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New ways to less soil pressure

Many machines encounter the limits of operation that is gentle on the soil, particularly when driving on moist soil. The fine line between economic constraints and soil protection requirements is particularly prevalent during liquid manure spreading in the spring. Dr Joachim Brunotte, Dr Marco Lorenz and Maike Weise of the Thünen Institute of Agricultural Technology in Braunschweig want to make this fine line more calculable for farmers.

Why is it especially important to deal with the effect of liquid manure spreading methods on the soil this year?

The regulations of the new Fertiliser Ordinance have led to a shift in spreading liquid farm manure from the autumn to the spring. The aim is to prevent emissions. As the subsoils are more likely to be moist due to winter rainfall, a strategy for adjusting to operation that is gentle on the soil is required in this case. Because liquid manure volumes are increasing and the spreading periods are specified, this adjustment can only take place via the technology.

Exactly how extensive is the problem of soil compaction in Germany?

Unfortunately, it isn't possible to provide a universal answer to the question concerning this problem, i.e. the prevalence of soil compaction. In some federal states and regions, there are status surveys and permanent soil observation plots that describe the soil condition based on soil physical parameters. Added to these are locally undertaken load tests that study the change in the soil structure caused by driving under individual machines and entire operational chains. None of the investigations reveal extensive harmful soil compaction; damage is only shown in isolated cases in tramlines, on the headlands or on arable fields where harvesting has been recklessly carried out on very moist soil. As a universal assessment of harmful soil compaction has always been desired, a series of models (e.g. pre-loading) have also been developed for estimation purposes. However, these are unable to simulate the real mechanical loads that occur in entire operational chains because they only give consideration to individual wheels. This is why we have developed a new concept to be able to estimate the compaction exposure of soils in Germany and to derive the mean regional number of days on which driving on the soil is possible for different degrees of mechanisation.

How does this new concept work?

Adjustment strategies are always required in an age of changes, and this case is no exception. It therefore depends on the train of thought. It is not the soil that reacts to the use of machines and entire operational chains; instead, the operational chains have to adjust to the soils' compaction sensitivity.So what is needed are adjustment strategies that are derived from the soil conditions and simultaneously enable the machines to be used profitably. Our planning concept gives consideration both to the properties of the location and to the vehicle parameters. In the first step, we determine the compaction sensitivity of arable land based on the type and moisture of the soil. By calculating the evaporation, the current changes in soil moisture at different depths can be shown precisely for each day and in five compaction sensitivity classes. The second step gives consideration to the mechanical load of entire operational chains based on long-term field measurements and the KTBL's (Kuratorium für Technik und Bauwesen in der Landwirtschaft e.V.) technology database. The load proportions consist of the wheel load, tyre inflation pressure, contact surface pressure, number of passes and track area percentage and enable the operational chains to be categorised in 5 load classes. In the third step,the mechanical load caused by agricultural machinery is then compared with the loadability of the soil in a scheme with five classes in each case.

You have tested this concept in practice. What was the outcome?

We compared three liquid manure spreading techniques under real field conditions: (1) a tractor with a tandem slurry tanker without tyre pressure adjustment system and with a tyre inflation pressure of 3 bar but without working in, (2) a self-propelled slurry tanker (1.5 bar) with working in and (3) a tractor with liquid manure distribution system (0.8 bar) and with working in. The measurements of the soil pressure and settling as well as the soil physics showed that not only the wheel load influences the soil condition but also the passage frequency and the tyre inflation pressure.

The result was that, despite only medium wheel loads, the use of tandem and tridem tankers is regarded critically due to the high number of passages and the high tyre inflation pressures needed for road transport. This technology is often found on farms with their own machinery – tyre pressure adjustment systems are commonly used by contractors. With this method, the liquid manure also has to be worked in in a separate operation with two additional passages.

Despite its high wheel load, the self-propelled vehicle with crab steering impairs the condition of the soil less due to only one single passage, as well as its lower tyre inflation pressures and high operating speed. The liquid manure distribution system performs most favourably because the liquid manure is temporarily stored on the headland and the spreading tractor only has a low wheel load and a tyre inflation pressure of less than 1 bar as well as only requiring two passages.

How do you determine the appropriate technology for the location?

To further improve the benefit for each individual farm, days on which driving on the soil is possible without impairing it are determined for spreading liquid manure in the autumn and spring. We have demonstrated this based on example locations with comparable soil types in the topsoil but different rainfall/soil moisture levels for spreading in the autumn and spring. Let us consider two lighter soils of an identical type in the federal states Mecklenburg (drier) and Upper Swabia (very moist). In the autumn, the entire period of 61 days (1 September to 31 October) is available on average over the years at the location in Mecklenburg with the self-propelled slurry tanker and the liquid manure distribution system variants. Only 35 days are available for a tractor with a tridem slurry tanker. At the very moist location in Upper Swabia, 34 days are available for the liquid manure distribution system in the autumn. The two other technology variants cannot be used in a way that protects the soil due to their excessive mechanical loads.

In the springfrom 1 February to 30 April, there only around two to four days for soil-protecting spreading regarded on average over the years at both locations for the tractor with a tridem slurry tanker. In Mecklenburg, the self-propelled vehicle can be used for 14 days and the liquid manure distribution system for 33 days. In Upper Swabia, soil-protecting spreading is only possible on ten to twelve days using a liquid manure distribution system.

However, the soil at both of these locations must not be driven on to spread liquid manure during this time because the subsoil is acutely at risk of compaction. In extreme years with minimal rainfall, the soils are drier and more days on which driving on the soil is possible are available. Conversely, maximum rainfall means that even fewer days are available.

And what is the benefit to farmers of these findings?

The benefit to farmers of these days on which driving on the soil is possible involves being able to plan investments and operating days for soil-protecting driving and not only being dependent on economic aspects. If these days are limited due to unfavourable soil conditions, time slots have to be provided for a break. Profitability can then be achieved through high capacity utilisation rates per total usage period and not through high capacity utilisation rates per year. We are currently developing an online application in which both long-term weather data and short-term weather forecast data are input in combination with individual farm machine data to enable spot-on operational planning. The costs have to be compared against the monetarised benefit of soil-protecting spreading methods, as the acceptance of such measures otherwise suffers in practice.

The questions were asked by Thomas Preuße