Introduction to integrated methods in the vegetable garden
Texture and structure of cultivated soils
Chapter : Crop soil
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⇒ Soil texture and structure.
Introduction to integrated methods in the vegetable garden
Chapter : Crop soil
Previous or next articles ; click on a title to go to the page
⇒ Soil texture and structure.
In pedology, texture defines the size of soil minerals in categories of size more or less related to the chemical composition of these minerals classified as clays, silts and sands. Organic matter particles, which vary greatly in size, are not included in this definition. The finest particles are mostly clay micelles with a diameter of < 2 µm. However, more precise analyses are able to show that there are also limestone particles of this size. The silt particles have a diameter of 50 µm to 2 µm, and those selected for sand of 2 mm to 50 µm. In laboratory analyses, silt and sand particles are often classified into two sub-categories according to their diameters. Silt, which consists mainly of fine quartz and feldspar particles, is the last stage of degradation of the parent rock produced by rivers or wind erosion. These silts may contain plant nutrients (it is known that the silts brought by the periodic flooding of the Nile are the origin of the ancient Egyptian civilisation). The composition of the sand includes at least 180 minerals from the parent rock or from living material such as shells.
A laboratory performs a particle size analysis according to a conventional classification after removing particles larger than 2 mm. The analysis report specifies the percentage of these rejects, and then the various preserved sediments are classified according to the size of their particles, from the smallest to the largest: clay, fine silt, coarse silt, fine sand and coarse sand. Most often, you will be shown a pie chart with slices indicating the ratio of these different elements in %.
The structure of the soil defines how its constituents, which are not only minerals, organise themselves to form aggregates. For example, a very clayey soil will tend to form compact layers that evolve into flakes after drying. Two main groups of aggregates are defined: macro-aggregates (> 250 µm) and micro-aggregates (< 250 µm). The association between these aggregates and the coarse elements (gravel, pebbles, large sand particles, shells and other debris) allows the formation of pores containing water and gases. A soil rich in clay, silt and organic matter is characterised by a large number of microspores with a diameter of less than 10 µm, which allow water and gases to circulate more easily.
A properly maintained cultivated soil usually contains 2-5% humus which tends to aggregate with clay or other mineral elements (colloidal silica, zeolite...) to form stable aggregates with the property of retaining plant nutrients. The fine, granular soils known to be most suitable for agriculture are made up of these stable, colloidal aggregates. The more colloidal fractions a garden soil contains, the more porous and light it is. The more structured the soil is, the more the soil can breathe, the more water can circulate, the more the root system of the plants can develop. The destructuring of the soil caused by a depletion of humus has negative consequences on its fertility potential.
One laboratory proposes a soil classification similar to the GEPPA texture triangle. This triangle was established from tests carried out on numerous materials and it situates the overall behaviour of the soil in relation to the 3 basic constituents: clay, silt and sand (there are other texture triangle models built on the same principle). For example, the laboratory will specify that the soil is sandy-clay with a medium presence of silt.
Crops need a soil structure that favours rooting, and more precisely a soil that is not compacted. Millennia ago, our ancestors quickly realised that by reducing compaction through tillage, plants germinate and take root more easily.
Various factors can contribute to soil compaction, in particular mechanical pressure (tractor wheels, trampling, etc.) that modifies the structural properties of the soil even if it is rich in organic matter. In periods of drought, compacted soil can become very hard, especially if it contains clay, which, when moist and because of its absorption capacity, improves the level of soil fertilisation.
In a natural environment or in a properly maintained cultivated soil, soil structuring is ensured by earthworms, which feed on plant debris, and by micro-organisms. Earthworms mainly help to aerate the soil, improve water circulation and recycle organic matter. Aerobic bacteria (which need oxygen) responsible for the humification of organic matter play a major role in the structuring of soils (anaerobic bacteria are mainly involved in the peatification process).
Source: Analyze your soil to get to know it better - Drome Chamber of Agriculture - Nov 2013
⒜ MOS = matières organiques stables.
Why such 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, because 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 sand, clay or limestone compositions. For example, asparagus and gherkins like sandy soils, garlic requires little mineral salts, although they must be balanced, and garlic hates soils rich in fresh organic matter. Cucumbers require a lot of manuring.
It is common to read in organic farming literature that soils well supplied with composts automatically regulate themselves to produce good garden soil, but this is not always true. The binding of clays 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 experience a destructured.
In a well aerated soil, ¼ of the volume of the topsoil is in the form of gas and ¼ is taken up by the volume of water. The water in the soil is never pure. It contains gases and dissolved mineral salts, some of which are plant nutrients. This water with its dissolved salts is referred to in agronomy and pedology as the soil solution.
The gaseous fraction of the soil is called the soil atmosphere. A properly aerated soil contains all the gases present in the atmosphere, but in different proportions. The values are approximately: 15% oxygen, 80% nitrogen, 3% CO2 and about 2% methane and other gases. However, these values can change according to the nature of the soil, the plant cover, the seasons, the climatic conditions, etc. These gases are largely produced by the biological activity of the soil and in particular by the processes of decomposition of organic matter by the microflora.
A compacted soil (battance in French) is characterised by induration on the surface and a decrease in porosity. The soil becomes slaked when rain or irrigation water no longer infiltrates the soil. The plough is sometimes blamed for this development of the soil. However, ploughing, because it aerates the soil and reduces compaction, favours the penetration of water at depth. It is above all the local geographical conditions, the climate, and the absence of sufficient humus and calcium inputs to correct the withdrawals, which are at the origin of soil compaction. This is the case, for example, with soils that are too clayey, although their structure can be rapidly improved by adding lime. Laboratory analyses are able to determine whether a soil is subject to compaction.
When talking about the causes of compacted soil one should be wary of making hasty judgements that do not take into account local particularities. The geographical position of certain fields favours the accumulation of water, such as sunken land near sloping structures. The compacted soil may also be subject to poor natural drainage or caused by infrastructure (passage of agricultural equipment, road or path blocking the flow of rainwater, etc.), or by the trampling of farm animals, repeated heavy rainfall, etc. A soil may also be naturally too rich in fine silt. Being very light, fine silt tends to come to the surface to block the porosity of the soil and form a compacted soil. It is therefore necessary to be wary of the photos presented on certain websites which, contrary to what is claimed by their authors, are not necessarily indicative of soil compaction caused by agricultural activities and in particular by ploughing.
Laboratory analyses, next page: Clay-Humus Complex and Cation Exchange Capacity
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