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Has organics over-hyped the benefits of compost? |
This is the original version of a comment article published in Organic Farming Magazine (see bottom of page for a scan of the published article) under the title of "Don't believe the hype"
Composting has been an integral part of organics since its inception and is considered a highly beneficial process that should be done as much possible. However, there is increasing scientific and practical farm experience that indicates that composting is not always as beneficial as its nickname of black gold indicates. I am therefore making, what many would consider to be the heretical statement, that in most 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.
Composting is an aerobic process where microbes (bacteria, fungi, etc.) break down fresh organic matter to produce a stable product that is mostly humus (biological material that is resistant to further decay). To put it scientifically; organic matter + oxygen (O2) + aerobic bacteria → carbon dioxide (CO2) + ammonia (NH3) + humus + energy [2]. The key issue is a considerable amount of the carbon (C), nitrogen (N) and energy in the starting material is lost as the gasses carbon dioxide and ammonia and as heat. Composting literally evaporates valuable nutrients and energy into the atmosphere. The ammonia also gets washed out of the atmosphere in rain which can lead to eutrophication of non agricultural soils and water.
While compost is undoubtedly good for soil, undecomposed crop residues on the soil surface are far better for soil health. Composting decomposes the starting material into hard to break down humus, therefore compost releases nutrients so slowly it is a poor food source for the soil flora and fauna. Fresh undecomposed crop residues break down much more quickly so it is a good food source for soil microbes so the soil ecosystem becomes more biodiverse with much larger populations. Large biodiverse soil ecosystems = healthy soil. Exactly the same goes for energy, instead of it being lost as heat from the compost pile it now passes though the soil food web building it up far more than compost ever can. In addition, the C and N that were lost during the composting process now also pass through the soil ecosystem which retains more of them than the composting process, i.e., crop residues have a higher fertiliser value. Importantly when ammonia is produced by decomposition in soil it’s almost immediately converted into ammonium then nitrites and nitrates which are trapped in the soil water solution and therefore not lost to the atmosphere. The end point of plant residues being decomposed by the soil is humus, just the same as composting, so the soil gains all the benefits of elevated humus levels too.
Undecomposed crop residues are the main food source for earthworms, compost is of very limited feed value to them. Earthworms are the keystone of healthy soils. A New Zealand study comparing home-made compost, a commercial bark, manure and paunch compost with freshly cut pasture from a mixed herb lay found the pasture treatment had much higher earthworm populations, higher microbial biomass, higher mineral nitrogen and mineralisable NH4+-N, which were correlated with the higher worm and microbial biomass and better water infiltration [3]. Unfortunately there are very few studies comparing the effect of composting vs. organic residues and to be of any value they have to be long term, e.g., minimum of six years, ten or more is best. However, indisputable proof comes from no-till farming. The key features of which are absolutely no soil disturbance of any kind with the exception of drilling which aims to cause the minimum level of soil and residue disturbance. The second feature is maintenance of high levels of surface residue. The result of this is astonishing, soil organic matter increases, even under continual cropping, earthworm populations and beneficial soil organisms are vastly increased [1].
The message is clear. Where possible we should be avoiding composting and applying manures in a fresh state, using technology like the trailing shoe to minimise N loss, and developing farming systems that maintain crop residues on the soil surface. This is the next step in maximising soil health.
1. Baker, C. J. and Saxton, K. E., eds. No-tillage Seeding in Conservation Agriculture, 2nd Edition. 2007, Food and Agriculture Organization of the United Nations: Wallingford, UK.
2. Day, M. and Shaw, K., Biological, chemical and physical processes of composting, in Compost Utilisation in Horticultural Cropping Systems, Stoffella, P.J. and Kahn, B.A., Editors. 2001, Lewis Publishers: New York.
3. Morgan, T., The effect of organic mulches on soil bio-physical and chemical properties under an apple orchard. 1992, Lincoln University: Christchurch. www.merfield.com/sol-morgan/
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