The task of the seedbed is to provide the seed with conditions resulting in fast and uniform emergence. This requires water, air, warmth and an environment free from diseases.
The ideal seedbed lays the foundation for high yield. The seedbed acts as a nursery for the germinating seed and must provide the right conditions in order to allow the crop to emerge quickly and uniformly.
The ideal seedbed
The most important properties of the seedbed are to:
- absorb heavy rain, providing stability against crusting and erosion
- act as a barrier to evaporation
- provide capillary water transport for germinating seeds
- act as a nutrient, water and oxygen reserve, promote root development
The ideal seedbed should have the appearance shown in the picture with, starting from the top, 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 all seedbeds need to provide the seed with these fundamental conditions:
- an environment free from diseases
Water around the seed
In the case of cereal, germination begins with the grain taking up water. The swollen grain germinates when the water content in the grain increases from 13-14% up to 45-60%. At least 6% plant-available water is needed around the seed to ensure a reliable water supply and emergence. To ensure that the grain will have access to water, it is also important to create good contact between seed and soil, since the water is taken from the soil around the seed. 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 aggregates in the seedbed should be less than 5mm in diameter in order to ensure emergence even if no rain falls after sowing.
Right depth for every species
Sowing depth is a compromise between sufficient depth to avoid drying the seed and sufficiently shallow depth to give high emergence and good plant density.
Sowing depth is also very important for water availability. The right sowing depth is a compromise between placing the seed at sufficient depth to find enough water for germination and sufficiently shallow depth to allow rapid emergence as in figure above.
As a rule of thumb, sowing depth should be approx./max. 10 times seed diameter. For example, rapeseed has a diameter of 1.5-2.0mm and thus the optimal sowing depth is approx. 15-20mm. However these figures are only a guideline – never compromise water supply for the seed and place it where there is sufficient moisture.
A good rule of thumb here is that the sowing depth should be 10 times the diameter of the seed, as illustrated in figure above. According to this principle, peas and beans are positioned deeper in the soil, where there is often moisture in the seedbed. Rapeseed, on the other hand, must be placed at shallow depth, where it is more difficult to guarantee that there is sufficient soil water. However, water supply should never be compromised by placing the seed at too shallow depth. Instead, the seed should be placed where there is moisture.
Air in loosened soil
Plants store nutrient reserves in their seeds, fruits or grains in the form of starch, oils or proteins. These reserve nutrients must last until the green plant parts can supply the plant with energy through photosynthesis. When the seed takes up water, this starts an enzymatic process that breaks down the nutrient reserves during respiration. This process requires oxygen, which is available in the air round the seed. Therefore it is 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 during respiration 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 solar radiation, but also indirectly by rain and air flows. The temperature in the seedbed has a great influence on how rapidly the seed germinates and on seedling growth. Wheat, barley and oats can germinate at around 3-5°C, but prefer an average temperature of around 20°C for fast establishment. The temperature of the soil is the result of an interplay between heat capacity, thermal conductivity and evaporation. 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 with a crop rotation
In order to ensure that the seedbed is as free from diseases as possible, a varied crop rotation should be used. The ideal is for monocotyledons and dicotyledons to be varied so that they occur alternately in the crop rotation. Another rule of thumb is to ensure that plant residues from the previous crop are decomposed before sowing. This decreases the potential disease pressure and ensures that the residues do not obstruct the emergence of the next crop.
Respiration = cell respiration is the process by which nutrients in the cell are broken down to create energy – in the case of seed, starch, proteins and oils are broken down to give the seed (or the grain) the energy for germination
Enzymatic process = enzymes are proteins that control chemical reactions in the cell through increasing or decreasing the rate of processes
Carbon dioxide = gaseous waste product (CO2) of cell respiration in the roots that is also the building brick together with water for sugars created by the plant through photosynthesis
Heat capacity = the amount of warmth/energy (kJ) required to increase the temperature of 1 kg of a material by 1°C
Thermal conductivity = the capacity of a material to conduct heat
Monocotyledons = plants that germinate from seed to produce a seedling with only one seed leaf (cotyledon), e.g. grasses and cereals
Dicotyledons = plants that germinate from seed to produce a seedling with two seed leaves (cotyledons), e.g. oilseeds, peas, beans, linseed, sugar beet, etc