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.

Rooting conditions (SQ3)

Rooting conditions include effective soil depth (cm) and effective soil volume (vol. %) related to presence of gravel and stoniness. Rooting conditions may be affected by the presence of a soil phase either limiting the effective rooting depth or decreasing the effective volume accessible for root penetration. Rooting conditions address various relations between soil conditions of the rooting zone and crop growth. The following factors are considered in the evaluation:

Adequacy of foothold, i.e., sufficient soil depth for the crop for anchoring;

available soil volume and penetrability of the soil for roots to extract nutrients;

space for root and tuber crops for expansion and economic yield in the soil; and

absence of shrinking and swelling properties (vertic) affecting root and tuber crops.

Soil depth/volume limitations affect root penetration and may constrain yield formation (roots and tubers). Relevant soil properties considered are: soil depth, soil texture/structure, vertic properties, gelic properties, petric properties and presence of coarse fragments. This soil quality is estimated by multiplying of the soil depth limitation with the most limiting soil or soil phase property

Soil phases that relevant for rooting conditions vary somewhat with source of soil map and soil classification used. In the HWSD these are:

FAO 74 soil phases: stony, lithic, petric, petrocalcic, petrogypsic, petroferric, fragipan and duripan.

FAO 90 soil phases: rudic, lithic, pertroferric, placic, skeletic, fragipan and duripan.

ESB soil phases and other soil depth/volume related characteristics: stony, lithic, petrocalcic, petroferric, fragipan and duripan, and presence of gravel or concretions, obstacles to roots (6 classes), and impermeable layers (4 classes).

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%.