Composting to achieve Soil Humus
One of the main reasons we compost plant material is to produce humus which is then added to the garden or farm soil.
Humus is an all-important component of soil, it encourages earthworms nature’s cultivators, arbuscular mycorrhiza soil fungi, supplies nutrients (Solaiman, et al., 2019) and builds soil structure which prevents crusting and caking on the soil surface. https://milespeter061.medium.com/soil-structure-1fa16f0d3e41
Humus is organic matter which has decomposed and is now composed of organic polymers, these are resistant to further bacterial decomposition but which may continue to decompose at a much slower rate. Humus can persist in the soil for 10 years or even 100 years if bonded to soil particles which helps to protect the humus.
Generally, humus is a dark brown to black colour and of a reasonably fine texture.
There is no distinct change between the states of compost and humus, there will most likely be small particles of not fully decomposed organic matter mixed in with the humus.
Before 1936 humus was defined chemically as comprised of three substances, fulvic acid, humic acid, and humin (Waksman, 1925).
Selman Waksman, a professor of biochemistry and microbiology at Rutgers University, USA (Britannica, 2020), published again in 1936 a more complete definition of humus “Chemically, humus consists of certain constituents of the original plant material resistant to further decomposition; of substances undergoing decomposition; of complexes resulting from decomposition, either by processes of hydrolysis or by oxidation and reduction; and of various compounds synthesized by microorganisms” (Waksman, 1936; Baveye & Wander, 2019).
Hydrolysis — addition of water to break a bond ie polymer to a monomer; oxidation — removal of electrons and addition of oxygen; reduction — addition of electrons and removing oxygen, reduces the charge. However, these last two occur together, reduction-oxidation = redox and no change in charge (Knox, et al., 2014).
The Utah State University Extension Service also describes humus as decomposed organic matter resisting further decomposition, can be up to 1,000 years old and very stable. Humus particles, like clay particles, are negatively charged, but with 30 to 40 times more ability to bond with positively charged nutrients (eg nitrogen, potassium, calcium, magnesium, some trace elements) than clay particles. This prevents positively charged nutrients from leaching out of soil and also improves soil pH stability. The addition of humus also improves the condition, including drainage and aeration, of either sand or clay soils (Pace, Miller & Farrell-Poe, 1995).
A summary definition of humus is it is an amorphous substance formed by chemical decompositions and most importantly formed by microorganisms (Baveye & Wander, 2019).
The capacity of soil to bond with nutrients is called the cation exchange capacity, see https://milespeter061.medium.com/soil-nutrients-1484bf51c1f1
Composting is the process of rotting down or decomposing plant material such as weeds, kitchen vegetable scraps (better when cut up), lawn clippings, sawdust, leaves, sawdust, a great range of plant material.
Woody pruning’s are better off burnt or if possible take them to a municipal facility that grinds up branches into mulch and then you can bring back a load of mulch for the garden.
An often-used method to create compost is to have 3 adjoining sites (Thomson, 2018) about 1 metre square each, these can be divided with wooden sides or brick or even hay bales (when the hay bales start to break down, they can be included in the next compost heap).
In the first square or bay spread an approximately 150mm thick layer of organic matter, if using lawn clippings make sure they are mixed with leaf litter, then a 50mm layer of animal manure.
Cow and sheep manures are good, poultry manure has more concentrated nutrients so add less, horse manure contains many intact grass seeds which if not killed by a hot compost can be a later weed problem in the garden, alpaca manure has quite a strong odour as they urinate in the same place, pigeon manure provides a wide range of nutrients as pigeon eat seeds from such a wide area but only add a little to your compost because it is also concentrated.
Use a layer of blood and bone fertilizer if animal manure isn’t available.
Then add a 25mm layer of sandy soil, followed by another 150mm of vegetable matter. Next add several handfuls of garden lime, this is to ensure that the compost heap doesn’t get too acidic.
Repeat this process, ensuring the manure and lime is used between each layer of organic matter, until you have a good size heap and the bay is full.
Keep the heap moist all the time but not water logged. Irrigate as necessary or cover if too wet because of rain. Compost is too wet if you can squeeze moisture out of a handful; and too dry if it feels dry in your hand. Water in a water logged compost heap will drive out the air in the pore spaces of the organic material, depriving the bacteria of oxygen and slowing the decomposition process.
After 1 month turn the whole heap onto the second site ensuring it is mixed and aerated well. Turn the heap again back to the first site after another month. After one more month it should be ready to use, total 3 months. While that heap is composting and being turned start another heap on the third site (Lord, 1978).
More frequent turning and hence aeration of the compost heap may make the composting process occur faster as the limiting factor in making compost is often oxygen for the microbes. If oxygen levels decrease to a low point, anaerobic bacteria will continue the breakdown of organic matter but at a much slower rate.
The composting process will start straight away if there is a good mix of nitrogen and carbon plant material. A good carbon: nitrogen ration is about 25:1. The microorganisms use carbon for energy and growth and nitrogen for making protein and for reproduction. In practice nitrogen rich organic matter include kitchen scraps, vegetable garden plants and lawn clippings, while carbon rich organic matter includes autumn and dead leaves, woody material, straw and hay (Pace, Miller & Farrell-Poe, 1995).
Eucalypt leaves contain much cellulose and lignin and are hard to compost and may be better off used as a mulch on the soil surface, provided you are not in a bushfire zone. London plane Platanus leaves can be plentiful but also don’t compost well, whereas oak tree Quercus leaves do, as told to me by older Italian gardeners, among the best of gardeners.
The compost heap temperature will rise, from heat given off by the respiration of bacteria consuming the organic matter and increase up to 50 to 60 degrees C. At this temperature many weed seeds are killed along with plant pathogens and fly larvae. A compost heap of at least 1m wide and 1m high is usually required to reach the desired temperature. The temperature should fall to outside temperature by the end of the second month indicating that the majority of the organic matter has been decomposed by bacteria (Pace, Miller & Farrell-Poe, 1995).
If you have worms in your compost heap this is not a good sign as the composting process should be so hot as to drive worms back into the soil below the heap. It is a sign that not enough decomposition is occurring, probably needing to turn the heap over. Worms may start to enter the compost heap as the composting process ends.
It is important to leave the heap for another month to ensure final composting of remaining organic acids and of the carbon and nitrogen in the organic matter.
The break down to humus and humic substances may take 12 to 14 weeks, depending of course, on the organic material used, the temperature reached, water content, the size of the heap and the amount of turning.
If your compost heap isn’t reaching a hot temperature, try turning it again to let in more oxygen, ensure the moisture content and size of the heap are suitable. If this doesn’t increase the temperature the composting process may well be complete.
Well decomposed compost and humus can be incorporated into the soil by digging it in but it can also be spread on the surface as a mulch, saving on the hard work, and letting the earth worms incorporate it into the soil.
Increasing soil humus and humic substances, that is compounds synthesized by microorganisms, is a very important process needed to improve soil structure and increase soil fertility. It is the basis of regenerative agriculture and fixes large amounts of carbon in the soil combating climate change.
References:
Baveye, P. C., & Wander, M. (2019). The (bio) chemistry of soil humus and humic substances: Why is the “new view” still considered novel after more than 80 years? Frontiers in Environmental Science, 7, 27.
Britannica. (2020). Selman Waksman. https://www.britannica.com/biography/Selman-Waksman
Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.
Lord, E. E. (1978). Shrubs and Trees for Australian Gardens (4th Ed.). Adelaide, S.A.: Lothian Publishing Co Pty Ltd. Book.
Pace, M. G., Miller, B. E., & Farrell-Poe, K. L. (1995). The composting process.
Solaiman, Z. M., Hongjun, Y. A. N. G., Archdeacon, D., Tippett, O., Michaela, T. I. B. I., & Whiteley, A. S. (2019). Humus-rich compost increases lettuce growth, nutrient uptake, mycorrhizal colonisation, and soil fertility. Pedosphere, 29(2), 170–179.
Thomson, S. (2018). Sophie’s Patch. (1st ed). Sydney, Australia: Harper Collins. Book.
Waksman, S. A. (1936). Humus. Origin, Chemical Composition and Importance Nature. New York, NY: Williams and Wilkins. Book.
Waksman, S. A. (1925). What is humus? Proc. Natl. Acad. Sci. U.S.A. 11, 463–468. doi: 10.1073/pnas.11.8.463
