Impact on soil compaction of driving agricultural machinery over ground frozen near the surface

Abstract

The compaction of arable soils caused by driving over them with agricultural machinery poses a serious problem in numerous agricultural regions across temperate climate zones. The risk of compaction is particularly high in early spring or late autumn when soils are wet. This is why driving over soils frozen near the surface is recommended in some cases in temperate climate zones to prevent soil compaction. However, no findings have been available about the thickness of frozen soil required to effectively prevent compaction when the soil is driven over. In one experiment, soil physical measurements were carried out on the topsoil after a single pass with a tractor (4100 kg wheel load, 80 kPa inflation pressure) over an unfrozen variant, a variant with 2–3 cm frost covering and a variant with 5–7 cm frost covering, with comparisons made with a control variant that had not been driven over. Driving over the unfrozen variant led to a significant compaction of the whole of the topsoil. By contrast, the frozen surfaces were able to significantly buffer the compaction. No appreciable differences were detected between the two depths of frost penetration. A depth of frost penetration of as little as 2–3 cm was therefore sufficient to reduce the risk of compaction with a wheel load of approximately 4000 kg and appropriately adjusted inflation pressure.

Introduction

Compaction of arable soils caused by agricultural machinery is a significant problem on arable land subject to intensive cultivation, particularly in the temperate climate area of Central and Eastern Europe (e.g. Fulajtár, 2000). Compaction severely restricts a number of important ecological soil functions. Air capacity decreases and gas exchange is restricted (Ball and Robertson, 1994, Horn and Rostek, 2000). Another likely consequence of soil compaction and the establishment of platy and coherent soil structure is reduced water infiltration with water run-off and erosion (Horn et al., 1995). In some cases even yield decrease has been observed (Voorhees, 2000). In early spring and late autumn, the water content of arable soils is in most cases near the field capacity and there is therefore a high risk of compaction if driven over (Arvidsson et al., 2003). In addition to numerous preventive measures, such as reducing the wheel load and internal tyre pressure or using specialised chassis, a recommended practice in Central European agriculture involves driving over soil which is frozen near the surface. This option is available for several agronomical measures, such as the timely spreading of mineral fertiliser early on in the year or the tillage of unused arable land during the winter, particularly in agricultural areas of Central Europe's temperate climate zone. During years with particularly wet spells of autumn weather, part of the grain maize harvest also occurs on ground which is slightly frozen, in order to guarantee the ability to drive over it. All of the measures named here make use of weather situations where a light frost (usually ≥ 2 °C) occurs at night and by day temperatures above freezing (usually > 5 °C). Hence this does not concern prolonged periods of frost, rather solely ground frozen near the surface for only a few hours. From an agricultural point of view this is necessary to ensure, for example, the solubility of mineral fertilisers in the soil, which by day is not frozen.

It is already known from other scientific disciplines that the strength of frozen soils overall can be very high (Yang et al., 2010), up to the point where crushing occurs. There are, however, no findings concerning the depth of frost penetration required to effectively prevent compaction when the soil is driven over by agricultural machines. The results of a field trial carried out in March 2010 to answer this question are set out below.