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Soil Organic Matter
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  1. Default Soil Organic Matter

    A useful post from the defunct Rural Network forum, by Dr Doug Edmeades.

    Soil organic matter – what is it and where does it come from?

    The importance of soil organic matter, or to use the colloquial term, humus, has been known for centuries – certainly long before the Organic Movement. Most text books will list the benefits of soil organic matter (relative to soil with little or none of it) as improved soil structure, water holding capacity, storage of nutrients and better heat absorption. Soil organic matter is also the home and food for the myriad of soil micro and macro organisms from earthworms and insects to bacteria, fungi and the tiniest protozoa.

    We are blessed in New Zealand because our well developed pastoral soils, taken in the international context, contain large amounts of organic matter. This is a consequence of our temperate climate, and our clover-based, grazed pastoral system.

    Soil organic matter (SOM) - humus - comprises the breakdown products of plant and animal (dung) material returned to the soil. The fresh plant material and dung returned to the soil is food (energy) for soil bugs which get to work in a sort of chain gang and break this material into increasingly smaller and more stable units, which are then often joined together (polymerised) into stable, large, complex organic substances. Humus is dark coloured and as a general rule the darker the colour and the deeper it extends into the topsoil the better the soil. It is this colour that enhances heat absorption.

    In our grazed, clover-based pasture, carbon (the major component of soil organic matter) comes into the system from the atmosphere (as carbon dioxide) via the plant (photosynthesis), some of which goes into the soil as plant residues (from tops and roots) and some via the dung. The amount of carbon added from these sources is of the order of 1-3 tonnes per hectare per year annually. Losses occur from the animal because it breaths out carbon dioxide (respiration) and belches methane, and from the soil via the oxidation of organic matter.

    The key point is this: if the sum of the inputs is greater than the sum of the outputs then carbon and hence organic matter is accumulating in the soil.

    So, how should we manage our soils to ensure that this happens or that at least we are not depleting soil organic matter levels and hence jeopardizing soil quality and contributing to green-house gas emissions?

    Soil organic matter – how should we manage it?

    Several studies in the 1950s and ’60s showed that soil organic matter accumulated (inputs are greater than outputs) following pasture improvement (i.e. clover + fertiliser + animal). This accumulation continues for about 20-50 years and then reaches a steady state (inputs = outputs). The time required to reach this steady state, and the amount of soil organic matter present at steady state depended on the climate and the soil group. Generally, the wetter and warmer the environment the more soil organic matter.

    Thus the management recipe was simple: clover (to add nitrogen), fertiliser (especially phosphorus, potassium and sulphur - PKS - to maximise clover growth), and the animal (to do the recycling) plus time equals more soil organic matter.

    The situation under cropping is very different. Cropping exploits soil organic matter (outputs are greater than inputs) and this is especially so when the crop residues are removed. So the second management lesson in terms of soil organic matter management is: do not crop if you can help it! Or if you need to crop make sure there is a good rotation from clover-based, grazed pasture to crop and then back again. Green manuring or heaps of compost are also helpful. Civilisations have failed by not following this simple rule.

    A number of more recent studies suggest we need to modify slightly our understanding of soil organic matter accumulation.

    Tate (1997) compared the soil organic matter contents of 43 topsoils sampled first in the 1960s and again in 1992. He concluded that there was no change over this period. This is consistent with the idea that the soils were at a steady state with respect to soil organic matter accumulation, as discussed above. In contrast, Schipper et al (2007) reported an average decline (about 1% or 1 tonne SOM/ha) in 37 sites over a period of about 20 years.

    How do we reconcile these studies – one suggesting no change the other indicating a small decline?

    Other researchers have dug deeper into this apparent paradox and have reported results which indicate that soil organic matter can be reduced by:

    Land-use intensification – management practices which increase pasture utilisation (e.g. better grazing management, increasing stocking rate, introducing irrigation) and hence reduce the proportion of plant material (litter) being returned to the soil and thus result in the steady state soil organic matter being reduced.

    Changes in the quality of the litter returned to the soil – it is suggested that some of our newer management practices (and this includes all those listed above plus the introduction of new pasture cultivars and the introduction of fertiliser N) result in a change in the chemical composition of the litter returning to the soil allowing it to be more readily broken down in the soil and hence less is conserved in the soil organic matter pool.

    Two points must be emphasised: First, even if the figures reported by Schipper are true, (i.e. a decrease in SOM at the rate of 1% per annum) there is no need for panic or alarm. As stated earlier, our developed pastoral soils already contain large amounts of soil organic matter.

    Pastoral agriculture in New Zealand is not on the verge of collapse. I stress this point because of our propensity in this PC and environmentally sensitive age, to seize on and highlight the negatives, especially on environmental issues. Second, in the scientific sense, the possibility that modern management practices are depleting soil organic matter levels is somewhat speculative. It is an emerging issue and more science is most definitely required.

    The above is a summary of a talk I presented to a special meeting of the NZIPIM. It is also the origin of a formal paper presented to the FLRC Conference (see Metherell, Edmeades and Ghani 2008, Massey University, Fertiliser and Lime Research Centre Workshop, February 2008) and an adaptation was published in the Fertiliser Review No 20.

    Dr Doug Edmeades is a soil scientist of independent fertiliser consultancy AgKnowledge www.agknowledge.co.nz/

  2. Default

    Doug, I take your point that more research is needed in this area. Is it possible that Tate(1997) saw no change in soil carbon levels yet they might have improved and declined again during that 30 years?

    There is a great variation in the depth and quality of topsoils seen around the Waikato. Are all of these soils at their steady (best) state?

    It would be very disappointing to give up on the idea of continually improving the soil and profitability on the land you own. And what a great story if we can show that net carbon is being continually sequestered in that very large mass that is NZ topsoils. Most of NZ is covered in grass, and the average energy conversion efficiency to grass sugars in NZ from the sun is just 2%. It’s nearly as low through ruminants, resulting in an average energy conversion from the sun’s total blackbody radiation/Ha to say milksolids energy from each hectare per year, of 0.06%.

    Is that the best we can do? On the average pugged or damaged pasture there might only be 50% green showing to the sun, the rest is dirt.

    Recent concentration on kgDM/Ha/yr could be a recipe for growing bulk poor quality grass at the expense of the soil, the environment and the bottom line.

  3. Default

    Dr Edmeades replied (excerpts):

    I think it is unlikely that soil carbon levels (as in the Tate study) would fluctuate in that manner. They (soil carbon levels) are normally quite stable and do not change either up or down very quickly.

    Are all Waikato soils at steady state? I think this is more likely than not but the only way to be certain in a given farm situation is to measure soil carbon levels over time and this would need to be done for long periods of time (decades) to pick up any trend. Also it would need to be measured down the soil profile.

    Yes, there has been until recently a focus on DM/ha for the simple and sufficient reason that our past science was saying more DM in more milk or produce out. This has now become more refined and now there is emphasis on the components of DM (fibre, carbohydrates etc). This is not a change in science direction - it is simply science becoming more specific and refinined.

    It would be nice to think, as you suggest, that soil carbon would continue to accumulate over time and hence become a never-ending sink for carbon - but sorry -I did not design the system. In any case do we all want to farm peat soils?

    Doug Edmeades | May 21, 2008 |
    Last edited by Graham; 13th August 2009 at 12:53 AM.

  4. Default

    Let's say we wanted to offset the entire country's greenhouse emissions by storing carbon in pastoral soils.. how practical would that be?

    NZ has about 39% of its land in pasture, or 10,538,500 Ha of grassland, most of it higher-producing exotic grasses. Our total emissions (agriculture, transport etc) are 75,500Gg of CO2-e per year, or 75,500,000 tonnes of CO2 equivalent.

    So we'd need to store 7.2 tonne of CO2-e per hectare, per year. That's 720grams per square meter. Grasses grow about 20 tonne of drymatter per hectare per year at best in NZ, which is 2000 grams DM per square meter. (The grass raw energy conversion efficiency is quite low at about 2%.)

    Since profitable pastoral farming relies on stock eating most of the grass produced, the sequestered amount of carbon needed, looks a bit too high. But ruminants only use up about 10-65% of the organic matter they digest, and excrete the rest.

    Of course, the soil organic matter is continually being consumed by soil organisms, and grass is one of the fastest materials to be used up in this way. Wood and bark takes a lot longer. And I'm not sure what mass of soil organic carbon is equivalent to a gram of CO2-e.

    However, everyone is in agreement: more soil organic matter (SOM) is a good thing in general. It holds more moisture in the soil, and retains minerals and elements ready for plant use, by encouraging biological life in the soil. SOM also brings with it, many of the nutrients needed by plants. More SOM allows soil to breathe, and generally attain a crumbly texture.

    The pastoral system relies mainly on leaf litter and dung/urine to be returned to the soil. Using fertilisers increases grass growth (NPK plus trace elements) and this helps the cycle. Using legumes to fix nitrogen is a preferred option.

    Many of the exotic grasses used here in NZ, have shallow roots in normal situations, and growth tends to drop off markedly in drought conditions. As a lot of soil carbon is built up from decomposing and shedded plant roots, it would make sense for these roots to be as bulky as possible for most of the year.

    While not a common practice here, yeoman ploughs will cut vertical slots in the soil and trim the lower part of grass roots off horizontally. This aerates the soil and breaks up any hardpan structure, doesn't lose a lot of SOM, and accelerates the production of more SOM from the dead roots. Presumably this work is done when moisture levels are good, and the grasses soon grow back down into the soil. Some startling reports (not peer reviewed) about increases of several inches of topsoil/SOM in just a few years are made.

    Since it is known that pasture soils can take many decades to reach equilibrium in terms of SOM, it just takes some new farming practices to perhaps set up a new higher equilibrium, and our farmers could then claim carbon credits for farming in this way. All the while their productivity should be increasing as well.

  5. Default

    Local farm consultancy business, eCogent in Cambridge, has been a champion in using Brix pasture and forage measurements as a farming benchmark. Headed by Peter Floyd, over 200 farms are using their system, which uses several "truths" including Brix measurements. Their approach so far has been to compare two similar forages, and feed the animals the highest Brix on a given day. Rather than quote lab results, their emphasis is on rising farm profits as the ultimate result.

    Biological farming techniques are high on their list too. Today, the Waikato Times reported that eCogent customers who "benchmarked soil carbon levels last spring, using protocols designed by Landcare Research scientist Graham Shepherd, had [soil carbon] increases from 5% to 10%, representing up to 10 tonnes of carbon per hectare".
    Peter is quoted as saying "The highest figures are extraordinary".

    Yes Peter, they are! This is right in the required range I calculated above. Based on this work, it should be perfectly feasible for NZ's pastoral farmers to completely cancel out all of the entire country's emissions! And make a lot more profit in the process. These findings deserve immediate testing and confirmation by the scientific community.

    I'm sure Peter would be happy to see them peer-review it to death..we should be so lucky. This basic work should have been done a long time ago, but I think it has been bypassed in favour of chemical fertilisers, treatments, special grass breeds, etc etc. I don't think any pastoral scientists even know how to use a Brix meter. My contention is that a Brix meter gives a very good indication of soil biological activity when used carefully, and on a trending basis. A lot easier than counting worms etc.

    Of course, setting soils up with lime and other additives is the important starting point, and this does take 2-3 years or more.

  6. Default

    I think Peter is way ahead of us on this point - he has already linked up with his old university, Massey, and his results are probably a big part of the reason Massey is heading down the biological farming path, recently announced.

    What will be your legacy? – Peter Floyd - Rural News 28 September 09

    It has been an amazing couple of weeks. Two very special events for me, and in a strange way they are linked.

    The birth of our first grandchild, Baxter, was indescribably wonderful, and I would never have believed what a difference such an occasion could make to our lives. Those of you who are grandparents will understand, I am sure, how overwhelmed Gillian and I have really been by the experience. Yes, mother and son are both well and facing the big wide world with plenty of vigour and determination.

    The second significant event was the partnering up of eCOGENT Farm Business Systems with Massey University's commercial arm, the Auckland based e-centre. This is an important step which will help us grow the business and make more New Zealand pastoral farms increasingly profitable and more environmentally sustainable.

    The partnership comes on the back of just-completed Massey University research showing that the majority of our members are seeing a clear improvement in farm profitability. Commissioned by Investment New Zealand, the independent survey also shows eCOGENT farms have reduced their environmental impact through less intensive stocking and through a reduction of over 60% in their use of nitrogen fertilisers. This was achieved against a backdrop of improvements to soil, pasture and animal health.

    By coincidence it is almost exactly 50 years to the week since I completed my studies at what was then Massey College. When I think back to those early learning days at Massey and reflect on the soil and pasture management lectures we were given, I can see how far current farm practice has moved away from those basic principles in the quest for more production.

    My memory of farming back then is one of abundant supplies of multi-species pasture, normally a night paddock and two day paddocks and the odd stack of hay. They were days of healthy animals, healthy and happy, low stressed people who had bank balances usually in the black.

    I think of all of the fashions that have been introduced since that time under the guise of improved management for more production, and the confusion and frustration they have caused and the incredibly high farming costs that farmers have had to bear as a result.

    And what is the legacy of this never-ending quest for greater and greater production? Depleted soils, monoculture pastures, sick stock, high animal health bills, high debt, high blood pressure, dirty streams, contaminated groundwater, loss of microbiology, increased emissions of nitrogen gases, and a reputation for farming as a “cost” to the country because of environmental damage.

    The lesson I have learnt over five decades since those Massey College days is that it just doesn’t have to be that way. We now have a structured, sustainable approach to farm business that hauls eCOGENT farmers back from the brink, removes much of the guesswork and risk from management decisions, and allows them to take control of their environmental and financial performance.

    A critical tool for doing this is financial software that forecasts the daily profit from the dry matter consumed by each class of livestock. This results in more objective and more profitable stocking and farm management decisions. This approach plus the link with Massey University e-centre gives me new hope that pastoral farmers will be able to achieve sustained environmental improvement, sustainable profits and a valuable asset to pass on to their children’s children.

    These goals have always been important to me but the recent new arrival has given them new significance. Both the eCOGENT Process and a better environment are part of the legacy I want to be able to pass on to the precious newest member of my family.

    Peter Floyd is the Managing Director of eCOGENT
    www.eCOGENT.biz ph 0800 433 276

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