Seedbed fundamentals

Seedbed preparations

There are several approaches that can be used in achieving a good seedbed preparation. The deciding factor in choosing approach is how the various techniques manage harvest residues.

The seedbed lays the foundations for crop establishment. However, there are different techniques that can be used to create a seedbed:

The technique used depends on many different factors, e.g. harvest residues, the equipment available, soil type, climate, labour requirement, etc.

Tillage warms up the soil and buries plant residues so that they do not obstruct seeding. However, tillage disrupts the soil structure and increases oxidation of the organic material. Without tillage, the organic material and the soil structure are retained, but the straw can cause problems with seeding and crop emergence if present in large amounts.

The seedbed and seeding using different techniques

1. Conventional tillage, separate fertilizer application - tillage of straw, cultivation to seeding depth with a tine/disc cultivator, fertilizer spreading prior to seeding.

2. Tillage of straw, shallow cultivation, seeding operation where seed and fertilizer are placed in the soil simultaneously.

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3. Minimal tillage – tillage of straw by cultivator, seeding operation where seed and fertilizer are placed simultaneously in the soil/straw layer.

4. Ultra-Shallow tillage – shallow burial of straw at the surface, seeding operation where seed and fertilizer are placed simultaneously in the soil/straw layer.

5. Direct seeding (zero-till) – seeding operation where seed and fertilizer are placed simultaneously without prior soil tillage. The straw remains on the surface.

 

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Managing plant residues

An important difference between the techniques is how they manage plant residues. Plant residues affect the possibility of achieving good contact between soil and seed. If there are large amounts of straw remaining, tillage is required to ensure that the straw does not impede the next crop and is broken down as efficiently as possible. The type of straw present from the previous year determines how fast it decomposes and how effectively it can be handled by machinery.

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Baled or buried

The straw can be baled and removed or buried in the soil. Decomposition of plant residues can improve the structure of a soil with poor structure. If the straw has an alternative value in animal production or for heating, it is often baled and collected in. This makes soil tillage easier in the short term. However, tillage may become more difficult in the long term if the amount of organic material decreases.

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Tillage sometimes justified

Quick and even crop establishment is a critical component of crop success.  If residue amounts from the previous crop are present in large quantities, it is necessary to manage the residue using a tillage operation. Another aspect to consider is the pressure from weeds. Mechanical weed control is necessary in organic systems and in fields where herbicide resistance has become an issue.  Finally, the ability of the seeding machinery to handle large amounts of plant residue is another factor to consider.

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Optimal reconsolidation

The aim of seedbed reconsolidation is to create good contact between seed and soil in order to provide an optimal supply of water, nutrients and oxygen to seeds and roots according to figure "Reconsolidation of the seedbed".

Too little reconsolidation, i.e. soil too loose around the seed, can lead to malfunction of capillary transport of water because the pores are too large. This means that the soil around the seed can become too dry. Too much reconsolidation means instead that the large pores are compressed and become less effective in regards to draining away excess water as well as transporting oxygen and carbon dioxide to and from from the seed. This can lead to oxygen deficiency for the roots. On lighter soils (medium and fine sand), light machinery has the best effect, whereas heavy clays need heavier machinery to cultivate and reconsolidate the soil.  

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Reconsolidation of the seedbed

Soil consists of around 50% solid material, with 50% pores. In the ideal case, half the pores are filled with water, half with air. However this varies widely depending on how much rain has fallen, the structure of the soil and how it is tilled.

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  1. Too little reconsolidation, i.e. soil too loose around the seed, can prevent capillary transport of water because the pores are too large. This means that the soil around the seed becomes too dry, so the seed dries out and the seedling wilts.
  2. Optimal reconsolidation provides good contact between seed and soil, so the seed is supplied with water through capillary transport. At the same time, there are enough large pores to transport oxygen.
  3. Too much reconsolidation means in contrast that the large pores are compressed and become less effective as regards draining away excess water and transporting oxygen to and carbon dioxide from the seed. This can lead to oxygen deficiency, which kills the roots.
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Dictionary:

Capillary = water that can rise upwards in the soil within the fine pores through binding of the water molecules in the pores, adhesion, but also through attraction between water molecules, cohesion. Silty soils have high capillarity and combine a large height of capillary rise with a high rate of capillary rise

Precrop = the precrop is the crop grown before the present growing season, i.e. the previous year’s crop, and it affects the current year’s crop through the amount of residues it leaves, the nitrogen release from these, soil structure, disease pressure, etc

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