Low pH leads to acidity related toxicities, e.g., aluminum, iron,
manganese toxicities, and to various deficiencies, e.g., of phosphorus
and molybdenum. Calcareous soils exhibit generally micronutrient
deficiencies, for instance of iron, manganese, and zinc and in some
cases toxicity of molybdenum. Gypsum strongly limits available soil
moisture. Tolerance of crops to calcium carbonate and gypsum varies
widely (FAO, 1990; Sys, 1993).
Low pH and high calcium carbonate and gypsum are mutually exclusive.
Acidity related toxicities such as aluminum toxicities and
micro-nutrient deficiencies are accounted for respectively in SQ1,
nutrient availability, and in SQ2, nutrient retention capacity. This
soil quality SQ6 is therefore only including calcium carbonate and
gypsum related toxicities. The most limiting of the combination of
excess calcium carbonate and gypsum in the soil, and occurrence of
petrocalcic and petrogypsic soil phases is selected for the
quantification of SQ6.
Soil qualities have been estimated for the sequence 1 soils in each grid
cell with as reference crop maize. The derived maps for the individual
soil qualities represent therefore the qualities of main soils only.