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Compost Maturity

Added Wednesday, April 2 2014

Maturity – When compost grows up

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One of the most confusing parts of selecting a compost is figuring out how a particular product will perform. The term compost can apply to quite a wide range of materials, everything from a barely pasteurised product through to the most stable old compost possible. Each of these products, even if made of the same materials, have quite different chemical, physical and biological properties. The term we use to describe these age based difference is maturity. Maturity is the single most important factor in understanding how a composted product will perform, which is why we will now dive into the deep end to try and sort it all out.

The term maturity comes from people recognising that the age of a compost makes a difference to how it performs in a soil. For humans the rule of thumb is that the older we are, the more mature we are. We have adopted maturity to describe the difference between an old and a young compost. Unfortunately the trouble with rules of thumb is that they are a handy method of estimation only, you can’t rely on them because there are always exceptions. We all know (or at least I hope you do) a 50 year old who shouldn’t be let out alone and a 12 year old you’d trust your retirement money with. Composted products are the same, an old compost that has been poorly formulated and maintained will be less mature than a young compost that has been carefully formulated and well aerated. So now we’ve established that a term based on age isn’t really just about age, what is it we’re actually trying to describe?

Maturity in composted products is, when it’s all boiled down, a description of the size and complexity of the organic molecules it contains. All other things being equal, a product that has been through a full composting process (however long that may have taken) will have larger and more complex carbon molecules than a product that has not been completely composted. There are all sorts of other indicators of maturity, like colour, temperature, nitrogen forms, carbon dioxide evolution and so on, but the factor that underlies all this is how far the process of microbes breaking down organic matter and building the components up into different compounds has gone. That’s all well and good, but you may reasonably ask what this has to do with you and your compost? Let’s have a look at some of the reasons why maturity is so important when choosing or making a composted product for your purposes.


The full suite of positive soil health attributes of composts will be covered in many other articles, so rather than repeat ourselves too often we’ll limit ourselves to crop nutrition in regards to maturity as an example. The conventional way of looking at the ability of a product (be it processed fertiliser, manures or compost) to fertilise a plant is to look at the total amounts of the various macro (required in large quantities) and micro (needed in small quantities) nutrients present in the material. This is not an unreasonable method when looking at processed fertilisers. They are designed so that the relevant nutrients they contain are provided in water soluble forms that are immediately and completely available to plants. In this context, looking at the total nutrient concentrations makes sense. With manures, a large part of the nutrients within are in quite soluble forms so, again, judging by total nutrient content is not completely off track.

Once you start to compost organic materials the story gets more complex (better get used to that phrase). In the initial stages of composting the organic molecules in the ingredients are broken down by microbes, releasing carbon and soluble nutrients. As the nutrients are released they are taken up by other microbes who use them for their own purposes. The carbon released is mostly used for energy before being released to the atmosphere as carbon dioxide (many microbes take in oxygen and let out carbon dioxide, just as we do). The remaining carbon is either incorporated into the organic molecules that make up the building blocks of the microbes, or released in other organic molecules as waste. These waste products are used by and passed through a succession of different microbial groups. Each of these have different specialities and their own waste products they are progressively combined into longer and more complex carbon chain molecules. Other elements are bound to the carbon atoms in the chains, giving these molecules many of their properties. These molecules are what make up the humates, fulvates and most other stable functional groups in soil chemistry. As we said at the start, compost maturity is a measure of the complexity of organic molecules, and these are the esteemed and redoubtable molecules in question. We will refer mainly to humates for the moment, partly as a way to cover the whole group, and partly because more is known about humates to explain. It is likely that much of the science around humates is measuring the effects of the whole group so we won’t get too tied up in debilitating angst about the semantics.

While made up of essentially the same elements, simple organic molecules and complex humate organic molecules have different chemical and physical properties. Simple organic molecules are relatively easy to break down by microbes, which in the process makes nutrients available to plants. Complex organic molecules are difficult for microbes to break down, and so they only release a slow trickle of nutrients. This is essentially a continuation of the composting process within the soil. As we saw above a less mature compost is towards the simple molecules stage of proceedings, and a more mature compost has its life sorted out and all its molecules in stable relationships. This brings us to the first part of how maturity is related to nutrition. A low maturity compost will contain less complex organic molecules, so it will likely have readily available nutrients. A high maturity compost will contain a large proportion of complex organic molecules, and so will have few readily available nutrients, but will release a trickle of nutrients over time.

The amount of nutrients contained in a composted product and the rate at which they are or aren’t released is only part of the nutrition story. The humates in more mature composts have a wide range of soil health properties that increase the storage and recovery of nutrients. This increases the efficiency of any nutrients applied to the soil. On the recovery side humates have been shown to increase root growth, which increases the amount of soil explored by plants, ensuring that they pick up more of the available nutrients. Mature composts can also increase the amount of symbioses between arbuscular mycorrhizal fungi (AMF) and plant roots. AMF effectively become extensions of the plant roots and can increase the amount of soil explored by orders of magnitude, again ensuring that plants pick up as much of the available nutrients as possible. AMF are also able to pick up forms of phosphorus and other nutrients that plants are not able to access directly, which can be very important when soluble fertilisers aren’t used. On the storage side, humates have a very strong capacity to store soil nutrients in a way that is resistant to being carried away by water flowing through the soil, but which plants are able to mobilise and use when their roots come into contact. In addition humates are able to make phosphorus that has been effectively locked up on clay particles in the soil available to plants again by inserting itself between the phosphorus and the clay. The phosphorus is still poorly soluble in water, but is able to be solubilised and used by plants.

Higher maturity composts also tend to foster diverse microbial populations that perform incredibly important functions in the nutrient cycle, such as transforming nitrogen from organic forms (molecules with a carbon atom) to inorganic forms (without a carbon atom) and decomposing organic matter, then predating on each other to release nutrients that plants can then take up. Most nutrient additions to the soil will stimulate microbial activity, but it is important to have a diverse community, or you can end up with negative effects, such as losing soil carbon, or having too many of the nutrients in the soil tied up in organic forms.

Each of these storage and recovery elements of compost when applied to soil revolve around humates. Humates are of course associated with high maturity composts, so we have a useful dichotomy. Low maturity composts are useful for nutrient supply, and high maturity composts are useful for increasing nutrient efficiency.

This is all fine and dandy. Using high maturity composted products seems like a pretty smart way of doing things, but if low maturity composts are more about nutrient supply, why not just stick with chemical fertilisers or manures? A pertinent question, let me summarise with an example:

Chemical = junk food
Manure = white bread ham sandwich
Compost = nice brown rice

Chemical fertilisers are designed to be easily soluble so that all of the contained nutrients are available as soon as there is enough soil moisture. There isn’t anything inherently wrong with this, until you get to practical considerations. If you can put on lots of small applications that provide for the immediate needs of the plants you are growing then you probably won’t get many negative effects. Logistics often make this difficult or expensive to do, so what usually happens is that a larger amount of fertiliser is applied in order to get the plants through a certain amount of time. In this situation there will be more than plants can rapidly take up. You are then at risk of nutrients being lost through being washed away or volatised into gases and lost into the atmosphere. This would mean you have to put on even more nutrients to make sure there is still enough left to carry the plants through, while also raising the risk of offsite environmental impacts to do with raising nutrients in water levels and causing algal blooms. A worse effect is that putting high levels of soluble nutrients into the soil can burn up soil carbon. This is quite a controversial concept, and will be discussed further in its own article, but the basic explanation is that soil microbes are often limited by nitrogen. If all of a sudden they have a wealth of it from highly soluble nitrogen, they go into a frenzy of activity that leads to large amounts of soil carbon being respired and leaving the soil. The prevailing wisdom has been that nitrogen fertilisers increase soil carbon by increasing the amount of plant growth and activity, increasing the carbon supply to the soil, but recent studies have shown that the real world effect is usually a net loss of soil carbon. Carbon is the backbone of life in the soil (richly deserving of its own article), and its loss leads to a whole suite of soil degradation processes. We would like to point out that we don’t think chemical fertilisers are the devil. They can be used to positive long term effect, so long as they are well understood in their benefits and limitations. We have had very good results from combining reduced amounts of fertiliser with compost applications, getting the best of both worlds.

Manures have got the edge over chemical fertilisers in that they contain reasonable amounts of carbon. This helps to offset the carbon losses possible from putting a large amount of soluble nutrients in the soil, and it means that the nutrients are a little more stable, as they are mostly bound in simple organic molecules. Nutrients are still readily released from these molecules and there remains the risk of nutrients running off site. The carbon in manure is similarly simple. It is readily broken down and lost as carbon dioxide. This is not a bad thing, but leads to one of the reasons why you might choose a low maturity compost over a manure.

As we have already established, high maturity composts are full of complex organic molecules that are mostly very stable. They do have slow release fertiliser value over time, but in high demand situations they might need to be supported with some amount of more available nutrients. Lower maturity composts have a greater proportion of nutrients that are readily available, and can be used on their own for plant nutrition. Where they vary from manures is that the composting process has pasteurised any plant or animal pathogens, and bound nutrients and carbon into more stable forms. There are also some degree of the highly complex organic molecules present in highly mature composts, but not to anywhere near the same extent. So what you get is reasonably stable nutrients balanced with reasonable stable forms of carbon that will give a steady flow of slow release nutrients over time. In this way plants get a continual supply of nutrients, without the risk of excess nutrients being washed off into local waterways, or losing your soil carbon to rambunctious microbes on a nitrogen bender. Of course, while each unit of low maturity compost gives off only a steady trickle of nutrient, if you over apply compost the trickle can add up to a flood that can still cause environmental issues. Along with prudent application rates a low maturity compost would perhaps be best used mixed with a high maturity compost just before spreading. In this way you could get both the benefits of the slow release fertilising of the low maturity compost, and the soil health and nutrient efficiency effects of the high maturity compost.

The benefits of complex organic molecules in the form of humates and associated chemical groups are in improving soil health. They do this by increasing the amount of soil water storage, increasing the ability of the soil to hold onto nutrients in a plant available form, and stimulating plant root growth. The take home message from this is that two composts made from identical ingredients with identical methods can have quite different characteristics if they are of different maturities.

Chemical fertiliser = No carbon, highly soluble nutrients
Manure = Simple carbon, soluble nutrients
Low maturity compost = Stabilised carbon, moderately soluble slow release nutrients
Medium maturity compost= a mix of low and high maturity properties
High maturity compost = complex carbon, increased soil health properties, lower solubility nutrients

How we measure maturity

Measuring the maturity of composted products is a vexed issue. While we have tried to present what we believe is the most relevant description, maturity doesn’t have a clearly agreed upon definition, which makes it difficult to decide on the correct measurement method. This isn’t surprising, as the combination of factors that make up the composting process are extraordinarily complex when you’re trying to view them as a whole. As a result, there are a range of measurements used as proxies for maturity.

The simplest measure used is temperature. During the composting process there is a huge amount of microbial activity, which produces a lot of heat. During the early stages of composting, when there is the greatest amount of easily available carbon and nutrients available, microbial activity is at its highest, so it follows that this is when the compost pile is hottest. As these easy resources are depleted and tied up in more complex molecules, different groups of microbes that have slower lifestyles based around food sources that are more difficult to get access to take over. These microbes are less active, and so they produce less heat. This progresses until eventually there are only highly complex molecules remaining, and microbial activity is very low. This process is what makes temperature seem like a reasonable proxy for compost maturity, a young, immature compost has high temperatures, and an old, mature compost has lower temperatures.

The problem with this measurement is that factors other than maturity have effects on compost pile temperatures. Ambient air temperature can have some effect, heat is lost more rapidly in cold weather than hot weather. Pile size is important for surface area and insulation reasons. A big pile has less surface area per cubic metre of compost than a small pile (it’s a funny property of physics), and there is more material for heat to travel through before getting to the edge of the pile. As a result a large pile retains more heat than a small pile of the same material of the same maturity. Water is essential for microbial activity. An immature compost that runs out of water will have a drop in microbial activity, meaning it produces less heat than a mature compost. How often a pile is turned, how it is aerated, and all sorts of other factors make temperature an unreliable guide to judge the maturity of the compost process, although if the temperatures are high (>65°C) in a pile it most likely is less than mature.

Related to temperature as a measurement is carbon dioxide production (and its corollary, oxygen use). It is also linked to microbial activity, but instead of temperature, the amount of carbon dioxide being produced by the compost is measured. Microbes produce carbon dioxide as part of making energy (via respiration, which is basically the same as what is happening when we breathe in oxygen and breathe out carbon dioxide), and as we established with temperature, a young, immature compost will have more microbial activity than a mature compost. Carbon dioxide is directly linked to microbial activity, so a young, immature compost will produce more carbon dioxide than a mature compost with less microbial activity.

Unfortunately carbon dioxide production as a maturity measurement is also confused by a number of factors. Just as with temperature, a shortage of water will restrict microbial activity. Less microbial activity means less carbon dioxide may be produced by a dry immature compost than a moist but more mature compost. The quality of the ingredients and the proportions in which they are mixed strongly affects the amount of microbial activity. One of the crucial factors in formulating a correct ingredient mix for composting is getting the carbon to nitrogen ratio correct. A shortage of either element will restrict microbial activity. For example a compost with too little nitrogen will stop composting when it runs out, leaving much of the carbon molecules partway through the transformation from simple to complex molecules. This compost will show low carbon dioxide production, implying a mature compost, when the complex molecules in a mature compost are not present. Even in a well formulated compost, microbial populations vary strongly according to the particle size of the ingredients. The amount of carbon dioxide is the result of activity per microbe x the number of microbes. Larger particles have a lower surface areas than smaller particles (the same funny property of physics as surface area in compost piles and temperature, we could go into it but we don’t think that you willingly signed on for that level of brain bending). Less surface area means less area to fit microbes onto for the same weight of material. A lower population means a lower amount of carbon dioxide compared to a compost with identical ingredients but finer particle size. This means that a coarse compost appears to be more mature than a fine compost. This is most relevant at the end of the composting process when the end product is often sieved into different size fractions. If split into a coarse fraction and a fine fraction, the coarse fraction will produce less carbon dioxide and appear more mature than the fine fraction from the exact same compost. In reality it will probably be less mature, as the larger particles take longer to break down and will have a lower proportion of complex humate carbon molecules. This is a factor that can be manipulated to try and create a product that meets a higher maturity rating and should be watched for. We have observed this directly at C-Wise, where a compost batch that was separated into fine and coarse fractions gave a lower maturity reading for the fine fraction and higher maturity for the coarse fraction, when the opposite is true for material that has come out of the same composting batch.

Carbon dioxide is the result of microbes effectively using oxygen to burn carbon. If you have a poorly aerated compost pile then the little oxygen that is there is going to be used up rapidly, leaving nothing behind for the production of carbon dioxide (one part carbon, two parts oxygen). You would think that this would mean that microbes would stop their activity as well, but many of them have a trick up their sleeve that allows them to use fermentation to produce energy instead, with carbon leaving the cell as methane instead of carbon dioxide. If you’re measuring carbon dioxide to look for microbial activity, methane won’t show up, making a compost process that is still active look more mature than it is. This has further problems in that fermentation (energy production without oxygen) is much less efficient with carbon than respiration (energy production with oxygen). The composting process takes longer, does not produce the same range of carbon compounds and makes less of them per unit of carbon in the initial ingredients. Having very tiny areas (microsites) that are anaerobic (lacking oxygen) within the compost pile actually appears to be a necessary part of making the best quality composted products, but having large parts of the compost pile being anaerobic is a poor situation, and likely to give misleading maturity results when using carbon dioxide production as a measure.

The factors we have discussed so far mostly revolve around the carbon cycle, so a way to add reliability to our measurements is to look at other nutrients. While the carbon related measurements we’ve discussed so far are quantitative, using the nitrogen cycle can give some qualitative indications of compost maturity. Ammonia production occurs when microbes break down nitrogen containing compounds (mostly proteins). They excrete excess nitrogen as ammonia. Ammonia is highly volatile and a certain proportion of it leaves the compost pile as a gas. The ammonia that remains gets transformed by other microbes into ammonium and then nitrate, neither of which are lost as readily as a gas (except under certain circumstances). This means that a younger compost is likely to have more ammonia production than a mature compost that has had most of the nitrogen either transformed to ammonium and nitrate or bound up in humates. This has the advantage over the carbon cycle based measurements already discussed in that it is attempting to measure the progress of a chemical change that happens with increasing compost maturity. Unfortunately it is also linked to microbial activity. In the context of compost ammonia is only produced by microbial activity, if this is depressed by any of the factors mentioned before you will get a misleadingly low measurement for ammonia gas.

Clearly maturity measurements based around measuring the current microbial activity are unreliable used in isolation. Ammonium and nitrate can be considered better indicators than ammonia in that they aren’t linked to current microbial activity at the time of testing. In a way they are the product of previous microbial activity, and are better indicators of the current state of maturity. The nitrogen cycle within the soil and in compost (when simplified and leaving out the back and forth between the atmosphere) goes organic molecule &rarr ammonia &rarr ammonium &rarr nitrite &rarr nitrate &rarr organic molecule. In theory a compost that has more nitrate than ammonium and more ammonium than ammonia would be towards the mature end of the spectrum. The problem with this theory is that it is only true on average. At any given moment an individual compost can vary significantly, as nitrogen can transform back and forth between all four states depending on the conditions of the day, and can also be lost as a gas from each of the non-organic stages. Unfortunately while ammonium and nitrate measurement are superior in theory in that they measure chemical changes associated with maturity directly rather than using microbial activity as a proxy, the individual measurements have a high rate of error.

An alternative to trying to characterise the compost directly is to test how it works with plants. The tests used to do this (generally known as bioassays) test how a plant will germinate and grow when placed in the material. This makes a lot of sense, why worry about trying to characterise the maturity of the compost when it is so problematic, just let the plant taste test itself. Once again we’re beset with complications. Bioassays are very effective at showing up a very immature compost. High levels of ammonia and several other factors will retard both germination and root length of seedlings, the two main bioassay factors. A compost of medium maturity should have no retarding effects on seedling germination and root growth, so all good so far. The problems start when you enter the realms of high maturity composts. These are relatively rare on the market place, so it is forgivable that this effect isn’t better known, but high maturity composts can fail bioassays. 

The reason is that a high maturity compost has a lot of organic molecules that act as osmolytes in water (dissolved molecules that affect the process of osmosis). This is a normal process in the soil, and plants are able to overcome it in normal conditions by raising amount of solutes within their cells. Water wants to move from areas with low amounts of solutes to areas with high amounts of solutes, and this mechanism (osmosis) is taken advantage of by plants to take up water. If the amount of solutes in the soil water is higher than the amount that a plant can produce within itself then it will be unable to take up water and will dehydrate in a moist soil (known as osmotic drought). This most often happens in saline conditions, where dissolved sodium and chloride are at high enough concentrations to cause this effect. It can also happen where there is an over application of compost, and high maturity compost in particular. The nature and volume of the organic molecules in a high maturity compost generate a strong osmotic effect that can overwhelm plants ability to produce their own internal solute concentrations, particularly small or young plants. Seeds will not germinate in these conditions, and if the hardy few do manage it, their root growth will be restricted. That means that a high maturity compost can fail a seed germination bioassay. This sounds like we’re misguided for trying to create high maturity composts. That perspective depends on your intended use. If you plant seeds into a 100% compost mix of high maturity, you will likely get bad results. Luckily this isn’t how this sort of product should be used, it should be used at relatively low concentrations in the soil. Its benefits come from stimulating soil processes to work more effectively, and in this context it will increase seed germination and root growth. So, looking at it this way, the bioassay isn’t telling fibs as such, but the methodology of the test isn’t relevant to the intended use. Low maturity composts (or medium for that matter) also aren’t intended for use as a sole growing medium. Diluted by appropriate amounts in the soil they will have positive rather than negative germination and root effects. Bioassays should be used with caution and with a good understanding of their relevance to real world conditions.

At C-Wise we use the Australian Composts, Soil Conditioners and Mulches Standard (AS4454 - 2012) as our reference. We have chosen two of the commercially available Solvita tests in combination to measure carbon dioxide production and ammonia production. The combination of a microbial activity measure and a chemical measure of maturity gives an acceptable reliability as a compost production tool. We still need to be aware of all the issues described above, and occasionally get unexpected results that we can usually track down to one of the described problems with each measurement.

Appendix N of AS4454 (2012) contains a rating system for compost maturity ranging from Maturity 1 for low maturity products, and Maturity 3 for high maturity compost. To meet Maturity 1 a compost must have passed through the minimum pasteurisation methodologies for composting, and to meet the higher ratings it must also pass chemical and bioassay tests appropriate to medium and high maturity composts. We use this system to label our composts in order to be able to easily describe our products in a way that other people can easily replicate, understand and use to judge our products.

The ideal maturity measurement would be to measure the presence of humates directly. If there are no humates, then you have a very immature compost, and if there are lots then you have a mature compost. This can’t be disguised or give false positives for a mature compost because it is a direct measure of the defining properties of a mature compost. Unfortunately there is a lack of consensus on how to test for humates. The definition of humates is a functional one based around acid extractions from organic matter. The actual molecules that fit within this functional definition can be very structurally diverse, making it difficult to design a chemical process to test for them. Most of the methods existing so far are expensive and/or use dangerous lab chemicals (humate testing and definition will be discussed in the humate article in the learning centre). Very few commercial labs offer humate measurement services, and the complication of the methods make it difficult for most compost producers to undertake them themselves. This is why maturity methods currently revolve around tests on proxies for compost maturity, and until humate testing in compost becomes cheap, easy, safe and reliable this will likely remain the case.

How to judge the maturity of compost with your own senses. Having just spent considerable effort painting a picture of uncertainty around putting a number on compost maturity, there are some practical rules of thumb that can help you when deciding to purchase a product.


A high maturity compost tends to be darker than a lower maturity compost. Unscrupulous compost producers (there are some out there, just like any industry) know this as well, and use iron oxide to darken their composts, so be careful. If a compost glistens, or looks wet, this could mean that it has been produced with too much water and is likely to not be very mature at all. This could be because of heavy rain or the compost being watered for whatever reason by a retailer, but it is something to take into account. If there is a lot of sand visible (and it isn’t clearly labelled as some sort of specific topdressing blend), then you are likely paying for a lot of weight and volume that isn’t doing anything. If you do see sand, make sure that you scrape the surface away to have a look underneath, it is possible to get windblown sand on the surface of a pile that makes things look much worse than they actually are.


Smell can tell you a lot about a compost, but you need to break open the pile and get a handful right to your face to make a proper judgement. A judicious sniff from a distance before putting your nose in the way of an olfactory right hook might be a good idea though. If a compost has a really terrible, manure/poo smell, then it probably hasn’t been pasteurised properly and may not be safe to use in the same way you would a compost. You would need to treat it more as a manure. It is not unusual for a low maturity compost to have a whiff of sharp ammonia smell (baby wee) about it. This isn’t ideal, but is okay so long as the use is appropriate. A high maturity compost will never have this smell about it. It should have either a pleasant, earthy odour, or almost no smell at all. If you get anything of the previous descriptions you should question the maturity of the product.


Allowing for particle size, a mature compost should be softer than a less mature one. Obviously a mature mulch will be rougher than an immature finer compost. Small pieces of wood should be readily broken or deformed in a mature compost, but may depend on the properties of the ingredients. A highly mature compost should be mouldable, in that is should hold a bit of the shape of your hand if you squeeze a handful. When you squeeze the handful, there shouldn’t be a stream of moisture that comes out, if there is, and there hasn’t been a lot of rain or irrigation by a retailer you should question the maturity of the product.

Using Look, Smell, Touch you can learn a lot about a product that is in front of you without needing to pester people for lab data. If you’re happy at this stage, then you should ask for the lab data, remembering to take everything with a grain of salt. Now that you have a good level of knowledge you should find your decision making process when looking for compost a lot more straight-forwards, and you should end up with the product that will suit your purpose.

If you have any questions or comments about this or other articles, please don’t hesitate to contact us.