Quality check test for compost

Composting has been talked about a lot on this blog. How to make a composter, what materials are suitable for composting … All these tips and procedures that we have been writing about for a long time are intended to make good compost. But … how to recognize it? How do we know if we have a mature or good compost ? There are a number of tests that tell us.


The processes that explain the composting process are complex and very numerous. A number of biochemical processes are involved from the organisms found in it. Fungi, bacteria, yeasts and other organisms (such as worms) are responsible for breaking down and transforming compostable waste into mature compost. Each group of microorganisms “attacks” compostable waste at different times and phases, depending on the decomposition phase of the matter and the conditions (temperature, pH, humidity…). The humification or humus formation process is one of the most complexwhich is still being studied extensively today. This process is vital for the proper development of plants and occurs in all fertile soil. In fact, valuing a soil as fertile is synonymous with a good humification process among other things.


If we had to discuss the humification process in detail, we would not have lives to write about it. It is complex, very complex, but we could define it in a very general way as the intermediate step between fresh organic matter and the mineral compounds into which it is ultimately converted. In a schematic and very general way we could establish:

The result of the complex humification process provides us with the famous and necessary humus, which can be more or less stable depending on the time spent in the humification process and, above all, the balance that is achieved between the organic matter rich in nitrogen (young parts and plant greens) and carbon-rich organic matter (cellulosic and lignified compounds from older parts). In fact, if you are somewhat involved in the world of compost, you will know that one of the values ​​that tell us if the compost produced is stable is the C / N ratio.

As a result of the composting process, depending on the time, we can obtain young compost or mature compost. None is better than another. They have different properties and uses from the point of view of the agricultural use of the garden.

It is understood that the mature compost is more stable and the elements that compose it can be used directly by the plants. It is what we know as mulch. If our plants need an immediate supply of nutrients available , this will be the best compost, for example, in germination processes or if there is a serious deficit of nutrients in the soil. The problem is that the contribution is so immediate that, in the long term, the soil will once again be undernourished (although to a lesser extent). This type of compost takes about 1 to two years to form.

On the contrary, young compost has a part of its elements degraded and usable by plants and there will be another fraction with complex compounds that still need to be decomposed. This compost, in the long run, will cause greater activity in the soil of our garden, because in it, there is still material that can be transformed by the organisms that feed on it. Let’s say the young compost can become a slow-release compost in our garden or vegetable garden soil. Young compost will take 3 to 6 months to form. All these times are relative. It depends a lot on the process conditions (temperature, season of the year, humidity, pH, aeration, balance of raw material …).

Different phases of composition
Fuente: regives.com


The theory is the theory and then you have to put it into practice. Composting well is a matter of learning, documentation, time and experience and things do not always turn out well even if we believe that we do everything as we are told or have learned. There are certain particular conditions of each situation and person who composes, which make this not such an exact science and sometimes it does not work out.

Remember that in the entry of types of composters you can consult the table with the main problems during the composting process. 

Therefore, to try to know if the composting process carried out has gone as it should, we are going to see certain tests or verification tests.


This is the first test to know if your compost is incomplete or not in good condition to be used. The characteristic smell of well-made compost is that of forest, humid earth. Conclusion, it should not be an unpleasant smell. If so (foul smells, acids, ammonia, putrefaction), something is going wrong. They can be anaerobic fermentations, excess humidity, lack of aeration, imbalance between primary components … The color must be a dark brown, in which we do not see or it is difficult to see recognizable remains of the initially incorporated residues.


Another way to check if the result is satisfactory is to check the texture of the compost. Some time ago we talked about the texture of the soil and how to know in a simple way what type of soil we were facing. We were doing the ring test. With compost, the valuation is somewhat similar. Just grab a fraction of compost and squeeze it tight in your fist:

  • If when released it crumbles and falls, we will be facing a dry compost
  • If, on the other hand, water sculpts from your hand and drips, we will have an excess of humidity (it may be lack of aeration)
  • If the handful of compost is maintained with good structure, it does not sculpt water, nor does it decompose, we will be facing a compost in good condition.

Compost ready to use
Source: Recycling Verde


Analytically, the germination tests together with the C / N ratio are the most objective methods to establish if our compost is of good quality and has formed as expected. The most accepted method is the Zucconi test. It requires a minimum of laboratory instruments to be able to do it correctly. If you want to know more about this procedure,   Germán Tortosa tells us in  detail in his article on the Zucconi Germination Test. With this test we will be able to establish a quantitative value called the germination index. If its value exceeds 80-85% it is enough to affirm that we are facing a balanced and stable compost.

As many of us do not have access to this laboratory material, we can always do a test a little more coarse but it can serve as a guide to know if the compost is as it should be. It involves sowing watercress seeds in a container and seeing their evolution. Depending on its adequate or inadequate growth, we can establish, although not quantitatively, if the compost we have is of quality.


In two glasses or pots you put: garden soil and compost in equal parts in one and garden soil in the other to compare. Cress seeds ( Lepidum sativum ) are sown in the seeds . It is important that you sow with relative equidistance and count the seeds you put in order to know with a quick count, which ones have germinated. Cover the seeds with a little more compost (1-2 mm) and water. Maintain the humidity of the substrate without flooding during the time it takes to germinate.

Watercress is a plant that takes a very short time to germinate. In just 3 days (if the conditions are suitable) we will have germinated watercress and up to a couple of centimeters high, therefore:

  • If all or almost all watercress seeds germinate quickly and on a very regular basis within 2-4 days, good news, your compost is promising! It is balanced.
  • If you see that germination takes more than a week to make an appearance, or it simply does not germinate, it is very possible that you have the compost still immature, unbalanced or with phytotoxicity problems. This does not mean that you cannot add it to the ground. Nothing will happen but it is advisable to continue composting.
  • If they germinate, observe the appearance of the plants. If the green of all of them is intense and regular, your compost has signs of being on the right track. If you see rottenness, bent stems, brown, if the leaves turn yellow and the growth is very irregular, then the compost may have phytoxicity problems caused by undesirable fungi for example.
  • Use the control pot without compost to see differences. If you see that the one with compost grows better, more vigorous and looks better than in the garden soil, we can see that the compost made is a great improvement. If, on the other hand, in the pot with garden soil it grows better, not only will we not be improving the nutritional quality of the soil with the prepared compost, but the result will be counterproductive.

We hope that with this data, you can better assess the state of your compost and therefore make better decisions in your garden.

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