Wet autumn – bad harvest?
”Never since measurements began in 1881 has there been such a high level of precipitation in Germany for twelve consecutive months.” This was reported by the German Weather Service (DWD) - in June 2024, mind you! In late autumn 2024, many farm managers were able to confirm that this weather situation has not changed fundamentally.
External surveys confirm that almost half of this year's winter cereal population was sown late. However, even timely seeding is often heavily influenced by wet conditions.
The soil moisture levels in the upper soil layers published by the German Weather Service show fully saturated soils at the beginning of November - with the exception of north-eastern Germany. The water in the soil also significantly determines our soil temperatures. While warm rain from the west in early spring can make the soil warm up more quickly, saturated soil in autumn leads to a lower proportion of air in the soil and thus to slower cooling.
More mass in the soil behaves more sluggishly because of the temperature fluctuations during the course of the day. Consequently, the plants actually should grow better if temperatures are more even. We can see why this is not always the case if we take a closer look at the necessity for oxygen in the soil. Bacteria and soil life in general need oxygen. Without oxygen, the soil falls into a reducing state and reduces the release and conversion of nutrients. The formation of fine roots depends on the oxygen content in the soil. Without oxygen, roots are reduced or rot. Excess CO2 from root respiration cannot escape and also leads to damage. This hypoxia finally has a lasting effect on the entire hormone balance of the plant and changes its aim in life from “generate yield” to “somehow survive”.
As more mass in the soil can lead to slower warming in spring, as the root system will be less well developed and as infiltration and drying will be slower, a number of challenges will be waiting for us at the start of vegetation. Nutrients that have to be mineralised (nitrogen, sulphur...) enter the vegetation cycle at a later stage and cannot be absorbed with maximum efficiency (phosphorus...).
Influence of tillage in autumn
To decide whether and how we can mitigate the situation with modified measures, we will first look at the role of tillage and take up the article in the last issue of terraHORSCH (Nostalgia or necessity?).
To be able to expect early mineralisation and even nutrient release, we first have to make sure that the starting materials degrade. “You can only take something out of the fridge if something has been put in first.”
Straw and other organic matter need air (oxygen), heat and moisture for microbial degradation (there are also other degradation mechanisms, but these are of less use to us, e.g. UV weathering). The deeper we dig into the cultivated soil, the fewer coarse pores we find, and the less oxygen can initiate the rotting process. Sandy soil usually still has sufficient oxygen in deeper layers due to the coarser particles whereas in clayey soils the ratio of coarse pores quickly decreases in favour of fine pores. As shown in the following illustration, material to be decomposed should be mixed in evenly but not too deeply depending on the soil type. In case of high water saturation, these zones move upwards as the oxygen in the coarse pores is displaced by the water.
Illustration according to Prof. Dr. Dr. B. Bauer, 2020
Coarse pores are important for the infiltration capacity of soils. However, this general statement is not sufficient for higher amounts of precipitation. The continuity of the coarse pores is important when it comes to leading large quantities of precipitation water into deeper layers. If coarse pores are missing, e.g. at the cultivation horizon caused by smearing cultivation, this becomes a bottleneck. Water accumulates in the area above and you cannot drive on the fields for a long time. This is most noticeable when there is too much rain between ploughing in slightly too wet conditions and seeding. The excessive number of coarse pores in the top 20 to 30 cm quickly become saturated and do not transport the water fast enough to deeper layers.
Water does not rise again in coarse pores during dry periods. In this case, the capillary power is too low. Only with sufficient consolidation and correspondingly small pore spacings, the capillary power is high enough that water and the minerals dissolved in it (free Ca²+, potassium etc.) can be expected to rise.
The role of tracks
One point that is given too little attention is the influence of tracks on infiltration. Outside the tracks, water does not only flow vertically downwards into the soil. Even if ususally no erosion of precipitation water is visible on the surface, it flows towards the lowest point even if the slope is minimal. Water that is not retained in the soil accumulates either at the lowest point or in ponding layers. Tracks compact the soil below and prevent the water from flowing away quickly and horizontally. Water accumulates in the topsoil at the surface of the compacted track and drains off downwards more slowly. After large amounts of rainfall, it can be observed that the track does not only not produce any yield, but the plants in the neighbouring area also develop worse. The heavier the axle load and the lower the load-bearing capacity of the track, the deeper is the compaction and the larger is the negatively affected area.
A fully saturated soil, like dry soil, cannot be sufficiently consolidated as the cement does not become strong enough for the soil particles to stick together. Soil is best cultivated and sufficiently consolidated when approx. 2/3 (effective field capacity: 60-70%) is saturated with water.
If we cultivate a too wet soil, we cannot consolidate it sufficiently resp. compress the cavities as the coarse pores are inelastically filled with water. Fine particles dissolved in the water drain off downwards after having dried off and block infiltration channels resp. are deposited like concrete in a deeper cultivation layer which acts like a filter. The soil becomes encrusted and collapses as stabilising particles (lime) are also washed downwards. The soil structure suffers and can only heal itself to a limited extent in the short term.
In our latitudes, we cannot rely on a sufficient frost as a natural regeneration mechanism. We therefore have to reckon with a poor soil structure until at least the next tillage pass.
Poor soil structure does not mean that our imaginary fridge suddenly becomes empty, but that we can no longer access all the compartments in it. However, in contrast, plants cannot steal secretly from other compartments. Fertiliser has to be applied as needed resp. the few compartments have to be kept as full as possible. Needs-based fertilisation means to identify the incorrect information we receive from the soil analysis, if need be by means of plant analyses. At best, the soil analysis gives us the contents of the entire refrigerator.
What pore distribution do roots want?
This question can be answered in a very complex way. Or you can look at the basic principle and derive the rest from it: Roots do not want intense soil density changes! Cavities are avoided just as much as smear layers. Smaller compaction zones are opened up over time. Roots also tolerate the jump from loose topsoil to firmer subsoil rather well as long as the transition is even. Interlocking the cultivation horizons with appropriate points helps in this respect. Free-flowing soil can be cultivated with wider points and a tighter tine spacing than coarsely breaking, very clayey soil. Cavities caused by dry and deep cultivation can only be reduced, especially in deeper layers, by using the appropriate packer for the site. The packer also reduces the above-mentioned seepage of fine particles into deeper layers.
Finally, we look at how our crops should ideally enter the upcoming dormancy period. It begins in Advent when the soil temperature consistently drops below 5 °C. The cell division of the crop plant starts to become very slow, and ryegrass and foxtail rarely germinate below this temperature. A real vegetation dormancy that lasts for weeks diminishes in our latitude and leads to only minimal growth breaks in crops with lower temperature requirements (rapeseed). Nevertheless, the right crop stage has to reached in the event of regular frost nights.
Rapeseed plants should be developed until the 10- to 12-leaf stage and form a good root structure. Moreover, the aim should be to achieve a deep taproot with a root diameter of 0.8 to 1 cm. Very weak populations, i.e. also those that are clearly overgrown and where the sensitive vegetation cone protrudes too far from the soil, are particularly prone to winter damage.
For wheat, too, the development stage of the plants is decisive for resistance. Wheat populations are very frost-hardy shortly before or shortly after the 3-leaf stage (conversion grain to root nutrition), and this stage is the best to enter the winter. This also applies to tillered plants. With their nutrient reserves, their regeneration capacity is very good. In principle, the aim should be tillering before winter as the shoots that are formed in autumn usually are somewhat higher yielding and more resistant than the shoots that are formed late in spring.
Due to the earlier stem extension phase, achieving a certain minimum target population density is even more important and yield-relevant for barley than for wheat. Here, too, the shoots formed in autumn achieve a higher yield. Therefore, the objective should be a good pre-winter development with tillering up to a maximum of EC 25. In this case too, overgrowth of the population leads to increased winter damage and higher disease infestation. In principle, winter damage can be prevented if the plants are healthy.
After a sufficient hardening phase, rapeseed and winter wheat can withstand temperatures of -15 to -20 °C and winter barley temperatures of -12 to -15 °C, even without snow cover.
If time is short for the timely establishment of the crop, the term ‘seedbed before seeding time’ applies until the point in time when sowing the crop really does no longer make sense. If there are no alternatives, one consequence may be to sow crops without prior primary soil cultivation, even if a previous tillage has been established on the site for many years. In the long run, a crop sown under less than optimal conditions is better than the wrong tillage. Seed drills without pre-cultivation tools that still achieve an optimum depth placement are predestined for this.
If cultivation has been carried out in wet conditions, it is important to keep an eye on the fields and, if necessary, to carry out repair measures in the coming years as well as to bear in mind the sluggish nutrient flows and the poor root system when carrying out population management measures in spring. This does not only mean, for example, that the initial input (timing, nutrients and quantity) has to be adjusted, but also that there is a potentially higher risk in case of a prolonged use of growth regulators before dry periods.
‘Wet autumn - poor harvest’ does not have to be true if we adapt.