6 Great Ways To Improve Your Soil Carbon Levels

Existential to soil health and soil itself, we like to refer to soil organic carbon as the magic stuff. Bringing tremendous benefits to farm profitability and landscape resilience, it is the key ingredient that makes soil not only fertile – but healthy. 


In the last decade investments into a broad range of research projects across both government and private sectors have been intensifying. The focus has been linking land management practices with soil organic carbon and the plethora of co-benefits that come with it.


Favourable developments have led to science being firmly embedded in evolving agricultural land management practices by some of Australia’s most innovative farmers. The result is the birth of a system that’s best for the landscape, the climate system as well as production; a system that is truly sustainable. 


This investment into science is paying off. In the preceding decades, Australian farmers have converted from intensive tillage practices to reduced or no tillage practices. Stubble retention after harvest has become the norm and nitrogen (N) – fixing crop rotations with pulses and legumes outside of pasture establishments, have also gained traction. These changes in land management alone have slowed the rate of soil organic carbon decline that has contributed to vastly degraded land across Australia's agricultural landscape. 


In this article, we will be exploring 6 great measures agricultural land managers can implement that will not only improve soil carbon levels, but will make significant improvements to farm productivity as a whole, whilst creating sustainability benefits that will reflect in your bottom line. 


1. Applying lime to remediate acid soils. 


Acidity itself is not responsible for restricting plant growth. Instead, biological and chemical processes favourable to plant growth can be negatively affected by acidity. It decreases the availability of plant nutrients, such as phosphorus and increases the availability of some elements to toxic levels for both plant and soil biota, particularly aluminium and manganese.


Acidity can degrade the favourable environment for bacteria, fungi and other soil organisms, and highly acidic soils can inhibit the survival of useful bacteria, such as the rhizobia bacteria that fix nitrogen for legumes. Below a pH level of 5.5 the aluminium present in soil becomes available as the ion Al(OH)2+ which, when charge in the soil is low, starts to become toxic to fungi and also leads to phosphorus fixation which causes deficiencies in crops. 


When the pH drops even further to about 5.2 in a low–charge soil (e.g. a sandy soil with low clay content), the dissolved aluminium species Al3+ becomes increasingly available in the soil water solution – causing toxicity not only to most soil fungi and some bacteria, but also many of our crops, whose roots are sensitive to 


Figure 1. Effect of pH on availability of plant elements (source).

dissolved aluminium ion Al3+.  Crop roots are inhibited by the dissolved aluminium,  and won’t grow into pores in which dissolved aluminium is present. The reduced root mass results in less  water uptake and thereby stunting plant growth. 


This same range of acidity is also associated with reduced phosphorus availability for plant growth – due to fixation with aluminium in the soil solution causing phosphorus deficiency, further affecting crop growth. 


Applying a fine agricultural lime with a high neutralising value is key to revitalising acidic soils. Chemically, the addition of lime raises the pH which precipitates the toxic aluminium out of the solution,  liberating significant amounts of acids which is then also neutralised by the lime.  This eliminates pore water toxicity increasing the effective plant available water.


In highly acidic soils (pH lower than 4.5), liming to achieve a pH level between pH 5.2 and 5.5 will allow plant roots significantly greater access to all pore water and reduces toxicity issues associated with aluminium and deficiency issues associated with phosphorus. 


However when considering the soil microbiome, achieving this range will not be sufficient to allow the biome to return to full function – as the fungi are still suppressed. Without fungi, organic matter is converted to CO2, soil structure is less aggregated with fine minerals infilling the pore space and the transfer of nutrients to plant roots are inhibited. For these reasons, crop production and carbon sequestration rates are not optimal.


Even liming above 5.5 requires stimulation of the biome, for example through inoculation with microbial species, in order for the biological active zone around the root to return to its full potential. The closer you get to a near neutral pH (a pH of >6), the less inhibited the soil biome will become and the more the entire system will thrive.


As you can see, acid soils limit the potential for plant growth due to plant nutrient imbalances whilst simultaneously inhibiting a fully functional soil biome. Liming can be a cost–effective way of removing these acid-soil constraints to farming systems, resulting in greater biomass production, increased crop and pasture yields – greatly contributing to the increase in soil organic carbon levels. Liming alone may not be enough and a system change in management to reduce the amount of manufactured fertiliser applied may also be required. 


2. Applying gypsum to remediate sodic or magnesic dispersive soils.  


Certain clay minerals, in the presence of an imbalance of the ratio of sodium to calcium (and to a lesser extent, magnesium to calcium), have a repulsive charge when wet that pushes clay particles in soil apart (Figure 2.). This unfavourable reaction is known as dispersive soil and, in the mild form, known as slaking.  Dispersed soils with a lot of clay shrink and swell, and dispersive soil with only a little clay, rapidly erode by piping, sheet and gully erosion. Whatever the reaction, soils in this state are practically unable to sequester carbon, and need to be repaired to function healthily. 


Figure 2. Soil Dispersion (source).


Sodic soils, because of the nature of clay, tend to be hard and compacted and thus difficult for roots to physically penetrate, and are low in organic matter and water holding capacity. These soils have poor yields. There are only two treatments; glueing the soil with organic matter (so literally adding fresh vegetative litter for decay into organic matter which will cover the repulsive charge and bind the soil); or to change the chemistry of the soil. From a chemistry standpoint, although sodium and, to a lesser extent, magnesium cause the clay to repel in the first place, these ions are actually less preferred by the clay, due to their weaker charge (Figure 3.). Because of its greater charge density, calcium is much more preferred by the clay and can, for example, be added to the soil in the form of gypsum. Gypsum, like agricultural lime, is a slow release calcium source – but it does not impact the pH, making it the better choice for this particular practice.  

Figure 3. Chemical reaction of gypsum in sodic soils (source).


When treated attentively, sodic and magnesic dispersive soils can regain a healthy biome and soil functionality, reestablishing a healthy production system where crop yields, biomass and  soil organic matter improve – which enables the soil to once more effectively sequester carbon.



3. Establishing and maintaining a diverse range of pasture species. 


Multi species perennial pasture blends that include grass species such as cocksfoot, fescue, prairie grass and legumes such as lucerne and multiple different clovers – which all seed at different times during the year, are great in maximising continual growth and renewal of the pastures. When managed together with a good rotational grazing strategy (discussed further below), a diverse range of pasture species will ensure peak soil and farming system functionality – resulting in optimal soil carbon sequestration conditions.


A diverse range of pasture species is hugely helpful in maintaining ground cover for 12 months of the year, which prevents soil from drying out and enables it to store moisture to help it persist through dry periods. Biodiversity of insect species and the soil biome thrive in such pasture systems and assist in pollination, nodulation, nutrient cycling and of course the conversion of pasture biomass into soil organic carbon – where it enters the carbon pool. 


Plant diversity also means beneficial root and litter diversity. The rhizosphere uses not only nodulated N-fixing bacteria (proteobacteria), but also fungi known as mycorrhiza to thrive. 


Mycorrhiza essentially invade roots to feed on exudates; mostly in the form of simple sugars formed from photosynthesis as well as sloughed–off dead root sheath – energy upon which this microbial system depends. In return, the Mycorrhiza help aggregate soil to create better passage of air and water, oxidise moderate to complex molecules (such as lignin, chiton and cellulose) and produce acid to dissolve rock – which liberates phosphate and other plant–essential nutrients (Figure 4.). 

Figure 4. Mycorrhizal symbiosis (source).

 

A resilient biome, which we all should strive for, is the result of plant diversity – and requires the inclusion of legume species such as lucerne and clover for improved nutrient cycling. These different pasture species all work well together because, as well as promoting a diverse soil biome, their root systems vary in depth (some having shallow roots and others having deeper tap roots) which ensure deep moisture retention and deep nutrient permeability. All these factors lead to improved soil health, capable of efficiently sequestering carbon.

4. Developing a grazing strategy that alters the stocking rate, duration or intensity of grazing.


Rotational Grazing, particularly mob or cell grazing (Figure 5.), allows pastures to rest and regenerate. It allows grasses and legumes to tiller and set seed – ensuring pastures last for many years without having to be re–sown. This also means that palatable grasses are not selectively removed – ensuring maximum ground cover which prevents the soil from drying out, and also secures stored moisture for dry-periods. What this also aids with is creating a diverse biome that optimises the cycling of nutrients – a biome highly capable of sequestering carbon. 


Figure 5. Cell grazing (source).


Frequency and duration of grazing, as well as paddock rotational order makes a big difference to soil health. Grazing livestock naturally fertilises pastures with manure, their grazing stimulates pastures to reshoot and their hoof impact incorporates pasture seeds into the soil for a continued germination cycle. Shifting livestock according to paddocks nutrient status  – with stock moving from higher nutrient soil to lower nutrient soil, can also evenly distribute nutrients and reduce the need for fertiliser inputs to the system. 


What a new grazing strategy like this requires however is a constant evaluation of appropriate stocking rate. During below–average and drought years, destocking or agisting is required to ensure the rotational system does not lead to bare ground caused by excess stock. During greater than average and wet years, restocking greater numbers of livestock is required to ensure the paddock is being evenly managed, however consideration needs to be given to allow pastures to spell after rain events with reduced grazing intensity.


When done right, rotational grazing can prove to be an effective land management and regeneration strategy, resulting in healthy and fertile soil high in organic carbon – and capable of efficiently sequestering carbon. 


5. Crop rotations that encompass a dynamic nutrient management strategy


Introducing a crop rotation system that encompasses a dynamic nutrient management strategy can lead to soil that is much more capable of sequestering carbon – particularly where there is a material nutrient deficiency. Typically this includes less reliance on fertilisers with the inclusion of rotational crops – such as legumes and pulses for nitrogen fixation, but can also include the lesser known use of certain fungal species, e.g. mycorrhiza and  penicillium bilaiae, to solubilise soil-bound phosphate.


Legumes and pulses have a symbiotic relationship with rhizobia (nitrogen–fixing root bacteria), which utilise the enzyme nitrogenase to catalyse the conversion of atmospheric nitrogen (N2) to ammonia (NH3) – which plants can readily assimilate to produce nitrogenous biomolecules for growth (Figure 6.). In this symbiotic relationship, plants provide sugars produced from photosynthesis to rhizobia, which then use it as an energy source for nitrogen fixation that then becomes available to the host plant for its growth.This considerably reduces the need for synthetic N fertilisers and machinery passes. Most of all it contributes to a healthy system of continuous nutrient cycling.


Figure 6. Nitrogen root nodules (source).


Phosphorus cycling is a very complex process involving P sorption the rate at which this occurs changes significantly with soil type. Only 5-30% of phosphorus applied as fertiliser is taken up by the plant in the year of application, a small portion is lost to runoff and groundwater and the largest portion becomes part of the soil reserve and is locked up in the soil as amorphous apatite. Various estimates indicate approximately 70–80% of P fertiliser added in the crop year becomes part of this soil reserve . Co-applying penicillium bilaiae with the fertiliser or the seed allows this bound portion to be liberated, as the fungi and bacteria release an organic acid to liberate phosphate before shuffling the phosphate along their fungal hyphae to the plants root hairs (Figure 7.)


Figure 7. Fungal and bacterial phosphorus acquisition (source).

 


The utilisation of plant-bacteria relationships (rhizobia) and plant-fungal relationships (penicillium bilaiae and mycorrhiza) are an extremely effective way to activate natural nutrient cycling of N and P and form part of a dynamic nutrient management strategy in rotational cropping systems. This diverse biome functions together by unlocking bound phosphate and improving its utilisation, whilst simultaneously also fixing vast amounts of nitrogen – all whilst crops are actively growing. We call this symbiosis “gains with grains”.

6. Undertaking earthworks to remediate the land.


This last point includes implementing various types of earthworks to remediate degraded farmland to rehabilitate the soil to the point of being able to sequester carbon again. It is important to have a thorough understanding of both your soil profile and your remediation objectives before attempting to undertake such measures, and we’ll explain why in the section below.


Remediation for hard pans

Caused by repetitive intensive tillage practices and significant shallow clayey B-horizons in duplex soils, hard pans create an impenetrable hard layer that prevents the roots from extending deeper into the soil profile (Figure 8.) – denying the plant from accessing essential nutrients and moisture, reducing water infiltration and ultimately limiting production (Figure 9.). The most effective remedy here is mechanical deep ripping to break through the hard pan.  


Figure 8. Hardpan (source).

Figure 9. Effects on roots (source).


Remediation for non–wetting sandy soils

Claying involves adding and incorporating clay-rich (30-50%) subsoil into water–repellent topsoil to overcome the repellency. Adding clay-rich soil provides a long-term solution to soil water repellence, and also increases soil water holding capacity and reduces wind erosion. Breaking the soil mechanically and mixing in clay subsoils with the sandier topsoil (Figure 10.) can improve the water holding capacity of the soil, particularly if a green manure crop is grown before earthworks.  This can be done by ripping, spading, delving or clay spreading, depending on depth–to–clay risk. 


Attention! Great care needs to be taken here not to bring hostile subsoils to the surface, namely dispersive, alkaline or acidic subsoils. Consideration of what mixing does, including adverse impacts, has to be carefully considered. This program is best undertaken with other strategies.


Figure 10. Mixing clay subsoils with sandy topsoil (source).


Remediation for soil erosion and surface runoff

Erosion occurs when raindrops hit the soil surface and displace soil particles – which then flow over the land surface. Water erosion reduces agricultural productivity by causing loss of topsoil which exposes potentially hostile subsoils, whilst also reducing effective rooting depth and plant available water. 


Erosion also causes loss of nutrients, which of course leads to reduced crop yields, downstream eutrophication and silting of dams and natural waterways and also damaged infrastructure and weed dispersal. Here, land managers can reduce the risk of water erosion by undertaking earthworks that control surface water run-off with contour banks. This system effectively intercepts, diverts and slows run-off, rather than permitting it to flow uninterrupted down the slope (Figure 11.).


Figure 11. Formation of contour banks for water erosion control (source).


Remediation earthworks such as these are most effective when combined with other management practices such as stubble retention, retained pasture cover and the introduction of both a comprehensive grazing management strategy and a system that builds up soil structure and minimises exposure of bare soil. 

In Conclusion


Existential to soil health, carbon brings tremendous benefits to both farm profitability and landscape resilience, and ought to be prioritised in land management practices, regardless whether one plans to enter the carbon trading market or not. There is no shortcut to healthy levels of soil organic carbon. It’s one of the few things that money simply can’t buy (at least not instantly) – you need to farm smart to farm carbon.

We pride ourselves in having the domain knowledge not only around cutting–edge IT, but soil science, farm systems and carbon farming. Reach out to us to find out more about how you can leverage Carbon Count to manage and maximise your soil carbon projects, improve farm productivity, soil resilience and benefit from the new income stream the carbon trading market has to offer.