Europe's Marginal lands

Adverse climate is composed of two factors as proposed in the JRC approach to map areas of natural constraints (van Oorschoven et al., 2014): low temperatures and dryness. It is clear that the change of the adverse climate factors will be most influenced by climate change and are therefore also most significantly influencing on the future location of marginal lands by 2050.

Very low temperatures exclude or limit growth of many agricultural crops. As an indicator for low temperatures the Length of Growing Period was used.

A location is marginal when categorised in the severe class which is determined by the Thermal-time sum or growing degree-days. The threshold for severe is at 1500 degree days for Growing Period defined by accumulated daily average temperature > 5°C.

The sub-severe threshold is set at 1575 degree days and this implies that if this threshold occurs in a pairwise combination with the factors ‘excess soil moisture’ and ‘heavy clay’ in the sub-severe class they are classified as severe and therefore included in marginal land.

For dryness the ratio of precipitation over potential evapotranspiration is indicative of soil moisture conditions for agricultural crops. In case of low rainfall and high evaporative demand then the soil moisture supply will be low and the growth potential for crops is low.

The indicator for dryness is assessed by taking 'the ratio of the annual precipitation (P) to the annual potential evapotranspiration (PET)'. The threshold is set at 0.5 (P/PET ≤ 0.5).

The sub-severe level is set at P/PET ≤ 0.6. If this sub-severe level occurs in combination with sub-severe level scores for the factors ‘coarse fragments and stoniness’, ‘sand and loamy sand’, ‘heavy clay’, ‘salinity-sodicity’ and ‘slope’ it is classified as severe and included in the marginal land.

Excess of soil moisture over prolonged time in the field is limiting for crops and for management. Access of the field with machines and the workability of the soil is hampered and lack of oxygen for root growth limits crop growth.

There are two factors that can determine the status in marginal or non-marginal:

1. water content above field capacity and
2. soil drainage status of the soil.

The first factor is defined as ‘soil moisture content exceeding field capacity for at least 210 days (7 months)’. The sub-severe threshold is set at 190 days. The sub-severe threshold is set at 190 days. If this sub-severe threshold occurs in pairwise combination with severe ‘slope’ the location is no longer severe, so excluded from the marginal land class. This pairwise combination is an example of a positive synergy.

The second factor, soil drainage status, is a morphometric parameter that reflects the combined effects of climate, landscape and soil. It is described in the field and is indicative for the wetness of a soil over longer periods of time (and that is reflected in the soil status, judged by a.o. soil colour and mottling). The poorly drained soils from WRB (at Soil Reference Group level and at the level of principle qualifiers) selected to be classified as ‘severe’ from the European Soils Database are: Gleysols, Histosols, Stagnosols, Plano-sol, soils with primary qualifiers Histic, Gleyic, Stagnic & marshlands with >60% clay. Sub-severe is >50% clay which if occurring in pairwise combination with severe ‘slope’ the location is no longer severe, so excluded from the marginal land class (positive synergy).

Also both factors will be influenced by climate change and are therefore of importance to identify the 2050 location of marginal lands.

Adverse chemical conditions are defined by two different factors. Either excess of salts (salinity or sodicity) or by toxic elements in the soil that hamper crop growth or may pose a health risk. The excess of salts affects crop growth in various ways: by toxicity effects, by reducing the water availability to plants through increased osmotic pressure and by causing nutritional disorders. Excess of salts occurs through salinity (access of free salts) and sodicity (saturation of the soil exchange complex with sodium), (Mantel and Kauffman, 1995).

Salinity is identified through units on the soil map of Europe (European Soils Database) which were mapped in the ESDAC project (Toth et al., 2018). Solonchaks soils and soils with a salic qualifier that cover more than 50% of the mapping unit area were ranked as highly saline (ECse > 15 dS/m).

Sodicity was mapped from the same source (ESDAC). It is derived from the mapping units that have more than 50% area of sodic soils (Solonetz) and soils with a sodic qualifier. Sodic soils are soils with saturation of the exchange complex with sodium (ESP) of more than 15%. The sub-severe class for both salinity and sodicity is determined at a below 50% salic or sodic area coverage of the mapping unit area. If this sub-severe situation overlaps with the sub-severe dryness class (> 0.5 P/PET < 0.6) it becomes severe in a negative pairwise combination.

There are several naturally occurring toxicities in soils that have a negative effect on crop growth. The acid sulphate soils were identified as toxic and these were identified through the Thionic qualifier of soils in the European Soils Database. The reason these acid sulphate soils are limiting agricultural use is because when they are drained, which happens when they are used for arable agriculture, they become extremely acidic, as sulfides react with oxygen to form sulfuric acid. Extremely high acidity, high sulfur availability and aluminium toxicity that result in drained acid sulphate soils are posing great limitations to land management for farming. In acid subsoils this may also be caused by high aluminium contents. Since aluminium toxicity is not well represented in the soil databases available it was not possible to include this factor in the marginal land map.

Low fertility refers to the availability of nutrients over time to crops. Soil nutrient availability is often highly variable in both space and time and depends on many variables. Here two parameters that influence soil fertility were ranked: soil reaction (pH) and organic carbon content. Soil reaction is an indicator for the availability of nutrients (poor in alkaline and in acid soils). Soils with pH (0-30 cm) below 4.5 or above 8 are considered (severely) limited. Organic carbon contributes to the nutrient buffering capacity of the soil and organic matter is a direct source of nutrients. Low carbon containing soils are indicative for low soil fertility and low biomass turnover. The severe threshold for SOC% is set at 1% and <1.25%.  If SOC% is sub-severe it will become severe in a pairwise combination with sub-severe scoring ‘slope’- and ‘dryness’- factors.

Root growth is directly related to possibility for uptake of nutrients and water and provides food for the crop. Several root growth constraining factors are used:

1. unfavourable soil texture
2. coarse fragments
3. organic soils
4. surface stones and rocks
5. shallow rooting depth

Unfavourable texture concerns the sandy soils and the heavy clays. Very sandy soils have a low water holding capacity and are often low in nutrient content and nutrient buffering capacity. Normal fertilization practices have limited efficiency on very sandy soils (Van Oorschoven et al., 2014). Heavy clays are limiting for crop cultivation as they have limitations in access for machinery during wet parts of the season, difficult workability and may have shrinking and swelling characteristics during dry and wet conditions that may damage plant roots. Water movement may be slow in heavy clays (due to low porosity) and water may accumulate on the surface in high rainfall events.  The threshold for severe is determined by the amount of sand, loamy sand defined as: silt% + (2 x clay%) ≤ 30% (implying  max 70% sand). If sand availability is max 60% the class becomes sub-severe and in a pairwise combination with sub-severe salinity/sodicity it is classified as severe.  

The presence of coarse fragments limits crop cultivation because the negative effect on workability. The volume occupied by coarse fragments limits rootable space and the volume of storage for water and nutrients in the soil. Surface stones and rocks are another limitation for soil workability and access of machinery. Furthermore, surface stones and rocks hamper seed germination. The threshold for 'severe' was set for a surface coverage (at depth: 0-30 cm) by coarse fragments at >35% and/or > 15% with rock coverage. For sub-severe level the thresholds of surface coverage were > 25% and/or > 10% respectively.  If these sub-severe classes occur in a pairwise combination with sub-severe scores for ‘sand, loamy sand’, and slope’ it is classified as 'severe'.  

Organic soils are soils with organic matter content ≥20% in a layer of 40 cm or more, either extending down from the surface or taken cumulatively within the upper 100 cm of the soil (histic horizon, IUSS Working Group WRB, 2006). Foothold for roots is limited in organic soils, especially for perennial crops. Peatlands are typical examples of where organic soils occur and they are both ecologically valuable and fragile. Cultivation of organic soils requires drainage and this causes oxidation of the peat and methane release. Peat soils are therefore best left uncultivated. There is no sub-severe class for organic soils.

Shallow rooting depth is defined as the depth from soil surface to coherent hard rock or hard pan. The rootable soil volume is a critical characteristic of land in relation to suitability for farming. It determines the foothold for roots, but most of all the total store of nutrients and water that will be potentially available to the plant during the growing season. Rootable soil volume may be limited by chemical or physical barriers. In assessment of marginal lands a shallow depth from the soil surface to an impeding layer (hardpan) or to bedrock of 30 cm or less is considered severe. Selected soils classified as severe are therefore Leptosols, Albeluvisols, Lithic, Petrocalcic, Fragipans, Duripans, Petroferric. There is no sub-severe class for organic soils.