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Soil diagnosis

Soil analysis and testing

Soil analysis and testing is fundamental for a high yield. With a spade and a careful inspection, it is easy to get a picture of the condition of the soil. This is because the appearance of the soil and its behaviour reflect its status and function.

The soil is a complicated system in which chemical, biological and physical processes have to act together so that the crop can grow and provide a high yield. For example, movement of water and air is an important function in the soil that is controlled by the structure.

Immediate insight

Therefore it is a good idea to keep a check on the conditions and status of the soil. This can be done quite easily with a spade and a close look, since conditions in the soil are often reflected in its appearance. It is often possible to get an immediate insight into the condition of the soil by looking at it and considering how it has been treated in the past.

The basic tip is to dig to around 30cm in order to get a view of the whole topsoil, while digging deeper into the subsoil gives an even better impression of soil properties. A number of different methods are used in different countries to make a diagnosis and assess the health of the soil. Some are rather advanced, but most are easy to perform. 

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Porosity

Much to look at

A feature in common for these and for most methods is that the soil is assessed on the basis of:

  • Texture – a roll test gives a quick answer regarding the clay content.
  • Structure – can be assessed by counting the number of stamps needed to force the full blade of the spade into the soil or by dropping a lump of soil onto a hard surface and examining the size of the aggregates formed as it breaks up
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  • Porosity – the amount of pores visible on breaking open an aggregate indicates how different the conditions for the roots can be
  • Colour, smell and taste (!) – the soil should smell fresh, a smell of sulphur is a bad sign.
  • Earthworms  – in one spadeful of topsoil there should be 2-4 earthworms. 0 earthworms is a warning sign – the more the better
  • Plough or tillage pan and other hard layers – a dense mat of roots in a zone is an indication of soil compaction

Digging deeper can also give an impression of hydraulic conductivity and a closer look at the nature of the aggregates. Rooting depth indicates what the soil is like as a growing site, as does number of roots and their diameter.

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Dig several pits

A good starting point is to dig a pit in at least 2 places in the field – one where crop growth is good and one where it is poor. Comparing the best spot in the field with e.g. a compacted headland can provide interesting information. It shows up contrasts and provides a better chance of finding differences in the soil that can explain differences in growth. Another tip is to dig at a point with a permanent plant cover outside the field where there is no machine traffic.

A pit that represents the average values for the field also provides information on the normal conditions. The number of pits dug depends on the time and energy available, but it is better to dig many pits and get a broader impression than to go into detail too much from the start.

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Dictionary:

Hydraulic conductivity = the amount of water that can infiltrate into the soil within a certain time is a good indicator of how well the soil is functioning from a soil physical perspective

Plough or tillage pan = the plough pan is the compacted boundary zone between topsoil and subsoil just below plough depth, which is often compacted by the plough and by wheel slip in the furrow. A tillage pan can develop at depth within no-till cropping due to repeated wheeling in wet conditions. A characteristic of both these is decreased soil permeability for water and air, since the largest pores are compressed together, preventing the roots from growing

Roll test = quickly rolling moist soil in a light grip between forefinger and thumb gives an idea of soil texture. A silt soil gives a roll of 4-6 mm, a light clay approx. 2mm and a heavy clay approx. 1mm.

Texture = refers to the proportions of mineral particles with different average diameter, i.e. the relative proportions of sand, silt and clay in particular according to table "Particle size distribution" in chapter The building blocks of soil.

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