceases. 

When this happens, labile carbon is mineralised rapidly before being converted to intermediate carbon; a more stable form of carbon in soil. The consumption of the labile carbon by aerobic microbes creates a sort of suction on the other carbon pools, particularly the intermediate pool. As a result of this suction, a gradient best expressed as a need for carbon is created; which results in all pools of carbon being reduced.

This gradient is a result of carbon constantly being exported from landscapes in the form of agricultural products.  This depleting extraction process of organic matter from the soil is no different to the extraction of minerals from rock – and is thus termed “mining”. 

As can be seen in Figure 2, organic matter is driven from the intractable pool to being mineralised as carbon dioxide in the air.  Organic matter is progressively lost to the atmosphere every year that the existing practices of agricultural production of the land continue.

Figure 2a: First two decades of depletion through mineralisation
Figure 2b: More than 5-7 decades
Figure 2c: More than 7 decades

A closeup on mineralisation

Mining of organic matter and mineralisation of newly added dead matter (litter) in soil is the result of practices that readily allow the aerobic microbial metabolic pathway that converts complex organic molecules to air and water, to dominate.

The processes of fermentation, putrefaction and humification do not occur or, if so, are greatly exceeded by this mineralisation process.

When the soil is allowed to dry at the surface and develop unfilled macro–porosity, have an unbuffered or poorly buffered pH (in the case of most No-Till), have bacteria exceed fungi in numbers (also in the case of most No-Till) or C/N/P exceed the ratio of 20/1/0.4; mineralisation is favoured.  

Though it sounds difficult, in circumstances of traditional tilled agriculture, all these factors are promoted. Even in circumstances where No-Till with stubble retention is being practised, where the soil may be covered and microporosity and drying does not develop initially and where the C/N/P ratio is maintained by manufactured fertilisers, the soil remains poorly buffered and bacteria numbers begin to exceed fungi, mineralisation is still favoured.  

Thus for most historical and recent agriculture practices, mineralisation will still occur if the newly created dead matter is mineralised and mining of the existing pools of organic matter occurs.

The first stage as shown in Figure 2 is a equi-loss of carbon from every pool and the freshly laid carbon via litter to the soil is mineralised and does not engage in the process of aggregation.

The second stage draws down from all the pools reducing the overlap between the pools. The degree of overlap represents the rate of movement between the pools and the arrow represents the direction carbon moves between pools.  

The final stage is loss of all organic matter except that so strongly bond to clay minerals with all litter going to labile carbon and back to the atmosphere as CO2. There is no aggregation and the soil has a massive structure for clayey soils and a loose for sandy soils. The soil has no resilience and needs annual input of fertilisers, lime and herbicides to maintain production.

Unfortunately mineralising dead matter such as vegetative litter and mining organic matter is comparatively easy, consistent focused management is not required.  Continued mining of organic matter leading to  the collapse of productive lands heralded the collapse of most past civilisations is unnaturally easy to do when the focus is solely on Financial Capital.

Why does it matter if we mineralise dead matter and mine organic matter? 

To address this question you need to ask yourself; why is organic matter important?  What good does it do for a farmer? How does it maintain or increase production from the land? Though we’ve discussed this in previous blogs (check out our Land Management blogs here), a refresh is considered worthwhile. 

Organic matter acts as a mulch and breaks capillarity, minimising direct evaporation. It reduces sunburn to the underside of leaves by not reflecting sunlight back. It stores nutrients in a series of different physical and chemical bonds that are released at different energy thresholds to plants.  

Organic matter is simultaneously the food for the soil biome, a water store for the soil biome, a nutrient highway and accommodation for the soil biome and also the product of a healthy soil biome. 

Without organic matter there is no living matter. The two are codependent.

But most importantly of all, organic matter creates soil aggregates; it’s the glue that causes aggregation in soil. A soil without organic matter–created aggregation has a lot of either micro or macro pores; while a soil with aggregation mainly has mesopores with almost no macropores. 

Aggregation means that though the soil can readily have a steel probe pushed into it with little resistance, (meaning it’s easy to plough and easy for plant roots to penetrate it) it is not readily eroded or broken apart. The soil can infiltrate faster, but can also hold more water in the energy range available to plants and thereby maintain moist conditions far longer than if it would have poor aggregation. Better aggregation also provides more recharge to groundwater which extends and increases stream base flow and results in less intense floods. 

Because of this organic matter reduces the environmental losses of fertiliser (resulting in less fertiliser being needed), improves water use efficiency, improves soil aeration after rainfall, allows farming equipment on sooner after rainfall, has plant available water for longer periods during dry spells, increases competition for invasive microbes which reduces crop damage, and many more benefits. 

Soil organic matter reduces cost of sales in every season and increases yields during average and bad seasons. It is the key to making the soil sustainably productive and creating a sustainable farm over many generations. Increased profit with time for most farms depends on the amount and the relationship of humification to mineralisation. 

Mining of organic matter negates all these benefits 

When mineralisation starts to dominate, the large microbial community contained in soil usually eats the shell of their “house” which is the intermediate carbon pool. This exposes labile carbon that reduces aggregates progressively to sub-aggregates and then sub-aggregates down to clay domains and intractable organic matter.

If mineralisation continues, even the intractable organic matter will be consumed, possibly all the way down to the char particles. As shown in Figure 2c, it is only the charge of the soil that stops the greedy consumption of all organic matter by the aerobic bacteria. As mineral charge strongly binds a portion of organic matter, stopping bacteria getting at it. Thus the amount of carbon that remains after prolonged carbon mining is attached with greater energy to the minerals than the microbes can provide. Soils that have a high clay mineral charge (also known as permanent charge) retain the most organic matter when subject to prolonged mining conditions.

Soil with little or no permanent charge rapidly loses nearly all pools of organic matter. These soils are poorly resilient and lose productivity quickly (within 30 years of cropping commencing or 50 years of intense pastoral activity) and require substantive chemical inputs increasing with time. Such soils include most of the grain belt of Western Australia.

But even highly resilient soil such as the loess of the Midwest USA can convert to low resilience once the highly charged topsoil loses aggregation and becomes highly susceptible to erosion.  Wind and sheet erosion can remove up to 30cm of loess during a single wind storm during a drought. Not many farms production can sustain losses of several metres of topsoil, as in many areas of the Midwest in which loess is of variable thickness.

Very few of the world's agriculture land is on swelling deep chocolate soil (Kraznazems) which is derived from deep fertile parent material (basalt) or of deep alluvium from basalt parent material.  Only those farmers on these soils can afford to be flagrant in their practices.

Even the best soil crumbles under bad conditions at one point

What’s important to emphasise is that microporosity (deep cracks) will occur even in the best soil during drought. The removal of vegetative cover, the aeration of the soil profile happening because of microporosity and the low relative humidity of the soil during drought, will result in mineralisation dominating while the drought lasts.

It’s what happens after the rain returns that decides the long term fate of the soil.

Once mineralisation dominates not just the drought years but the good years, it becomes very hard to reverse the avariceness of the aerobic bacteria when they are left rampant, unrestrained by fungal dominated communities. Without a change in practice or a great run of good seasons soil carbon mining will continue albeit at a slower rate. Soil carbon mining occurs across most agricultural land, the only difference is the rate.  

How do you recognise when mining is occurring (without conducting regular soil testing) when mineralisation dominates in even the best seasons?

Not surprisingly, organic matter diminishment with time is easily recognised in system responses which can be measured/assessed  by simple tests easily done by farmers.  

  • Colour: The soil becomes lighter in colour,  from the exterior of clods in.  
  • Penetration Test: The soil is harder to work and a push rod or penetrometer does not go in as far. Any rod can be used, as long as pressure inserted is the same with time (ie. the same person pushes as hard as they can)
  • Aggregate in water test: When placed in a glass of water an aggregate of soil slakes or de-aggregates faster than previously.  Observe an aggregate over 12 hours, record a photo or what is observed every 2 hours. 
  • What happens to decomposing plant matter? Decomposing plant litter disappears faster and leaves no trace. There is little evidence of decomposing plant matter after a season. 
  • Clod formation and appearance of dust: The soil forms hard clods more easily when ploughed and readily dusts.  

Colour, push rod test and the aggregate test all measured every season with time directly indicate whether mineralisation and mining is continuing, becoming worse or has been conquered by humification.  

In order to successfully remove carbon dioxide from the atmosphere and into the earth, the humification  process needs to become dominant over the mineralisation process one once more. 

Step number one in our journey to carbon–rich healthy soil is understanding how we can swing this process back in motion, as well as understand what the early signs of successful humification are.

The next blog is how to reverse mineralisation to humification.  How to promote it, recognise it and measure it.

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