Rewriting PTR salinity, alkalinity, chemical toxicity and drainage based on the wrb-standard
Description of the current PTR used to derive chemical toxicity
This rule does not exist in CGMS but is in fact integrated in the salinity rule (Table 4-1).
In the salinity rule, two categories of soils concerning chemical toxicity for plants are distinguished:
- soils having a high amount of gypsum (Gypsic soils (FAO-UNESCO 1974 legend));
- soils having a high amount of sulphur (Thionic Fluvisols (FAO-UNESCO 1974 legend)).
It is proposed to create a new rule for chemical toxicity to plants in the knowledge base of the SGDBE version 4.0. Several chemical elements can be toxic for plants: sulphur, aluminium, gypsum and CaCO3, manganese, etc. (FAO, 1988). But considering the information available in the SGDBE, it is proposed to consider only sulphur and gypsum. The soils with sulphur are soils generally permanently waterlogged. For cultivating them, drainage is mandatory but then, the sulphides contained in the soil transform into sulphuric acid by oxidation and the pH is thus decreased below 3.5. For gypsum, as it is not very soluble, the bad effects on crops appear when it is highly concentrated like in a gypsic horizon. The toxicity of gypsum will then depend on the concentration of gypsum (more than 40%) and the depth of appearance of the gypsic horizon.