On the basis of soil parameters provided by the Harmonized World Soil Database (HWSD) seven key soil qualities important for crop production have been derived, namely: nutrient availability, nutrient retention capacity, rooting conditions, oxygen availability to roots, excess salts, toxicities, and workability. Soil qualities are related to the agricultural use of the soil and more specifically to specific crop requirements and tolerances. For the illustration of soil qualities, maize was selected as reference crop because of its global importance and wide geographical distribution.

Workability (SQ7)

Diagnostic characteristics to indicate soil workability vary by type of management applied. Workability or ease of tillage depends on interrelated soil characteristics such as texture, structure, organic matter content, soil consistence/bulk density, the occurrence of gravel or stones in the profile or at the soil surface, and the presence of continuous hard rock at shallow depth as well as rock outcrops. Some soils are easy to work independent of moisture conditions, other soils are only manageable at an adequate moisture status, in particular for manual cultivation or light machinery. Irregular soil depth, gravel and stones in the profile and rock outcrops, might prevent the use of heavy farm machinery. The soil constraints related to soil texture and soil structure are particularly affecting low and intermediate input farming LUTs, while the constraints related to irregular soil depth and stony and rocky soil conditions are foremost affecting mechanized land preparation and harvesting operations, of high-level input mechanized farming LUTs. Workability constraints are therefore handled differently for low/intermediate and high inputs.

The workability soil quality SQ7 includes physical hindrance to cultivation, and limitations to cultivation imposed by texture/clay mineralogy. The soil quality SQ7 is derived by combining the most limiting soil/soil phase attribute with the average of the remaining attribute coditions. Soil phases considered in the quantification of SQ7 are stony, lithic, petric, petrocalcic, petroferric, fragipan and duripan (FAO ‘74), and lithic, petroferric, rudic, skeletic, duripan and fragipan (FAO’90).

Note that the classes used in the Soil Quality evaluation are:

1: No or slight limitations

2: Moderate limitations

3: Sever limitations

4: Very severe limitations

5: Mainly non-soil

6: Permafrost area

7: Water bodies

Remember that classes are qualitative not quantitative. Only classes 1 to 4 are corresponding to an assessment of soil limitations for plant growth. Class 1 is generally rated between 80 and 100% of the growth potential, class 2 between 60 and 80%, class 3 between 40 and 60%, and class 4 less than 40%.