To plow or not to plow?

The advantages of different methods of cultivating soil

Although soil cultivation methods are considered aggressive by their detractors because they alter the soil structure using tools, these techniques nevertheless have advantages that are recognised even by those who practise organic farming. Here is a non-exhaustive list of these advantages:

• Ploughing does indeed increase oxygenation of the soil at depth, but this allows beneficial aerobic bacteria to colonise a larger volume of topsoil. These bacteria play an important role in the balance of the soil ecosystem and its fertility. Colonies of rhizobia (nitrogen-fixing bacteria) need oxygen, and ploughing therefore increases their presence in deeper layers of the soil.
• Increased oxygenation through ploughing strengthens the aerobic microbial agents involved in soil health. For example, Pseudomonas spp, a strictly aerobic bacterium that thrives in the rhizosphere, is known to protect plant roots (see the article on the rhizosphere and suppressive soils). It creates an adhesive, protective biofilm called microbial mucilage. This bacterium is also known for its ability to solubilise iron

• Manure used directly as fertiliser has the disadvantage of losing some of its nitrogen through the volatilisation of ammonia during its conversion to nitrate. To limit losses through volatilisation, immediate burial is an effective technique, as is the case for slurry.
• Deep ploughing followed by tilling to reduce clods prevents root vegetables such as carrots and chicory from degenerating (forked roots).

Carottes fourchues cultivées dans un sol insuffisamment émietté.

• Potatoes are very demanding in terms of soil preparation, especially as they are fast-growing plants: 90 to 120 days; it is therefore important to encourage root development by mechanically working the soil to a depth of 15 to 20 cm. To obtain well-loosened soil, deep tillage using a chisel plough is particularly desirable in clayey and loamy soils. Due to the specific requirements of potatoes, a thin planting layer of approximately 10 cm must be created, most often achieved in large-scale farming using scarifiers or vibroculteurs.
• The renewal of bacteria on the surface is very rapid. The distribution of anaerobic and aerobic bacteria in the different horizons is corrected very quickly.
• Deep tillage creates numerous pores and microcracks, allowing roots to explore the active zone of the growing medium more easily. Some crops are very demanding in terms of soil structure quality. For example, sunflowers are very sensitive to areas of compaction. Any obstacle to their development can cause them to lose 5 quintals/ha and reduce their oil content (18). Deep tillage is therefore necessary, as it is for certain vegetable crops (melons, leeks, endives, carrots, etc.). It is much easier to achieve deep tillage with mechanical tools than by leaving this work to earthworms, whose action can be hampered by uncontrollable factors, particularly climatic ones.

• Legumes have nodules on their roots that contain bacteria that fix atmospheric nitrogen. These bacteria also consume oxygen to form nitrates, which help enrich the soil with nitrogen. This is why nodules are more numerous in the top few centimetres of soil. Maintaining the soil by working it deeply to facilitate gas exchange with the atmosphere increases the thickness of this zone of biological activity and, at the same time, the soil’s fertility potential.

Nodosités sur des racines de haricots

• Very fine particles suspended in water tend to be carried away by rain and concentrated in layers of varying depths, which in turn causes surface erosion. After a few years, hardened areas form at depth and biological activity decreases. Deep tillage eliminates these hardened areas and repositions the very fine particles in the topsoil. The way in which clay is carried deep underground is described in another article by clicking here.
• In ploughed soil, which is more porous, there is less fertiliser loss due to runoff. Tilling facilitates the flow of irrigation water or rainwater to the roots, which at the same time receive more nutrients carried by the water. Unabsorbed mineral salts are more easily fixed deep in the soil by CAHs, whereas they would be lost through runoff if the soil were not sufficiently tilled.
• Ploughing is very useful for controlling weeds by burying them, which helps to enrich the soil with humus. Ploughing is known to be effective against bird’s knotweed, for example. 90% of brome seeds (locally harmful in winter cereals, known to be favoured by simplified cultivation methods) disappear after ploughing. Ploughing to a sufficient depth alters the establishment of woody weeds (phanerophytes, chamaephytes) and herbaceous species with stumps (hemicryptophytes) such as rumex (19). The effect on annual plants is more or less nuanced, as ploughing brings seeds produced in previous years to the surface if they have withstood climatic and biological disturbances.
• For perennial species, destruction can be partially offset by the germination of rhizome fragments. Success will depend on how these propagules are treated (depth and frequency of ploughing, collection of rhizome residues, etc.) and on climatic conditions. For example, ploughing in dry conditions in summer promotes the drying out of bindweed and creeping thistle rhizomes. After ploughing in summer, a toothed tool can be used to remove the dried rhizomes. Mechanised ploughing has made it possible to relegate once-common rhizome weeds, such as creeping bentgrass, to the edges of fields.
• Ploughing is an effective tool for disrupting the life cycle of pathogenic microorganisms such as fusarium, a devastating disease that affects soft wheat and certain vegetable crops (provided that the residues are finely shredded before being buried and that crop rotations are implemented to avoid incompatible combinations). No-till crops are more vulnerable if potentially contaminated residues are left on the surface and no treatment has been carried out before harvest to combat fusarium.
• Autumn ploughing facilitates the destruction of the eggs and pupae of harmful insects that have burrowed into the soil to protect themselves from the winter cold, exposing them to birds or frost. Some pests, such as wireworms, burrow deep into the soil to withstand the cold. Another example: the larvae of the celery fly (Philophylla heraclei) overwinter in the soil at a depth of 5 to 10 cm
• Shallow ploughing of a plot allows winter frost to destroy celery fly larvae without the use of pesticides. However, some beneficial ground beetles that overwinter in the soil as larvae or adults are also destroyed, but their population can be largely maintained by preserving semi-natural habitats near crops.
• It should be noted that shredding plant matter left on the surface, as suggested by some proponents of direct seeding, is not sufficient to eliminate pests, as the soil is not turned over to bring the pests or their eggs to the surface

While it is true that mechanisation in large-scale farming has led to deeper ploughing exceeding 30 cm, in recent years there has been a return to shallower ploughing in order to reduce organic matter losses. To this end, manufacturers offer stubble ploughs with low traction power that work the soil to a depth of 10 to 15 cm. Some stubble ploughs with ploughshares also allow mechanical weeding to be carried out while respecting the deep structure of the soil

Cross-section of the soil in the author’s garden at Oraison showing the thickness of the topsoil maintained by ploughing and certain soil conservation techniques (pseudo-tillage and shallow tillage)

When it comes to maintaining cultivated soil, there is no single ideal technique that supersedes all others; only techniques that are suited to the soil and climate conditions of the location.

For amateur gardeners, soil cultivation alternating between ploughing, pseudo-ploughing and shallow cultivation is still the most practical method of soil maintenance, provided that a minimum amount of compost is spread each year to compensate for humus losses.

Deep soil cultivation carried out correctly with periodic additions of medium and long-term composts ultimately produces topsoil corresponding to the thickness of the cultivated soil. This layer of topsoil improves every year, and it is easy to ensure that this is the case.

If we remove the topsoil layer, we can see a colour difference between the worked soil and the lighter underlying layer. The photo above shows a hole about 40 cm deep dug in my vegetable garden, revealing the topsoil layer about 30 cm deep and the lighter underlying layer. Fifteen years ago, the original soil was even lighter in colour, as the underlying layer had since received organic matter from the topsoil through infiltration