stimation of humus losses in cultivated soil

How to determine the annual loss of humus from cultivated soil

The volume and rate of humus loss in cultivated soil should not be taken lightly. This information allows amateur gardeners to determine the amount of compost they need to purchase or make themselves each year to compensate for humus loss. Reliable information on humus loss can only be obtained from periodic laboratory analyses. General studies of the region carried out by INRA or other organisations such as regional chambers of agriculture can also be used as a basis. However, these general studies are obviously less accurate than laboratory analyses, which can vary from one plot to another due to soil and climate characteristics, the nature of the crops grown, etc.

Generally speaking, for 300 kg per m² of arable land with a thickness of 20 cm, it is accepted that in the PACA region, cultivated soil must contain at least 3% humus, or 900 kg/acre. With an average humus loss of 3% per year (2% for regions further north), 27 kg/are of humus (40.5 kg for a topsoil thickness of 30 cm) must be added to compensate for this annual loss. With an estimated average loss of 80% during the composting process, approximately 135 kg of fresh organic matter (202 kg for a topsoil thickness of 30 cm) must be collected to obtain these 27 kg of humus. The humus produced by underground litter (particularly roots left in the soil after harvesting) should be deducted from this figure. For some crops, the ratio between the volume of organic matter produced by the roots and the crop residues returning to the field is known.

Do not forget the humus produced by the roots of cultivated plants

For cereals, the ratio of underground litter to surface litter is approximately ½ for wheat, slightly more than ½ for rapeseed, 1/3 for barley, and more than 1/3 for grain maize. For young green manure, this ratio is slightly more than 1/1, and slightly more than ½ for advanced green manure (4). For vegetable crops, the data is less precise. At harvest time, it can be seen that certain vegetables such as leeks, tomatoes, celeriac and carrots produce a lot of roots, but this is not the case for lettuces. In general, vegetable crop residues produce little recoverable dry matter.

Advanced green manure is probably the most accurate model for vegetable crops. For an area of 100 m² used entirely throughout the growing season, and taking into account that approximately 80% of the mass is lost, the humus produced by the roots of this green manure should be approximately 27/2/80% = 2.7 kg/are. This gives a good idea of what remains in the soil for market gardening after harvesting and what needs to be added in the form of compost to compensate for the total losses. The humus ratio from compost is around 24.3 kg/are (36.45 for a topsoil thickness of 30 cm). This value is approximate, as some crops consume more humus than others. In addition, other uncontrollable factors, such as climate, influence humus mineralisation. Humus losses therefore vary from year to year.

To meet the total needs of certain exportable plants that are high in mineral salts, more organic matter must be used as a base fertiliser, from which the mineral fertiliser input must be deducted (see article: Examples of sustainable fertilisation for some vegetable crops). For other plants that require less mineral salts, fertilisation with less organic matter is required in order to prevent mineral salts from leaching into the water table.

It is estimated that around 50% of the mineral salts absorbed by plants end up in the harvest, with the rest ending up in underground litter and crop residues that can be recycled. At least 80% of this waste will be lost during composting, but this loss can be reduced by adding organic structuring materials. Experimental studies by CTIFL (5) have shown losses of around 50% when composting bio-waste from fruit and vegetable crops with added structuring materials (wood, straw, etc.). Fresh organic matter containing a lot of cellulose (e.g. hedge trimmings) will result in less loss of humus volume during composting, in the order of 50 to 60%. On the other hand, kitchen waste produces very little humus (around 5%). As for manure, losses vary between 80 and 92% (6).

For agricultural businesses that farm large areas of land, the quantity of organic matter to be composted quickly becomes problematic. The average size of French farms is 55 hectares (7), and in order for the soil to maintain a humus volume of 3%, approximately 148.5 tonnes of humus are needed just to limit annual losses, or 742.5 tonnes of fresh organic matter. It is easy to understand that on large areas, it is practically impossible to compensate for humus losses solely by adding organic matter in the form of manure and compost if the farmer does not have cattle, sheep, poultry, etc. producing sufficient waste. If they want to maintain their yields, farmers have no choice but to use mineral fertilisers, which also have the advantage of increasing recyclable plant waste (including underground litter).

Restructuring soil deficient in humus

For a complete restructuring of soil lacking humus, much more fresh organic matter is needed. Let’s imagine a 100 m² plot of desert land to which we want to add 3% humus to a depth of 20 cm. The amount of humus to be added would be approximately 900 kg, or 4,800 kg of fresh organic matter. This is generally the volume of humus that needs to be replenished for most vegetable gardens that have never been analysed or restructured in terms of soil biodiversity, including vegetable gardens that have never been fertilised with mineral fertilisers and have been blindly treated with organic fertilisers that are not suited to the characteristics of the soil.

This is a very common situation, which often explains the origin of the many problems encountered. For a 55-hectare farm, approximately 4,950 tonnes of humus or 24,950 tonnes of fresh organic matter are required. For amateur gardeners, as for farmers, it is not easy to find such large quantities of fresh matter unless the soil is restructured over several years. Collecting and processing 5 tonnes of fresh organic matter over 2 to 3 years is still feasible for an amateur gardener if they make the effort to seek out organic residues: collecting manure from a riding centre or sheep farm, grass clippings and hedge trimmings from neighbours, collecting dead leaves in autumn, etc.….