
Fungi are widely spread on land. These aerial hyphae are growing on a piece of biodegradable plastic. (1750µm width.)
Biodegradation Requirements
The rapid decomposition of biodegradable plastics requires the right combination of microbes, moisture and a suitable growth medium.
Microbes
Bacteria are the most numerous organisms in the environment, but in soil, fungi make up the greater biomass. Although both groups are involved in biodegradation, on land, it is generally believed that fungi do most of the work.
(In aquatic environments, conditions are very different, and bacteria are predominant. There is only limited research available on the biodegrading ability of yeasts, algae and amoebae.)
Although fungal spores get dispersed widely, the resulting diversity & quantity can be quite variable even over short distances. What species thrive (and where they do so) depends substantially on soil composition, level of compaction, local (micro)climate, and to some extent, what species are already present.
In extremis, one end of a biodegradable plastic item may be successfully degrading, and yet it can remain largely unaffected at the other.
Media
Of the terrestrial conditions, well-rotted compost is perhaps the best medium, although good quality neutral or slightly alkaline organic soils also seem to work well. Forest and woodland soils are quite rich in organic matter and generally good for biodegradation, whereas field soils can be quite poor in nutrients and therefore not so good.
(Undisturbed field soil has, however, been reported to be more biologically active than garden soil, although, unsurprisingly, the use of fertilisers and other chemicals can significantly affect the microbial community in both cases.)
Under duress, most microorganisms can consume organic compounds anaerobically, but the metabolic process is considerably less efficient and leads to the production of methane. Well-aerated conditions therefore present the most favourable location, especially the first 2-3 inches of top-soil where air permeation is highest.

This rapstrap was partially buried in soil for a few months; Only the buried portion has started to degrade. Biodegradable plastics are generally quite stable above ground and will degrade only slowly when lying on the ground. They really need to be covered for good results.
Moisture
Along with suitable microorganisms and appropriate nutrients, the third essential requirement is the ongoing presence of water. This is particularly true for polyesters, which decompose via hydrolysis, a chemical process that deconstructs the long polymer molcules back into short fatty acids.more
Over the course of our own testing, we have generally found that continually damp conditions work best.
If the media dries out the microbes will go dormant, and although biodegradation will resume when enough moisture returns, the process takes a little time to restart. Conversely, too much water can impede biodegradation since it can make conditions unsuitable for fungi (the primary degraders) and reduce their oxygen levels.
A common method of improving water availability is the incorporation of hydrophillic starch into hydrophobic biodegradable polymers. Starches naturally contain about 15% water under ambient conditions, but this can rise considerably when soaked.
Biodegradable polymers incorporating up to 30% starch (such as our bioplastic i-Ties) can saturate at 10-12% water content. In wet environments (such as damp soil), this helps swell the plastic and carries microbial enzymes deeper into the material. Biodegradation can be significantly faster in these conditions.more
Nutrient Requirements
Like plants and animals, microbes require essential elements. Primarily, all require N, P, K, Mg, Ca, Cu, Zn, Mn and Co, and, in addition, plants also need S, Fe, B and Mo. Fungi, however, need only the base elements plus Na and Ni. Since the deficiency of just one of these elements can significantly impede microbial activity, biodegradation is more reliable in “organic” soils (e.g., compost, muck, forest soils). These contain higher levels of NPK salts and trace elements compared to “mineral” soils (e.g., sand, field soils)
Along with carbon content, typical NPK levels for different soil types are as followsmore (given in g/kg of dry matter).more
Medium | Carbon (C) | Nitrogen (N) | Phosphorous (P) | Potassium (K) |
---|---|---|---|---|
Sand | 2.5 | trace | trace | trace |
Field soil | 50 | 0.4 | 0.4 | 0.15 |
Lab “Standard Soil"more | 20 | 3 | 1.5 | 3 |
Forest soil | 114 | 16 | 4 | 16 |
Cow Manure | 276 | 18.4 | 7.9 | 18.4 |
Mushroom Compost | 334 | 26.5 | 6.9 | 24.4 |
These may be compared with typical elemental compositions:
Source | Carbon (C) | Nitrogen (N) | Phosphorous (P) | Potassium (K) |
---|---|---|---|---|
Plants | 440 | 13 | 1.6 | 9 |
Fungimore | 286 | 22 | 5 | 22 |
Animals | 575 | 65 | 27.5 | 5 |
As a general principle, plants concentrate the nutrients they extract from the soil, and then fungi & animals feed on the plants to concentrate them further. The subsequent release of these nutrients back into the environment is then reflected by the high levels seen in the more biodiverse conditions.
Because fungi like high nutrient locations, it is not surprising they grow well in forests, compost heaps and on piles of manure. These would also make good locations for the breakdown of biodegradable plastics, especially if they are kept damp.
Of course, this does not preclude their ability to biodegrade is less favourable conditions (moulds are particularly hardy), but it is perhaps telling that in formal laboratory testing of biodegradability, the microbrial inoculant is frequently sourced from a good quality forest soil.
Temperature

Biodegradation of PCL occurs in two main temperature ranges, with rapid biodegradation only happening above about 40°C. Below this level, changes in temperature do not have such a large effect. (Although very cold or very dry weather can temporarily halt the process, the decomposition rate (i.e., the gradient) is actually very consistent.)
A rule-of-thumb in chemistry states that reactions will double in speed if the temperature is increased by 10°C.
However, biochemistry does not strictly follow this trend, and although higher temperatures can improve biodegradation, the effect can be marginal below about 40°C.more
The main reason is the microbial species responsible for biodegradation are only adapted to particular temperature ranges. Those that operate well in ambient conditions struggle as things get too hot, so biodegradation does not significantly improve.more
Only at very high temperatures, where thermo-tolerant species out-compete the mesophiles, does biodegradation proceed more rapidly. (This transition normally seems to occur around 40-50°C.)
The phenomenon is noticeable when composting biodegradable plastics. Home compost heaps do not usually attain temperatures much above ambient (20-30°C), and their performance is often not much better than good quality soil.more
Industrial facilities, however, commonly exceed 50-60°C, and only heat-adapted species are present (such as the "hot mould" thermomyces). These take full advantage of the hot conditions (and lack of competitors) and biodegradation is substantially faster. In fact, conditions are so aggressive that even stubborn PLA will rapidly break down.
Poly Lactic Acid
After thermoplastic starch, poly lactic acid (PLA) is reportedly the most widely used biodegradable polymer. However, it has been consistently shown to only degrade at temperatures above 50°C. This essentially rules out ambient environment disposal and consigns PLA to industrial composting.