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Is composting un-organic?

July 07

This article was published in Organic NZ with a 'twin' article by Holger Kahl forming a debate on the pros and cons of compost. A JPEG version of the thesis of Timothy (Sol) Morgan who conducted the research on Bob Crowders apple orchard trial is avalible here (14 MB zip file).

Composting has been an integral part of organics since its inception and is generally considered a highly beneficial process that should be done as much possible.

However, there is increasing scientific and practical on farm experience that indicates that composting is not always as beneficial as its nickname of black gold would have us believe.  I am therefore making, what many would consider to be the heretical statement, that in a range of cases we should not be composting but rather applying fresh organic matter to soils.  The explanation of why is going to take us through some soil biology and the process of composting.

To have a healthy, biological active soil, requires healthy, active soil organisms which requires that they have good quality food.  Food has two principle components; nutrients and energy.  Life needs nutrients to make tissues, and energy to power the process of making tissues.  The problem with compost is that it’s the diet version of soil food.  It’s got nutrients, but it’s low on energy.  If our soils were overweight this would not be an issue, but I have yet to come across an obese soil, while I have seen way too many starving soils. We therefore, have a bit of a problem.

What’s the answer?  Simple, stop composting and put the raw compost materials directly onto the soil, just as nature does.  Next time you’re off for a walk in the woods, take a spade and dig a soil inspection pit (a hole).  You will see lots, and lots, of dead leaves and organic matter on the surface and then lots of dark crumbly soil underneath.  Same thing for grassland but less pronounced.  Try this on cultivated soil and there will be little or no organic matter on the surface, or anywhere for that matter.  When fresh organic matter is left on the soil surface, the soil gets not only the nutrients in the plant matter but the energy as well.

If you want to get a picture of how much energy is at stake here, think back to the bad old days when farmers could burn the straw off their fields.  These are so hot you can’t stand anywhere near them, and they often generate their own clouds, there is such an upwelling of air.  The composting process also demonstrates how much energy is at stake.  When aerobic composting is done well, the heap can reach and maintain temperatures of 60°C for two to three weeks. It takes a lot of energy to keep something that big that hot for that long.  All of that energy is stolen from the soil organisms when biological material is composted.  Less energy = less organisms = less healthy soil.  

So what can happen when you leave straw and crop debris on the soil surface?  The only mechanised farming system that has achieved this on a long term basis is no-till.  The results from hundreds of scientific experiments (Baker & Saxton, 2007) and even greater practical farmer experience is that soil health improves dramatically.  Under continuous no-till cropping (i.e., no pasture phase or livestock), soil organic matter levels build to the same levels as a long-term pasture.  This is unheard of in tillage farming where soil organic matter always decreases over time.  This is of course a result of both the retention of crop residues on the soil surface and stopping tillage, the latter of which has a huge impact.  But it clearly demonstrates that leaving residue on the surface is a viable alternative to incorporation.  In no-till there is also much greater soil biological activity, especially earthworms.  This is because earthworms need fresh, not composted, material to live on, and they like it left on the soil surface where they can gradually pull it down into the lower soil levels, mixing soil as they go and they don't like getting chopped up by cultivation.  Now, no-till has a number of problems including utter dependence on a very small number of systemic herbicides, so I am not advocating that we drop organics and go no-till, however, no-till beats organic tillage hands down when it comes to a healthy soil.  As creating a wonderfully healthy soil, full of earthworms, is a fundamental aim of organics, we need to sit up and take notice of the soil health that no-till is achieving.

Bob Crowder, the doyenne of New Zealand organics, also showed that fresh organic matter was essential for high earthworm populations.  He conducted a long-term organic experiment in an apple orchard comparing compost with fresh cut green grass as fertiliser, they had the same nutrient content and he found vast numbers of worms under the fresh grass treatments and very few under the compost.  This clearly shows that fresh organic matter promoted high earthworm populations compared with the compost treatment while having the same yields.  Now, had that grass been composted it would of lost a load of energy, nitrogen, carbon and other nutrients which would of resulted in lower yields and lower earthworms all for the time, expense and energy consumed making the compost. 

But wait, it gets worse.  Not only are we robbing the soil of massive amounts of energy when we compost, we are also stealing its nutrients.  Making compost reduces the volume and weight of the material composted, even taking into account the varying moisture content.  How is this?  All that heat is caused by the compost organisms respiring (breathing) like crazy.  A key feature of respiration is that carbon is combined with oxygen to produce carbon dioxide.  So the ‘solid’ carbon in the compost heap literally floats away as gas.  Why is that a problem?  Well carbon is a key ingredient of soil organic matter, so we are losing a great big wad of super valuable soil organic matter, literally as hot air.  As a principle aim of effective nutrient management in organics is efficient recycling of nutrients on the farm this is not good news.  Now it’s true that the same will eventually happen in the soil and the organisms there respire the carbon to gas, however, its much colder and therefore slower, so there is potential for the carbon to be retained in the soil over longer periods.

But wait, it gets worse still.  Nitrogen is also lost during the composting process, especially if the carbon:nitrogen ratio is below 20:1 as there is too much nitrogen to be ‘mopped up’ by the carbon so it floats away as ammonia.  One study found nitrogen losses of around 50% when composting material with a ratio of 20 to 25 which is pretty common.  Even for higher ratios there is still an appreciable loss of nitrogen just due to the heat of the pile vaporising volatile ammonia as it is produced by the decomposition process.  Losing some carbon could be considered careless but losing significant amounts of hard won nitrogen in an organic system is inexcusable. 

On top of all of this is the extra hassle of making compost.  You have to turn it, cover it, make sure no leachate gets into the soil or water, and store it.  At home, this is not much of a problem, but on farms it means higher labour, building and machinery, costs most of which also consume fossil fuel.  So composting, makes for a less healthy soil, increases the losses of carbon and nitrogen from the farm nutrient cycle, increases production costs and burns fossil fuel.  How many strikes before you’re out?

Now, to clear up potential misconceptions, I’m not saying that no compost is better than compost.  Adding compost to soil is always better than not adding compost, but it would be far better added as fresh rather than composted material.  There are also a lot of times where composting is required, for example, if there are lots of weed seeds present, or food scraps, especially animal products which would give rise to a health and safety issue (although it would still be better for the soil).  Sometimes it’s just not practical to put material straight on the fields, e.g., it’s too wet.  But I strongly argue that for the rest of the time composting is a bad idea and the organic matter should be put on fresh.  The worst scenario is the gathering of post harvest crop residue from the field surface, composting it and then returning it to the field.  This practice is so bad for the soil and wider environment it should be explicitly banned from organic systems. 

So next time you’re clearing the FYM from the sheds in spring, don't spend all summer composting it, stick it straight out on the fields to give them some food just when they need it.  If you mowing the lawns at home, don't put the clippings on the compost. Either leave them on the lawn to feed it, or sprinkle them on your vegetable plot to feed it instead.  That way you will have more soil organisms, greater recycling of soil nutrients and better soil structure all for less work.  As Bob Crowder so rightly pointed out, its time to kill the sacred organic cow of composting. 

Baker, C. J. & Saxton, K. E. (Eds.). (2007). No-tillage Seeding in Conservation Agriculture, 2nd Edition. Wallingford, UK: Food and Agriculture Organization of the United Nations.

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Copyright 2008 Charles N. Merfield.