As every arable farmers know the art of seedbed preparation is a vital part of securing a healty crop and high yield. In this blog we run through the requirements of a good seedbed and how best to create one.
Seedbed requirements for good crop emergence
The perfect seedbed provides the seed with conditions that will result in a fast and uniform crop emergence. This requires water, air, warmth and an environment free from diseases.
A good seedbed starts the pathway to a high crop yield. The seedbed is a nursery for the germinating seed and must provide the right conditions in order to allow the crop to emerge quickly and uniformly.
The most important properties of the seedbed are to:
- Absorb rain and weather without erosion
- Act as a barrier to evaporation
- Transport water to the germinating seeds
- Act as a reserve of nutrient, water and oxygen.
The ideal seedbed should start with a layer of coarser aggregates, including organic material that protects against crust formation, followed by a layer of finer aggregates that prevent soil moisture from evaporating off and create good contact between seed and soil.
Water is transported to the germinating seed through capillary transport from below, which requires good contact between seed and soil. On light soils, but also on heavy clay soils, this capillary transport of water is weak and it is particularly important to utilise the moisture that is present in the soil from the start.
Four basic seedbed requirements
Seedbeds can vary in different ways, but in order to accomplish their task of providing good growing conditions all seedbeds need the following:
- An environment free from diseases
- Water around the seed
In the case of cereal crops, germination starts with the grain soaking up water. The grain germinates when the water content in the grain increases from 13-14% up to 45-60%. Since the water is taken from the soil around the seed a good seed to soil contact is essential. This means that the soil particles around the seed should not be too coarse. A good rule of thumb is that at least 50% of the particles in the seedbed should be less than 5mm in diameter in order to ensure emergence even in dry conditions.
Drilling depth is also very important for water availability. The right drilling depth is a compromise between sufficient depth to avoid drying the seed and a shallow enough depth to give high emergence and good plant density
A good rule of thumb here is that the drilling depth should be 10 times the diameter of the seed, for example, rapeseed has a diameter of 1.5-2.0mm and thus the best drilling depth is approx. 15-20mm. According to this principle larger seed such as peas and beans are positioned deeper in the soil, where there is often moisture in the seedbed. On the other hand small seeds must be placed at shallow depth, where it is more difficult to guarantee that there is sufficient soil water. However this rule should never compromise the water supply by placing the seed at too shallow depth. Instead, the seed should be placed where there is the best moisture.
- Air in loosened soil
Plants store nutrient reserves in their seeds or grains in the form of starches and proteins. These reserve nutrients must last until the plant is self-sufficient through photosynthesis. When the seed takes up water, this starts an enzymatic process that breaks down the nutrient reserves, this process requires oxygen, which is available in the air round the seed. Is it therefore important that the soil covering the seed is loose enough to allow air and oxygen to pass through. It is equally important that the carbon dioxide formed can be transported away. If a soil becomes waterlogged by heavy rain after sowing, this can cause oxygen deficiency and subsequent problems with germination.
- Warmth speeds up emergence
The soil is warmed up in spring mainly by the sun, but also indirectly by rain and air. The temperature in the seedbed has a great influence on seed germination and on the growth of the seedling. Wheat, barley and oats can germinate at around 3-5°C, but prefer an average temperature of around 20°C for fast establishment. Dry, porous soil warms up more easily than damp or waterlogged soil. The higher the soil water content, the slower the soil temperature rises in spring.
- Decrease diseases
A varied crop rotation will help to keep the seedbed a free from disease as possible. Another important factor is to ensure that plant residues from the previous crop are decomposed before sowing the next crop. This decreases the potential disease from old crop residue and ensures that last year’s trash do not obstruct the emergence of the next crop.
We covered different tillage methods in previous blog post however we touch on them again here as we look at the different methods of creating a seedbed.
- Conventional technique - ploughing in of straw, cultivation to sowing depth with a tine/disc cultivator, seed drilling.
- Minimal tillage – tillage of straw by cultivator, seed drilling usually with a cultivation drill such as a Vaderstad Rapid/Horsch Pronto DC.
- Shallow tillage – shallow burial of straw at the surface, seed drilling with a cultivation drill such as a Vaderstad Rapid/Horsch Pronto DC.
- Direct drilling – seed drilling directly into the stubble and harvest residues using a disc drill such as a Moore Unidrill or tined drill like a Claydon.
The technique used depends on many different factors, e.g. harvest residues, the equipment available, soil type, climate, labour requirement, etc.
Ploughing warms up the soil and buries trash/harvest residues so that they do not cause problems when drilling. However, ploughing disrupts the soil structure and increases oxidation of the organic material. Without ploughing, the organic material and the soil structure are retained, but on the flip side the straw can cause problems with sowing and can transmit straw born diseases.
Managing harvest residues
An important difference between the techniques is how they manage harvest residues left from the last crop. Plant residues affect the possibilities to achieve good contact between soil and seed. If there are large amounts of straw remaining, more tillage is required to ensure that the straw is broken down and does not impede the establishment of the next crop. The type of plant residue left from the previous crop determines how fast it decomposes and which machinery handles it best.
To bale or not to bale?
If the straw has an alternative value i.e. in care of animal stock, it is often baled and collected in. Alternatively the straw is buried during cultivation and the decomposing harvest residues can improve the structure of a soil. Removing the straw can make tillage easier in the short term but it may become more difficult in the long term if the amount of organic material decreases too much.
Justifying tillage costs
The choice of tillage technique can also be affected by the value of the following crop. If the crop being planted has a high value it can balance out the costs of ploughing, if ploughing leads to better establishment. Also if there is a high risk of transmission of straw born disease full inversion tillage may be necessary. Problems with difficult grass weeds such as blackgrass may also need to be controlled by ploughing.
The balance of reconsolidation
Getting the perfect reconsolidation can be tricky, optimal reconsolidation is 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.
Too little reconsolidation, i.e. soil too loose around the seed, can lead to lack of capillary transport of water because the pores are too large. This means that the soil round the seed can become too dry so the seed dries out and the seedling wilts.
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. The soil can become water logged and there can be a lack of oxygen, which kills the roots.
Reconsolidation is very dependent on the soil type, on lighter soils light weight machinery can have the desired effect, but on a heavy clay soil much heavier machinery is needed to cultivate and reconsolidate the soil.