What Quantity of Heavy Metals Ends Up in Agricultural Soils?

Agroscope Science 123, 1−99, 2021

Calculations carried out by the Swiss Soil Monitoring Network (NABO) over more than 30 years reveal that fertiliser applications and plant-protection products can lead to excess heavy metals in agricultural soils.

The Swiss Soil Monitoring Network (NABO) was set up to observe the long-term development of soil health and fertility. To this end, it has conducted a monitoring programme since 1985, which at present regularly examines the soil at 112 sites. Seventy-eight sites are farmed, 28 are located in forests, four are on protected sites and two in city parks.

Inputs and outputs calculated on plots

At 46 agricultural sites – arable production, grassland and special crops – scattered across Switzerland, data is gathered on both soil properties and management. With the help of the management data, inputs and outputs of the heavy metals copper, zinc, cadmium and uranium as well as of the nutrients nitrogen, phosphorus and potassium were calculated at plot level from 1985 to 2017. Fertilisers and plant-protection products were taken into account as inputs, and atmospheric deposition was also considered for copper, zinc and cadmium. Output was calculated using harvest quantities. For uranium, only inputs via mineral phosphorus fertilisers were quantified. 

High copper and zinc levels with high livestock densities

With the exception of viticulture, copper and zinc inputs were mostly due to the use of farm manure. Both of these heavy metals are essential micronutrients, and pass from feeds and feed additives into slurry and dung via the digestive systems of livestock. Copper and zinc inputs were significantly correlated with the number of livestock per utilised agricultural area on the respective farm, which can be explained by larger farm manure inputs with larger livestock herds.

High copper inputs in viticulture

Net copper flows (inputs minus outputs) were highest on vineyard plots, owing to the use of plant-protection products containing copper. Here the median of the net flows from 1985 to 2017 ranged between 1400 and 2400 g copper per hectare and year. On arable and grassland plots with high livestock densities per utilised agricultural area, the medians reached up to 400 g copper per hectare and year.   

Cadmium and uranium from mineral phosphorus fertilisers

While in general net copper and zinc flows did not decrease and in the case of intensively managed grassland plots even increased, net cadmium and uranium flows decreased from the late 1980s to the mid-1990s, and have varied little since then. The reason for this is the decline in phosphorus mineral fertiliser inputs, which introduce cadmium and uranium into the soil; in addition, there is a decrease in atmospheric deposition in the case of cadmium. Consequently, which fertilisers are used to ensure the nutrient supply of a plot or different crops is a critical factor in which heavy metals are introduced into the soil, and in what quantities.

In a subsequent step, the surface balances are compared with the measured development over time since 1985 in order to better estimate and forecast the long-term influence of management on heavy-metal content in soils.