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Soil analysis & protection

Since you soils constantly is in a changing process, it is important to be able to asses in which condition it is in. How does the soil look? And which structure does it have?

To dig up a piece of soil and perform a soil analysis, is something you will benefit from during your future farming. We would like to share the knowledge we have collected throughout the years. Below you find guides and facts about soil analysis, soil compaction and erosion.

Soil diagnosis

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.

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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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Soil compaction

Soil compaction results in compression of pores that would otherwise transport water and air. This impedes root growth and can cause oxygen deficiency. Soil compaction can lead to a severe decrease in yield. 

Soil compaction by definition means that the density of the soil increases when it is compressed. In other words, the soil becomes denser and every kilogram of soil weighs more when the pores are compressed. It is often easy to understand and gauge the effects of soil compaction from watching a tractor tire roll over loose soil in wet conditions.

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Roots impeded

Soil compaction results in root growth being restricted and affects mechanical resistance to root development in two ways:

  1. Compaction decreases the number and size of large pores, macropores. As a result, there are fewer pores with diameter larger than roots in which the roots can grow freely, without mechanical resistance
  2. Compaction increases the mechanical resistance of the soil through pressing soil particles more closely together
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Water transport restricted

Soil compaction also restricts the movement of water down through the soil. This causes water saturation in the upper layers, which in turn can lead to oxygen deficiency for the roots according to image above. In addition, soil aeration status affects the availability of various plant nutrients, e.g. nitrogen and manganese. Under anaerobic conditions, denitrification can lead to severe loss of nitrogen in the form of nitrogen oxide or nitrogen gas to the atmosphere. Soil compaction can thereby decrease nitrogen availability in the soil.

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Water transport restricted due to compaction

Essential air: Guideline values for soil air-filled porosity: >25% air means good aeration, 10-25% air can result in limitations in certain conditions, and <10% air is characteristic of oxygen deficiency.

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Possible remedial measures

To avoid soil compaction, it is important to employ remedial measures that can contribute to better soil structure in the long term. Such measures include drainage, structure liming, keeping the soil covered with vegetation and supplying external organic material. These measures, which result in the soil being drier, decrease soil compaction at depth.

Cropping system and soil tillage are also critical for soil compaction. The most important consideration is to avoid tillage when the soil is too wet. A dry soil has greater bearing capacity for loads, while a wet soil is compressed under a similar pressure. Having a large contact area with the help of broad tyres or dual wheels results in a lower wheel load. Number of passes as shown by the results in image below is also important, as is keeping the total weight of field equipment as low as possible.

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A trial (L2-7118) at Önnestad, Skåne, in 2000 examined how soil compaction before spring sowing affected the yield of different crops (barley, wheat, oats, sugar beet and peas). The compaction consisted of different numbers of passes with a heavy load over the field before sowing:

  1. Sowing without compaction
  2. One pass + sowing
  3. One pass with a slurry tanker + sowing
  4. Three passes with a slurry tanker + sowing
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The results showed that yield of spring wheat, barley and oats was positively affected by some compaction, i.e. one pass + sowing. For sugar beet and peas, however, yield was decreased even by this treatment. When the number of passes and thus the amount of soil compaction was increased, yield of all crops decreased. The crop most negatively affected was peas, which are very sensitive to oxygen deficiency. 

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Soil erosion

Soil erosion from arable land by water or wind can transport away large amounts of soil. Plants or plant residues on the surface provide protection and for that reason minimal tillage is a measure against erosion.

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With a cover of vegetation, the soil gains a protective layer and is not directly exposed to the effects of rain and wind. However, cultivated and bare soil can be severely affected by water erosion and by wind erosion.

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Soil affected by water erosion

Water erosion

On sloping land, water erosion can be severe when rain beats down on bare soil without a plant cover or plant residues on the surface. The clay particles carried off by rain are accompanied by particle-bound phosphorus, which collects at the lowest point in the field or is lost to drains.

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Soil affected by wind erosion

Wind erosion

On dry, bare soil, wind erosion can cause damage when bouncing sand grains whip the emerging crop. As this photo shows, slurry can reduce wind erosion by binding the soil and preventing soil movement.

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Water carries away P

Heavy rain and intensive snowmelt can transport soil away from arable land. This water carries away soil particles, plant nutrients and organic material to streams, lakes and seas. Sloping fields are the worst affected. The steeper the slope, the greater the erosion since the speed of water runoff increases.

A doubling in runoff speed down a slope gives water four times as great erosive force on the soil. Losses of phosphorus (P) can increase sharply with water erosion, since much of the phosphorus in soil is bound to the surface of clay particles.

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Bouncing in the wind

Wind can also shift particles around the size of a grain of sand or less when the soil is bare and dry. Material <1 mm bounces along over the surface, while material <0.1mm can be carried freely in the wind. Loess soils are formed as wind-borne deposits.

Wind erosion can be decreased by planting shelterbelts, adding manure to the soil surface and growing protective crops. The wide open prairie areas in e.g. the USA and Canada for example are severely affected by wind erosion.

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Minimal tillage helps

In such prairie areas or in areas of sloping arable land, various forms of minimal tillage are recommended and applied as a countermeasure to both wind and water erosion. A shallower tillage depth means that harvest residues are accumulated in the uppermost layer of the soil. This increases the amount of organic material in the upper soil layer, which in turn improves aggregate stability and renders the soil more resistant to the impact of raindrops and the drag of wind.

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Plant cover protects

Harvest residues that are left on the soil surface are another contributing factor to why minimal tillage protects against erosion. In general, both plant and residues on the surface effectively slow the rate of movement of water and wind over the soil surface. In purely physical terms, a growing plant cover or plant residues also actively protect against the impact on raindrops on the soil surface as one example.

Among our usual crops, a forage based cover crop provides the best protection, while cereal offers moderate protection. Row crops such as sugar beet or corn leave part of the soil bare and thus provide weaker protection. The worst option from an erosion perspective is a cultivated fallow with nothing growing and no crop residues left on the soil surface.   

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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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Dictionary:

Pores = soil pores are the spaces, channels and cracks in the soil, which are filled with either water or air depending on the actual water content of the soil

Denitrification = denitrification is a process that occurs in the soil at low oxygen concentrations, in which denitrifying bacteria convert plant-available nitrate (NO3). into nitrogen gas (N2). If denitrification is not complete, nitrous oxide (N2O), which is a potent greenhouse gas, is formed

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Dictionary:

Loess soil = porous, wind-deposited soil that often has particles in the size range around silt (see more in table "Particle size distribution" in chapter The building blocks of soil). Loess soils are found in Eastern Europe and Ukraine and can extend to a depth of over 100 metres

Contour ploughing = involves ploughing along the height contours of the land

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