The size of aggregates in the seedbed controls the amount of water that can evaporate. With an aggregate size of approx. 2mm, water evaporation is minimised. Straw at the soil surface also decreases water losses through reflecting solar radiation and preventing the soil surface from heating up.
If rain does not fall after sowing, the water present in and under the seedbed is critical for how well the new crop can establish. It is important to retain this water and manage it carefully if the seed is to germinate.
Sun heats up the soil
When the sun rises and begins to shine on a newly sown field, the energy from the sun’s rays heats up the water in and under the seedbed. Some of the water molecules acquire enough energy to convert to gaseous form and attempt to move out of the seedbed and into the air as water vapour.
This evaporation of water can often be seen with the naked eye as wet soil is heated by the sun’s rays, as shown above. In principle, this is the same phenomenon as when a saucepan of water boils on a hob and loses water in the form of steam.
Silt allows water losses
Evaporation of water from the soil surface after sowing is mainly controlled by the size of the aggregates in the seedbed.
Soil particle. Source: Heinonen, R. 1985. Soil management and crop water supply. Dept of Soil Sciences, Swedish University of Agricultural Sciences, Uppsala. Figure 26, page 71.
Figure above illustrates the fundamental relationship between water evaporation and soil particle/aggregate diameter. A first maximum in evaporation occurs at particle size 0.005-0.02mm. This is approximately the particle size area for silt and reflects capillary transport of water to the soil surface from the seedbed. On such silt soils it is important to interrupt capillary transport so that water is not lost.
Coarseness creates turbulence
A second maximum in evaporation rate is reached when the aggregate size exceeds 50mm, which is often the case in soils with high clay content. With such coarse aggregates in the seedbed, the air flows become turbulent and the seedbed dries out. Between these peaks, there is a minimum water evaporation case where the aggregates have a diameter of around 2mm. These aggregates are not small enough to permit capillary transport of water, but not large enough to create turbulent air flows. With aggregates of this size in a seedbed, a lid is put on and evaporation of water is minimised.
Winter wheat, 3 weeks after drilling
A: Aggregates < 2 mm give 95% emergence
B: Aggregates 2-5 mm give 60% emergence
C: Aggregates > 5 mm give 35% emergence
This can be demonstrated using pure aggregate sizes in a model experiment, see image above. In other words, it is aggregate size that regulates evaporation of water from an open soil.
Straw reflects rays
Plant residues such as straw also affect the amount of water that evaporates from the soil. Straw at the soil surface affects water losses in at least two ways:
- The light-coloured straw reflects the sun’s rays, while the often dark-coloured soil absorbs solar energy
- The straw can interrupt upward capillary transport of water
Together, these two factors mean that the soil surface does not heat up as much in the spring and evaporation of water is restricted.
Reduced tillage can often reap the benefits of this effect. Better water retention in combination with better protection against erosion means that reduced tillage is the dominant tillage system in dry agricultural areas such as the prairies of the USA and Canada.