This year’s dry autumn brought us an interesting feedback regarding the sowing of autumn cereals. According to the story, after disc harrowing the stubble on the planting area, there was only enough time to prepare the seedbed on part of the area, while the seeds were planted without preparation on the other part. Unfortunately, there was no precipitation after sowing. The experience was that the plants emerged on the prepared seedbed, but not on the rest of the area. Even more interestingly, the cereals did not emerge from the neighboring no-till stubble either.
Obviously, this is the BUSA website, and we are glad it turned out this way, but instead of making a religious issue out of it, let’s try to understand why one happened and the other didn’t.
The story is particularly instructive because it highlights the importance of maintaining soil moisture. While this example highlights the advantages of traditional seedbed preparation, instead of exclusively promoting this method, it is worth examining the underlying reasons and mechanisms behind the processes.
In the autumn of 2024, water continues to be the bottleneck in Southeast Hungary. The short explanation is that there was enough moisture in the prepared seedbed to initiate germination, while in the other case, there wasn’t. Let’s take a closer look at how soil moisture and soil moisture processes play a role in such situations.
Let’s start with the simplest: relative humidity and dew point.
Air can hold different amounts of water vapor at different temperatures; the higher the temperature, the more it can hold. At a given temperature, the maximum water vapor content is the dew point, at which the air reaches saturation. When the air temperature drops below the dew point, condensation forms, for example, as dew.
This process can appear in a seedbed in two ways:
- Dew formation on the surface clods: The loose, small clods’ surface can cool rapidly at night, reaching or dropping below the air’s dew point, causing the water vapor in the air to condense on the clods’ surface.
- Condensation on the top of the seedbed base: In the cooler lower part of the seedbed in the morning, warmer, moisture-laden air may meet the cooler surface layer, and moisture can condense on the top of the seedbed base from the water vapor in deeper soil layers.
Vapor diffusion resistance
This describes how easily moisture can pass through a given layer. Denser soil has higher resistance, while looser soil is more permeable. In terms of soil, this means that if we create a denser layer (seedbed base), the moisture moving upwards from the deeper layers to the air passes more slowly through the seedbed base, ergo it accumulates before it, while the loose layer above dries out. This basically provides an opportunity for the aforementioned condensation, when the upper part of the seedbed is colder, and relative warmer and more humid air condenses below the seedbed base.
Vapor pressure
Moisture moves from areas of higher vapor pressure to lower vapor pressure. Air with the same relative humidity but warmer has higher vapor pressure, and air with the same temperature but higher humidity has higher vapor pressure. For example, air at 7°C 80% RH has the same vapor pressure as air at 14°C 50% RH, but air at 10°C 50% RH or 3°C 90% RH has lower vapor pressure than the former. Thus, the former two types of air want to give up moisture to the latter two. In autumn, the average air temperature typically cools faster than the average soil temperature, which can only result in moisture gain for the soil based on vapor pressure if the soil is drier, but usually results in moisture loss.
Based on this, the situation in the example is complex but understandable. The moisture appearing in the seedbed probably partially enriched from below to a usable extent at the bottom of the dense seedbed base, due to the vapor brake effect of the seedbed base. Partly, nighttime dew condensed on the top of the insulating seedbed base. On the uncovered soil, on the other hand, the moisture presumably escaped unhindered towards the cooler surface and air due to the lower vapor pressure.
Similarly, the direct sowing area presumably cooled rapidly, the positive moisture balance of dew did not compensate for the losses, and there was no replenishment from deeper layers.
An interesting thought experiment is that the other end of the spectrum, a good mulch, is not mentioned in the story. Its mechanism is surprisingly similar, explaining why it can be beneficial. A sufficiently thick and covering mulch is a good thermal insulator, thus reducing soil surface temperature fluctuations, and the vapor diffusion resistance of plant residues is higher than that of soil, so it quasi-retains the moisture. The slower-cooling soil surface/bottom of the mulch, with higher humidity, absorbs less moisture from the soil but increases the moisture above the sown seed. This sounds good, but not trivial, as seeds under a thick mulch would not emerge, and those emerging would not have a thick cover.