Estimation of humus loss from a cultivated soil

How to determine the annual loss of humus from a cultivated soil.

The volume and rate of humus loss in a cultivated soil should not be taken lightly. This information allows the amateur gardener to determine the amount of compost purchased or made by him/herself that he/she should apply every year to correct the humus loss. This information on humus loss can only be relied upon through periodic laboratory analyses. It can also be based on general studies of the region carried out by INRA or other organisations such as the regional chambers of agriculture. But these general studies are of course less precise than a laboratory analysis, which can vary from one plot to another due to soil characteristics, climate, the nature of the crops grown, etc.

Generally speaking, for 300 kg per m2 of topsoil on a 20 cm thickness, it is accepted that in the PACA region, a cultivated soil must contain at least 3% humus, i.e. 900 kg/are. With an average humus loss of 3% per year (2% for the northern regions), 27 kg/are of humus (40.5 kg for a 30 cm thickness of topsoil) must be added to correct this annual loss. With an estimated average loss of 80% during the composting process, about 135 kg of fresh organic material (202 kg for a 30 cm topsoil thickness) must be collected to obtain this 27 kg of humus. From this figure, the humus produced by the subterranean litter (notably the roots left in the soil after the harvest) should be deducted. 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 below-ground to above-ground litter is about ½ for wheat, a little more than ½ for rape, 1/3 for barley, more than 1/3 for grain maize. For young green manure, this ratio is a little more than 1:1 and it is a little more than ½ for advanced green manure (4). For vegetable crops, the data are less precise. At harvest time it can be seen that some vegetables such as leek, tomato, celeriac and carrot produce a lot of roots, but this is not the case for lettuce. In general, vegetable crop residues produce little recoverable dry matter.

The advanced green manure is probably the most approximate model for vegetable crops. For an area of 100 m² fully used during the whole growing season and taking into account that about 80% of the mass is lost, the humus produced by the roots of this green manure should be about 27/2/80% = 2.7 kg/are. This gives a good idea of what is left in the soil for vegetable crops after harvesting and what needs to be added in the form of compost to compensate for the total losses. The ratio of humus from compost is about 24.3 kg/are (36.45 for a 30 cm thickness of topsoil). This value is approximate, as some crops consume more humus and others less. In addition, other uncontrollable factors are involved in humus mineralisation, such as climate. Humus losses are therefore always changing from one year to the next.

To satisfy the total needs of certain exportable plants that are greedy for mineral salts, it is necessary to provide more organic matter used as a base fertiliser, from which the mineral fertiliser contribution must be deducted (see the article: Examples of reasoned fertilisation for some vegetable plants). For other plants that require less mineral salts, it is necessary to plan a fertilisation that is less rich in organic matter in order to avoid losses of mineral salts to the water table.

It is estimated that about 50% of the mineral salts taken up by plants end up in the crops, the rest in the ground litter and in 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 the CTIFL (5) have shown losses of about 50% with composting of biowaste 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; about 50-60%. On the other hand, kitchen waste produces very little humus (about 5%). For manures, losses would vary between 80 and 92% (6).

For farms with hectares of land, the amount of organic matter to be composted quickly becomes a problem. The average surface area of French farms being 55 hectares (7), and for the soil to retain a humus volume of 3%, approximately 148.5 tonnes of humus are needed to limit annual losses alone, or 742.5 tonnes of fresh organic matter. It is easy to understand that in 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. that produce sufficient waste. If he wants to maintain his yields, the farmer has no choice but to use mineral fertilisers, which have the advantage of also increasing the recyclable plant waste (including underground litter).

Restructuring of humus-deficient soil

For a complete restructuring of a humus-depleted soil, much more fresh organic matter is needed. Let's imagine a 100 m2 plot of desert land to which we want to incorporate 3% humus to a thickness of 20 cm. The amount of humus to be incorporated would be about 900 kg, or 4800 kg of fresh organic matter. This is generally the volume of humus that needs to be reconstituted for most vegetable gardens that have never been the subject of a soil biodiversity analysis and restructuring project, including vegetable gardens that have never received mineral fertilisers with blind applications of organic fertilisers that are not adapted to the soil characteristics. This is a very common situation and is often the cause of many of the problems encountered.

For a 55-hectare farm, about 4950 tonnes of humus or 24950 tonnes of fresh organic matter are needed. For the amateur gardener, as for the farmer, it is not easy to find such large quantities of fresh material unless the soil is restructured over several years. Collecting and processing 5 tons of fresh organic matter over 2 to 3 years is still feasible for a home gardener if he takes the trouble to look for organic residues: recovery of manure from a riding school or a sheepfold, lawn clippings and hedge trimmings from neighbours, collection of dead leaves in the autumn....