Category: Opinions on Agriculture

Soil carbon sequestration is like a poker machine

Microbes do most of the work to decompose organic matter which is added to the soil. They take carbon from the soil organic matter and respire about two-thirds of it as CO2, which then escapes to the atmosphere. The microbes use the rest of the carbon to grow and divide and so the microbial biomass builds up in the soil and essentially become part of the long-term pool of soil organic carbon.

The carbon going into the soil organic carbon pool is impossible to see and difficult to measure, in the same way that the money flowing from the poker machine to the bank is hidden. In contrast, the carbon which is lost as CO2 escaping to the atmosphere is easy to quantify with simple colour-change tests, just as the pay-out from poker machines can be seen by the punter because it is trumpeted by bells and whistles.

The peculiar aspect of the analogy is that CO2 emissions to the atmosphere affirm that the microbes are active, and that soil carbon sequestration is happening. Great news – which provides encouragement enough to keep adding organic matter to the soil. And as long as organic matter is added, the microbes will keep taking small amounts of carbon into the long-term soil organic carbon pool. It is a perverse winning strategy.

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Compost Restarts Soil Carbon Gains

The way in which carbon is built-up in soil and how it can be managed to achieve productive agriculture may soon become the farming dogma.

Applying compost to generate more productive agricultural soils is gaining traction among farmers. This change is not surprising as compost-rich soils through their colour, feel and smell evoke marvellous human emotions. Grab a handful, compost-regenerated soils promise bountiful riches.

The measured benefits of compost are also well-regarded.  Humus is a game changer that improves soil structure, water-holding capacity and nutrient availability, although in truth those benefits accrue mostly in the surface soil and the nutrient amounts are quite small. Lots of compost must be applied regularly.

There are other fine qualities of compost that cannot be seen nor felt nor smelled. One such acknowledged benefit is for composts to improve microbial activity. In reality composts bring microbial biomass to the soil and the activity of that biomass will depend on how much labile carbon remains in the compost (not much if it is mature) and how much organic carbon exists in the soil (not much if it is a typical Australian agricultural soil).

The microbial activity of bacteria and fungi in the soil is most important. Bacteria and fungi use carbon directly and transfer energy to the soil food chain thus driving the soil biological processes and carbon accumulation in soil. The carbon mineralisation process releases nutrients to growing plants.

Microbial activity cannot be seen but it can be easily measured. The main feedstock of microbial activity, the soil labile carbon, can too be easily measured. Those data can provide a guide as to how to supply compost and labile carbon to promote crop growth. Better crop growth results in more plant residues, and more residual biomass fuels the composting process in situ and the creation of more soil carbon.

The opportunity for compost makers is to spread their limited compost products wider to a bigger farmer group. New technology can be developed to use less compost at each site but overall to create more organic carbon and more productive soils. This approach will create more value for everyone and have a bigger impact on Australian agriculture.

There are two books on the topic that I have enjoyed reading.

The first is Earth Plant and Compost by William F. Brinton (Thomson-Shore Inc.) a general and easily understood guide to composting which AgSight can supply to you.

The second is a refined perspective on Soil Health, Soil Biology, Soilborne Diseases and Sustainable Agriculture by Graeme Stirling, Helen Hayden, Tony Pattison and Marcelle Stirling (CSIRO Publishing).

© Stephen Ockerby 2020

Profiting from atmospheric carbon dioxide

Putting carbon into the soil can rectify two of our global problems.

In resolution of the first plight, it is the carbon which can be taken from the atmosphere which is important. In respect of the second issue, it is the effect of carbon in organic residues on soil biology which will sustainably increase food production.

Those matters have been widely reported and are generally accepted, and it is not the purpose of this article to sanction them. Instead it is intended to say that the premise that carbon can be added to agricultural soils by organic residues should have gained real traction and investment but it hasn’t.

Let me say it again, the crucial matters are the carbon dioxide concentration of the atmosphere, and the cycling of carbon from plants to the soil to in-turn promote plant growth.

Putting a price on carbon (however it is derived) may serve to limit carbon emissions but it does not remove carbon dioxide from the atmosphere, and it does little to focus agriculture towards systems that use carbon-rich soils to produce abundant foods.

Putting carbon into soil is real ‘carbon capture and storage’ and while it is necessarily a long game it is a biologically-perfect process. As organic material decomposes in the soil part of the consumed carbon is respired by soil microorganisms to the atmosphere, and part of the carbon is reassimilated in higher-order soil organisms (that in-turn are consumed) until humus and other soil carbon fractions that are resistant to decomposition are formed.

The amount of decomposable (labile) organic matter in soil is the determining factor in both sequestering atmospheric carbon into soil and maintaining diverse and active soil microbes. Microbial activity is crucial because it results in carbon and nitrogen mineralisation and the release of other nutrients essential for plant growth.

Measuring respiration from soil microorganisms is now easy and inexpensive, and it is a critical piece of information that farmers and other land managers can use to promote carbon storage in soils and sustained production.

Two recent research papers give insights about the processes by which carbon ends up in soil. The first by Munoz-Rojas et al. (2016) describes how in restored mining topsoils, vegetation cover begets carbon in soil and microbial activity, which in-turn begets vegetation cover. The second paper by Walela et al. (2014) describes how residues whether from straw, pastures or woodlands vary the amount of active organic compounds and the rate of carbon turnover. The collective point is that carbon sequestration to soil is manageable and should be pursued in mining, pastoral and agricultural situations.

My own work (Ockerby et al. 2014) and that of my colleague (Yang et al. 2014) demonstrated the responsiveness of floral development in plants to growth.  Soil microbial activity affects plant growth and plant growth determines carbon capture from the atmosphere.

© Stephen Ockerby 2017

Truths and Benefits of Soil Biology

There are several reasons for me to write an opinion piece about soil health, not least that (for a crust) I market in Australia a technology called Solvita™ – which I think is one of the few tools available in the world that comes close to revealing the relevant processes of agricultural soil biology. Unfortunately it could be a long and technical sermon….

Instead I will describe a scene that I think will be played-out on thousands of farms Down-Under in the not-too-distant future. In a sense, I want to make only the point that it is the putting-into-practice aspect of soil biology which is important for us to master at this time.


It was the end of a long day. George switched off the tractor and jumped to the ground.

Max, his agronomist, was waiting for him in the shed leaning against his new, gleaming-white utility.

“You know, Max, we’re paying you too much.”

“Your crops have never looked better, George, and this will show you why.” He pulled out the box of jars that had become part of his standard kit and placed them in two lots on the workbench. “It’s not that big a secret – just changed the way you manage nitrogen so you’re now getting the benefits from those cover crops.”

George grabbed a couple of beers from the nearby fridge, opened them and gave one to Max.

“I took these soil samples from the fields you’ll sow to maize, and ran the CO2 respiration tests overnight. This one is from the back paddock where you grew the cover crop and this one from the bare-fallow block. Can you see the difference?”

George knew exactly what he was looking at; the probe in the cover-crop sample was a greenish-yellow colour (a reading of about 3.5) so the soil was well supplied with carbon and had good microbial activity, the other probe was grey (1.7) so the fallow soil was poor in carbon and had not much microbial activity.

“A no-brainer,” George muttered, “less fertiliser following the cover crop.”

“Yes, but I also ran the new SLAN test.” Max picked up another jar from each lot. “SLAN measures the nitrogen that is easily released when the organic residues break-down. You see the yellowy-green colour of this probe – it gave 178ppm Labile Amino-N on the Colour Reader. So after the cover crop there is sufficient soil N for the maize at sowing but I think we should still retest at side-dressing. The fallow soil had nearly no ammonia in the jar, the probe’s still yellow – so the maize will respond to fertilizer N at sowing.”

George thought for a moment. “So the CO2 test is for the process of N release through microbial activity and the SLAN test is for the amount of N that can be released from organic matter. Does that all makes sense to you, Max – you know – the science and technology behind the tests?”

Max nodded, “I reckon it does…, the best in the world probably. And I’ve sent soil samples to the lab. With the early season rain we’ve had, they should come back with elevated nitrate-N levels in the cover crop block. So it will all add up. I’ll work through the data: growth and yield targets, crop N requirement, soil N supply and C:N ratio of the residues; and then calculate the amount and type of fertiliser the maize will need – you’ll save some…, but it’s not just about saving money on fertiliser, it’s also about getting N into the crop when it uses it.”

George smiled a wry smile, “Your shout,” he said to Max, who grabbed a couple more beers.


Innovative farmers are starting to trial these ideas:

Sunflower and soybean growing in the inter-row of Simon Mattsson’s sugarcane crop at Mackay in Queensland. The intercrops will be felled using a crimp-roller before the sugarcane has achieved a full canopy covering the inter-row. The idea is that polyculture changes the way carbon and nitrogen is cycled in the soil, improving both soil health and the timing of nitrogen supply to the sugarcane crop. More efficient nitrogen use could result in better fertiliser strategies in the sugarcane crop (Photos courtesy of Simon Mattsson).


And how a scientist may explain it:

The primary purpose of agriculture is to produce plants, and soils should be managed with that outcome in mind. Nearly everywhere in Australia farmers need to grow soil carbon. They need to add plant residues and organic matter (OM) to the soil with the focus on developing the activity of soil microbes to turnover carbon and release nitrogen (N) to match the needs of crops. Only then can farmer effectively apply fertiliser N to feed the soil biology and crop, and grow yields.

The truth is that soil biology will adjust to the purpose and the way in which the soil is used. There is no point to infusing bacteria and fungi into a soil if their activity is not underpinned by a supply of carbon from OM, nor is there immediate value unless the amount and timing of released N matches the uptake pattern of the species growing in the soil.

In well-managed soils, the benefit of soil biology to farmers and land managers is that nutrient supply to crops becomes easier to work out. Two pools of nutrients become active: firstly and more importantly the soil OM pool can absorb then release N to the crop, and secondly the pool of nitrate-N in the soil solution can be topped-up with fertiliser to supply the crop and manage the rate of OM decomposition. When the nitrogen in the OM pool is managed well, the benefit to the environment is that fertiliser N surplus to what the crop needs is not applied, and that fertiliser is not causing pollution of the groundwater.

The agronomy of crop N supply is not rocket science, but generally in agriculture we drag the chain on this issue. Calculations of N fertiliser uptake by crops remain intractably just above the half-efficient mark. Is that figure real or does it reflect our inability to measure the dynamics of the organic N pool? Granted, measuring N immobilisation into OM in real-time is difficult and, before now, measuring the availability of N from OM to crops has been time-consuming and expensive. The technology from Solvita™ changes that circumstance. In the future we should expect that the amounts and timing of N released from OM will become numbers in the head of the farmer. Ask your agronomist for advice.

© Stephen Ockerby 2015

Grow the Crop to get more Food

Farmers are organized people and modern farms are precisely laid-out and strictly regimented. Crops stand to attention in straight rows each plant the same as the next. Farmers create this parade-ground by planning, analysis and execution. They have tractors that steer perfect rows, machines that sow seed precisely and yield monitors that apportion next year’s fertilizer even before this year’s grain hits the silo. Just like army generals, farmers and agronomists use sensors and data and graphs and computer models to analyse past crops and trial scenarios.

Scientists bolster this system of crop production. Empirical models now integrate crop performance against weather and resources over time and those models find minute inefficiencies in production and economics. The technology involved in farming is phenomenal and getting better, and farmers are adopting it and getting better at farming.

Plants also achieve perfection. They grow leaves and stems and flowers and seed with architectural symmetry, and they flower with imperfect synchrony to the seasons. How can that be? Plants have no way of planning; they have no memory nor can plants imagine the future. And despite widely-held beliefs to the contrary, plants do not house internal clocks, so they have no way of telling time. Plants only respond to the here-and-now, but despite that circumstance they stand to attention like soldiers, imperfectly the same as every other plant in the field.

There is a growing misalliance between farmers and their crops; they go about the same business but in completely different ways. In Australia where I do most of my work, I observe that this bifurcation often constrains the scope for agronomic improvements. One could easily fault the farmer, but it may be that the plants too have a narrowing focus, taking defensive traits against environments seen by their makers to be increasingly-degraded and hostile.

My opinion is that farmers and scientists need not focus so much on applying genes and water and fertilizer to gain a marginal increase in crop yield. Instead they should create in their minds a picture of what a productive crop looks like and work out what is required at all of its stages of growth to sustain its biological yield. An approach based on that rationale will surely reveal ways to enhance production from agricultural fields. Tend the soil but feed and water the plant.

To take the plant’s view of agriculture, agronomists will need to relearn the fine sciences of the 1980s and well-before, when botanists and physiologists dissected leaves and flowers and grains to study crop growth and yield. With those methods and today’s never-before-dreamed-about technologies, discoveries to reinvigorate the biology of farming systems will surely abound.

Agriculture in the future may be about the specialization of the gene pool, but it will almost certainly progress more soundly if it is grounded in the skills and prides of farmers and scientists to sustain the growth at each-and-every moment of each-and-every plant standing in the fields.

© Stephen Ockerby 2014

Yellow Canopy Syndrome in Sugarcane

Sugarcane farming systems on the east coast of Australia appear to be in serious decline. The recent heralding of Yellow Canopy Syndrome as a major issue for cane is worrying for the industry. As a crop scientist I concur, but whereas specialists in the industry seem to be proclaiming little insight as to its cause, to me the leaf symptoms resemble those of deficiencies in the macro nutrients: nitrogen, phosphorus, potassium or sulphur.

The recent widespread incidence of poor crop growth in cane is reasonable. Current sugarcane farming systems are essentially long-term monocultures and investigation would surely reveal declining soil quality, limited amounts of soil organic biomass and declining mineralisation and dissolution of the soil nutrients essential for crop growth. For decades sugarcane crops have been dependent on the application of industrial fertiliser to sustain crop growth, and the recent trend towards using lesser amounts of industrial fertiliser could explain the onset of symptoms of nutrient deficiency in cane leaves.

Achieving better crop nutrition yet using less fertiliser is a difficult proposition for farmers. It can be done by matching nutrient supply to the time that the crop is using nutrients for growth. It requires not just a basic measurement of what nutrients may be lacking in the soil as gained from the standard soil tests; but also precise knowledge about how those nutrients are held in the soil and the rate at which they are released into the fine root zone of the sugar cane plant. It is not fertiliser in the soil but rather nutrients in the plant that determines crop growth.

Changing the soil’s capacity to hold and then slowly release nutrients to the crop is perhaps the single most-effective crop management tool that is not currently practiced in the sugarcane industry. Farmers need data and advice to substantially increase the amount of labile (active) organic carbon in their soils so they can manage fertiliser application and sustain crop growth rates.

Organic carbon in sugarcane soils is generally less than 1% and mostly exists in forms that are resistant to decomposition by soil microbes. The amount of labile soil carbon which is active in the processes of microbial decomposition, immobilisation and mineralisation (nutrient storage and release) is much less and undoubtedly limiting to crop growth. Soil carbon levels are so poor that even in a system that retains all cane leaves as a green-trash blanket and grows legume crops such as soybean in the plough-out regaining enough carbon to restore soil fertility and sustain sugarcane growth may take decades.

My view is that farmers need to invest time, effort and money to increase the level of labile organic carbon in their soils. Then they can recalibrate their fertiliser management to sustain maximum yields. How they can do it requires new technology. For example farmers may undertake ley carbon farming on a portion of their land; or grow carbon crops and spread the biomass or manufactured, resilient, carbon products onto their sugarcane soils. Novel research is needed.

© Stephen Ockerby 2013

Progardes™ as a break crop in the sugarcane farming system

Progardes™ is a blend of different types of Desmanthus sp. a tropically-adapted legume that has been newly-released for over-sowing into the vast, grassy, clay-soil plains of northern Australia. Progardes™ are sub-shrubs with thin stems, fine leaves, woody base and deep tap-roots. As forage it has nutritive value similar to that of lucerne.

Seed for sowing the legume pasture is currently being grown in the Burdekin River Irrigation Area and at Mareeba. The seed crops are sown at the start of the wet-season and harvested in the period from late-April to July. Consequently, seed crops are a little too late-maturing to be grown during the plough-out period of the sugarcane crop. Instead, Progardes™ could be grown for tropical hay or more likely as a green-manure crop.

As a green manure, Progardes™ can accumulate 15 tonnes/hectare of above-ground dry biomass. About one-third of the biomass is carbon so 5 tonnes/hectare of carbon in the form of mostly woody organic matter can be incorporated into the soil. And about one third of that carbon can be converted to humus which results in an increase of about 0.15% soil carbon per crop over the long-term. The amount of carbon in stable organic matter generated by a Progardes™ break-crop is small but any incremental increase in soil organic carbon is an important change to the health of sugarcane farming systems.

The immediate benefit from incorporating organic matter into the soil comes not from increasing stable soil carbon, but from changing the amount of labile or active organic matter levels. The labile organic matter includes both the fresh incorporated residues and decomposing organic matter. Organic matter is important to crop productivity because it provides benefits to soil tilth but it also affects the soil’s water holding capacity and the soil’s capability to store and moderate the release of nutrients. Carbon in organic matter is important because carbon is the element that is consumed and re-consumed by microbes in their active processes of decomposition.

While organic carbon is consumed and transformed into stable humus, and lost from the soil as carbon dioxide, other elements: nitrogen, phosphorus, potassium and sulphur; are gradually released into the soil solution and become available to be taken up by plant roots.

The benefit to be gained from incorporating residues comes not only from the other nutrients that the residues contain, but from the rates of organic matter decomposition and nutrient release. Residues that are poor in nitrogen can also absorb nitrogen from the soil or fertiliser, and re-release that nitrogen later-on when they decompose.

The skill of farmers is to manage the organic matter levels in the soil, rates of nutrient release and fertiliser application to match the demand for nutrients to sustain crop growth. That’s not easy.

Progardes™ offers farmers a different break-crop to those such as soybean. It has more residues which are much woodier and has much greater capacity to store and slowly release nutrients.

Progardes is a relatively easy crop to grow; seed is sown with a small amount of starter fertiliser; weed control; supplementary irrigation and before seed-set the crop is foraged or mulched. With a compatible sowing configuration, sugarcane can be planted with minimal cultivation. Progardes™ that set seeds will persist in the field because it is hard-seeded, but it is a broad-leaf and a weak competitor, and the experience of growing cane after seed crops has presented no problems to the sugarcane.

Enquiries about Progardes™ can be directed to Agrimix Pty Ltd.

© Stephen Ockerby 2013