How Do Different Arable Crops Affect Erosion Risk?

To calculate erosion risk in Swiss field crops, the effect of different arable crops on the risk of erosion was determined. Calculations for the whole of Switzerland show that although erosion risk is on average low, it is not negligible everywhere.

Erosion is one of the greatest threats to the soil identified by the United Nations Food and Agriculture Organization (FAO). For one thing, erosion leads to the loss of precious topsoil on the affected site itself; for another, soil erosion can result in water pollution and damage to buildings, roads, railway lines, etc.

Because it is difficult to measure soil erosion directly, however, various indicators have been developed to calculate erosion. One of the most complex yet most sensitive indicators is the soil cover and management factor (C factor), which describes how agricultural practices (e.g. crop rotation and tillage practices) affect erosion. The value of this indicator lies between 0 and 1, with 0 representing the maximum possible protection against erosion and 1 the greatest erosion risk (permanent bare fallow).

Agri-environmental monitoring yields extensive dataset

Since 2009, the agri-environmental monitoring service of the Federal Office for Agriculture FOAG and Agroscope has been recording and centrally analysing data from around 290 farms (Swiss Agri-Environmental Data Network (SAEDN)). These data are used for the calculation of various agri-environmental indicators, including the C factor. In this study, we considered C factors for the total of 33,078 plots with arable land for the years 2009 to 2019. The calculation of the C factors takes account of ‘the sequence previous crop-main crop, the region (plain versus hill), eight different winter-use variants, four tillage methods and three correction factors for certain farm-specific crop combinations. The extensive SAEDN dataset enabled the calculation of crop-specific C factors for all arable crops in Switzerland. As expected, field vegetables (0.357) and potatoes (0.241) had the highest average C factors, whilst temporary ley had the lowest (0.024).

Temporary ley and low-impact tillage lower erosion risk

Extrapolated to all of Switzerland’s arable land, the average C factor is 0.100 (including field vegetables) or 0.093 (excluding field vegetables). It is thus significantly lower than in Germany (0.124, excluding field vegetables). The differences between the two countries are mainly caused by the high proportion of temporary leys in the rotation (32%) as well as the relatively high proportion of conservation tillage practices (18%) in Switzerland.

Erosion risk calculated at municipal level

Using nationwide statistical data for Switzerland on the distribution of arable crops, an area-weighted average C factor was calculated for each municipality and represented cartographically. Regions with high C factors were Seeland, Orbetal, the Rhone Valley at Lake Geneva, the Zurich Lowlands and wine-growing region and the St. Gallen Rhine Valley; the Alpine foothills and Jura had low C factors.

Erosion control necessary on around 10% of arable land

Linking the C Factor map with the pre-existing map of potential erosion risk yielded a map of the actual erosion risk in Switzerland, according to which the calculated actual average soil loss is 0.81 t per hectare and year. This results in a total soil loss of around 314,000 t for all Swiss arable land in 2020. According to the Swiss Ordinance on the Pollution of Soil (VBBo), the tolerable limit for soil erosion is 2–4t/ha/yr. In Switzerland, approximately 8% of arable land falls into this range. On 1.8% of the arable land, the maximum tolerable rate of 4 t/ha/yr is exceeded. Erosion protection measures would thus be essential on around 10% of arable land.