Texture and structure of cultivated soils

Why such wide ranges, especially for humus content, which varies significantly from one source to another? It is difficult to reach a consensus on an optimal value for each element that would be valid for everyone, as this value depends in particular on the nature of the crops that the soil must support. Some vegetables thrive in humus-poor soils or require different compositions of sand, clay or limestone.

For example, asparagus and gherkins like sandy soils, garlic is not very demanding in terms of mineral salts, although these must be balanced, and garlic hates soils rich in fresh organic matter. Cucumber cultivation requires substantial fertilisation. It is common to read in organic farming literature that soils well supplied with compost automatically regulate themselves to produce good garden soil, which is not always accurate. The binding of clay with organic matter can be slowed down if the soil is low in iron. Soil that is too wet, poorly drained or low in calcium, even if it receives a lot of organic matter, will eventually become destructured.

Water and gases present in the soil

In well-aerated soil, ¼ of the volume of topsoil is in the form of gas and ¼ is taken up by water. The water present in the soil is never in its pure state. It contains gases and dissolved mineral salts, some of which are nutrients for plants. This water with its dissolved salts is referred to in agronomy and pedology as soil solution.

The gaseous fraction of soil is called the soil atmosphere. Properly aerated soil contains all the gases present in the atmosphere, but in different proportions. The values are approximately as follows: 15% oxygen, 80% nitrogen, 3% CO₂, and about 2% methane and other gases. However, these values may vary depending on the nature of the soil, vegetation cover, seasons, climatic conditions, etc. These gases are largely produced by biological activity in the soil, particularly the decomposition of organic matter by microflora.

Soil compaction

Crusted soil is characterised by surface hardening and reduced porosity. Crusting occurs when rainwater or irrigation water no longer infiltrates the soil. Ploughing is sometimes blamed for this soil change. However, ploughing aerates the soil and reduces compaction, promoting deep water penetration. Topsoil compaction is mainly caused by local geographical conditions, climate, and a lack of sufficient humus and limestone inputs to correct nutrient depletion. This is the case, for example, with soils that are too clayey, even though their structure can be quickly improved by adding lime. Laboratory analyses can determine whether a soil is prone to compaction.

When discussing the causes of soil compaction, it is important to avoid making hasty judgements that do not take local conditions into account. The geographical location of certain fields encourages water accumulation, such as hollow ground located near sloping structures.

Crimping can also be the result of poor natural drainage or caused by infrastructure (agricultural machinery, roads or paths forming a barrier to rainwater runoff, etc.), or by trampling by farm animals, heavy and repeated rainfall, etc. Soil can also naturally contain too much fine silt. Being very light, fine silt tends to rise to the surface, blocking the soil’s porosity and forming a hard crust. It is therefore important to be wary of photos presented on certain websites which, contrary to what their authors claim, do not necessarily reveal hardpan caused by agricultural activities, particularly ploughing.