So why all this interest in soil carbon?

World soils constitute the third largest global C pool, comprising of two distinct components: (i) soil organic C (SOC) estimated at 1550 Pg, and (ii) soil inorganic C (SIC) pool estimated at 950 Pg, both to 1-m depth. Other pools include the oceanic (38,400 Pg), geologic/fossil fuel (4500 Pg), biotic (620 Pg), and atmospheric (750 Pg) (Lal, 2004a). Thus, the soil C pool of 2500 Pg is 3.3 times the atmospheric pool and 4.0 times the biotic pool. However, soils of the managed ecosystems have lost 50 to 75% of the original SOC pool. Conversion of natural to managed ecosystems depletes SOC pool because C input into the agricultural ecosystems is lower, and losses due to erosion, mineralization and leaching are higher than those in the natural ecosystems. The magnitude of SOC depletion is high in soils prone to erosion and those managed by low-input or extractive farming practices. The loss of SOC pool is also high in soils of coarse texture and those with a high initial pool. Most agricultural soils have lost 20 to 40 Mg C/ha due to historic land use and management.

The maximum soil C sink capacity, amount of C that can be stored in it, approximately equals the historic C loss. In other words, most agricultural soils now contain lower SOC pool than their capacity because of the historic loss.

N.B. 1 Pg or pentagram = 1 Gt = 1 gigaton = 1 billion metric tons.

Emphasis added. We worry about carbon in the air due to theories of a greenhouse effect. Whether you find those theories compelling or not the loss of soil carbon to the air is a concern since it affects our ability to produce food, fiber and fuel from agriculture. This isn't news to practitioners and many of their management methods address the issue, but it may be news to the public and policy makers who fail to grasp the consequences of removing carbon from fields.

This is an old concern for those of us in the grazing business. One of the stories we tell one another is about the difference between farms in the Ohio valley resulting from their management. Up until the mid twentieth century many of the farms grew hay for export to the populous cities of the east coast such as New York where transportation and cartage was still mainly done with draft animals. They harvested their fields and shipped the hay down the rivers, canals and lakes on barges.

While New York was awash in animal dung - a problem that created a furor much like the one we now have about climate - the fields where the hay came from slowly subsided. If you stood at the fence line separating a hay field from a permanent pasture such as dairies kept, you looked down from the pasture to the hay field a foot below. Bit by bit, year after year, the hay farm had in essence sent its soil to New York.

Land use and soil management techniques which lead to C sequestration are retention of crop residues, NT [no-till] farming and incorporation of cover crops in a diversified rotation cycle (together also referred to as CA [conservation agriculture]), INM [integrated nutrient management] techniques of using compost and other biosolids, erosion control, water conservation, contour hedges with perennials, controlled grazing, etc. An average long term rate of SOC sequestration with these techniques is 200 to 1000 kg/ha/yr for humid temperate regions and 50 to 250 kg/ha/yr for dry tropical regions. In addition, the rate of SIC sequestration as secondary carbonates is about 5 to 25 kg/ha/yr in arid and semi-arid regions. In contrast, depletion of SOC pool occurs with the use of excessive plowing, residue removal and biomass burning, and extractive farming practices where nutrient balance is often negative.

The lesson for growers is that managing soil carbon has increasing returns while failing to do so has decreasing returns. This is no secret, no mystery, it's basic farming knowledge. It's just one of the considerations in making and harvesting a crop rather than a consuming obsession as it is for climate worriers and some fringe growers, which is how we get to the current state of soil carbon depletion. The carbon was in effect traded off - like the hay sold down river - to make ends meet and make it to the next year and begin all over again, perhaps vowing to do better next time.

It isn't just carbon that is lost when materials are removed from fields.

Positive impacts of crop residue retention on soil quality are partly due to nutrients recycled into the soil. On average, crop residues contain ~ 0.8 % N, 0.1 % P and 1.3% K (Table 4). Therefore, amount of NPK contained in crop residues produced is about 11 x 106 Mg in USA and 81 x 106 Mg in the world. (Table 5). Consequently, the long-term impacts of residues retention on soil quality are both due to elemental cycling and to providing food (energy source) and habitat for soil biota, especially micro-organisms and earthworms.

Residues here means things like corn stover and wheat straw, the non-crop portion of what is grown. It's common to hear such materials called "wastes", which leads uninformed policy makers to think of residues as junk free for the taking, as for making biofuels. However, taking them depletes soil of its carbon as well as embodied nutrients, not just NPK but also minerals of agronomic value. But it's not enough.

Crop residues are also a principal source of C, which constitutes about 40% of the total biomass on dry weight basis. Therefore, total amount of C assimilated in crop residues is about 0.2 Pg/yr in USA and 1.6 Pg/yr in the world. Increase in rate of application of biomass C increases the SOC pool (Fig. 3). The magnitude of increase in SOC pool, however, depends on other management input used in conjunction with crop residues mulch (e.g., depicted as Management 1,2 and 3 in Fig. 3). Components of the management packages may differ with increase in the rate of input of biomass C into the system. Because C is only one of the building blocks of stable humus and humic substances (which are enriched with N,P,S and other elements compared with crop residues), application of N and other elements can enhance the humification. Jacinthe et al. (2002) observed that fertilization of wheat residues with N increased humification of biomass and enhanced the C sequestration rate of the soil in central Ohio, USA.

It takes money to make money. It takes nutrients to make stable soil carbon which is in some ways like money in the bank. You have to pay the soil microorganisms who do the conversion work. There's no free lunch. The dynamic of this system is that retaining crop residues can seem like it depletes soil fertility rather than increasing it, though this is actually a time shift issue since the nutrients locked in the residues will eventually be released. Timing matters to a grower since nutrients that aren't available when a crop is growing are of only theoretical value.

There are about 850 million food-insecure people in the world (Borlaug, 2007), and an additional 3.7 billion are prone to hidden hunger and malnutrition due to deficiency of minerals (e.g., Zn, Cu, Se, I, B) and vitamins because food is grown on degraded soils. Most of the food insecure people live in Sub-Saharan Africa (SSA) and Asia. The Green Revolution of the 1960s and 1970s that saved hundreds of millions from starvation in Asia and elsewhere by-passed SSA because soils of the region are severely degraded (Lal, 2008c). Resource-poor farmers and small land holders of SSA and Asia remove crop residues for use as fodder, fuel, construction material and other competing uses. Furthermore, most soils have extremely low levels of SOC concentration (< 0.5% in contrast to critical level of 1.1%). In addition, the negative nutrient budget of – 30 to – 40 kg/ha/yr of NPK on continental scale is exacerbated by residue removal and the attendant increase in soil erosion hazard. Adverse agronomic and environmental impacts are confounded by the use of animal dung as household fuel rather than as soil amendment. The data in Table 7 from a long-term experiment conducted on Alfisols in Western Nigeria, show that even the use of NT farming on a relatively flat terrain (< 1% slope gradient) caused severe decline in soil quality because of the residue removal. Despite the application of chemical fertilizers at recommended rates and use of improved crop varieties, soil quality deteriorated (e.g., decline in SOC concentration, soil pH, exchangeable cation, CEC). There was also a strong increase in bulk density and decrease in infiltration rate and AWC (Lal, unpublished data). Consequently, maize yields in the first growing season, after 13 consecutive years of growing 2 crops per year, was 2.7 Mg/ha with residue retention compared with 1.5 Mg/ha with residue removal (Table 7; Juo et al., 1995; 1996). It is apparent therefore, that the agronomic benefits of improved crop varieties and fertilizer input cannot be realized unless soil structure and hydrologic properties are improved with residue retention and application of other biosolids as amendments.

When you put all of this together you can propose an improved agronomic system that not only sequesters carbon but also produces more and better food for the large and still growing population of food insecure and nutritionally compromised people. It's not some airy-fairy organic subsistence farming wheeze - that's what they are doing now - it's a science based system informed by knowledge of soil. You need to retain crop residues for several reasons rather than using them for energy or building materials, and you need to amend the soil with imported nutrients. The soil is sick, it needs medicine. It's not going to get better on its own.

Ideologues laboring under some pseudo-naturalist theory often argue that the needed nutrients can be imported as waste organic matter from elsewhere. I've even heard wackos argue that the already over worked and under nourished subsistence farmers should go into the jungle and gather leaves to carry back to their fields, and that this would amend the depleted soils. It is true that you can haul in organic matter from another place to enrich exhausted soils, but that just exhausts another place. To truly fix the problem you must grow your own on site, you must increase the amount of organic matter produced not just shift it around from one spot to another.

That doesn't mean that there is no role for imported "biosolids". The ideal system would collect the residues of the crops taken from the fields - even human dung, urine and deceased carcasses - and return it to where it came from. But there are complications in this, not least for control of disease and toxic contaminants. Only a fraction of what is taken will ever in practice be returned despite the best of intentions.

It won't happen. We may know how to fix the problem in the abstract, but putting it into practice is impractical. It would require changing the way people live and massive subsidies. Who would pay them? Why? We can natter about taxing the rich to pay the poor to manage their fields well - at long last - and justify it with some complicated narrative about carbon sequestration and climate change, but if that is truly the motivation there are far better and cheaper ways to do it. The economic problem is the same as the agronomic problem. Redistribution - impoverishing one place to enrich another - doesn't actually help. Just as farmers must grow their own organic matter to truly amend their fields, they must also grow their own wealth to truly relieve their poverty.

It's easier said than done. The first need is knowledge - real knowledge not the deceits of activists, advocates, NGOs and other external parasites. Applying that knowledge requires billions of acts of individual genius by motivated thinkers with skin in the game, and time. It won't be quick and it won't be pretty but it is what it will take to do something useful. It may be that the whole subsistence small holder system is unworkable given the current conditions of poor soil and poverty. They have an extractive economic model not a productive one. They are closer to being hunter gatherers that merely consume rather than producing anything, and their range is not sufficiently rich to allow progress.

Prediction and prescription is a fool's game best left to professionals. However, the existing model that seems most likely to succeed in making productive use of these exhausted soils is one of long term investment in expectation of future rewards commensurate with the risks and delays - venture capitalism. It might well be state capitalists - such as the Chinese - who are best positioned to invest and hold the mark long enough to reap rewards. And, given the governance problems in the area, it might take a state with significant resources to keep the local kleptocratic governments from simply confiscating any improved systems. This is one of the impediments to improvement for local individuals now, and if the kleptocrats will steal pennies from the eyes of dead men they wouldn't pass up richer opportunities unless faced with retaliation of some sort.

It may be that the best that can be done is a mixed system of large and small producers. Many very small subsistence growers who had been mining their soil and growing ever more unproductive will be displaced. It seems it would be better to anticipate a rural exodus - most likely to fringe cities - and look to provide services there than to continue with a fantasy idea of "sustainable" peasant agriculture subsidized by a world tax on carbon. Even if this worked, which is extremely unlikely, it wouldn't work for long and the problems would be worse when that rickety support collapsed. It's an irresponsible policy proposal, though that seems to be a fashionable behavior.