Percentage silt respectively in the in the subsoil Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. Silt size is between 0.002 and 0.050 mm (USDA classification) and between 0.002 and 0.0625mm (ISO and FAO classification). In the database no difference is made between the two, but reported figures are used, whatever the source.

Reference Bulk Density of topsoil.Reference bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. The calculation procedures for reference bulk density can be found at http://www.pedosphere.com/resources/bulkdensity/index.html Bulk density, as a soil characteristic, is a function rather than a single value (USDA-NRCS, 2004 #3078, p. 73) as it is highly dependent on soil conditions at the time of sampling: changes in (field) water content will alter bulk density. The SOTWIS database provides estimates of bulk density values derived from available analyzed data, and thus consider differences in soil texture, organic matter content and porosity. Careful review of these values also by comparison with calculated reference bulk densities has revealed substantial differences. For reasons of data quality and consistency of the HWSD, reference bulk density values – calculated using equations developed by Saxton et al. (1986), have been used here: these equations represent a statistical estimate and reflect only the textural influence.

Available water storage capacity in mm/m of the soil unit For the soil units of the Soil Map of the World (FAO-74) and for the revised legend (FAO-90), FAO has developed procedures for the estimation of Available Water Capacity in mm/m (AWC) (FAO, 1995). The AWC classes have been estimated for all soil units of both FAO classifications accounting for topsoil textural class and depth/volume limiting soil phases. The following AWC classes are used 1 - 150 mm/m 2 - 125 mm/m 3 - 100 mm/m 4 - 75 mm/m 5 - 50 mm/m 6 - 15 mm/m 7 - 0 mm/m

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%.

Reference Bulk Density of top- and subsoil.Reference bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. The calculation procedures for reference bulk density can be found at http://www.pedosphere.com/resources/bulkdensity/index.html Bulk density, as a soil characteristic, is a function rather than a single value (USDA-NRCS, 2004 #3078, p. 73) as it is highly dependent on soil conditions at the time of sampling: changes in (field) water content will alter bulk density. The SOTWIS database provides estimates of bulk density values derived from available analyzed data, and thus consider differences in soil texture, organic matter content and porosity. Careful review of these values also by comparison with calculated reference bulk densities has revealed substantial differences. For reasons of data quality and consistency of the HWSD, reference bulk density values – calculated using equations developed by Saxton et al. (1986), have been used here: these equations represent a statistical estimate and reflect only the textural influence.

Percentage sand in the in the subsoil. Sand comprises particles, or granules, ranging in diameter from 0.0625 mm (or 1⁄16 mm) to 2 millimeters. An individual particle in this range size is termed a sand grain. Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour). Sand is commonly divided into five sub-categories based on size: very fine sand (1/16 - 1/8 mm diameter), fine sand (1/8 mm - 1/4 mm), medium sand (1/4 mm - 1/2 mm), coarse sand (1/2 mm - 1 mm), and very coarse sand (1 mm - 2 mm).

Percentage clay respectively in the in the subsoil. Clay is naturally occurring firm earthy material, composed primarily of fine-grained (diameter less than 0.002mm) that is plastic when wet and hardens when heated and that consists primarily of hydrated silicates or aluminum. Clay is mostly composed of clay minerals which are phyllo-silicate minerals and minerals which impart plasticity and harden when fired or dried. The definition of "finegrained" used above is particles smaller than 2 μm, colloid chemists (and Eastern European soil scientists) may use 1 μm. In the database no difference is made between the two, but reported figures are used, whatever the source; these values are also used to determine the “USDA texture class”

This field gives the soil reaction of top- and subsoil. pH, measured in a soil-water solution, is a measure for the acidity and alkalinity of the soil. Five major pH classes are considered here that have specific agronomic significance: pH < 4.5 Extremely acid soils include Acid Sulfate Soils (Mangrove soils, cat clays). Do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still. pH 4.5 – 5.5 Very acid soils suffering often from Al toxicity. Some crops are tolerant for these conditions (Tea, Pineapple). pH 5.5 –7.2 Acid to neutral soils: these are the best pH conditions for nutrient availability and suitable for most crops. pH 7.2 – 8.5 These pH values are indicative of carbonate rich soils. Depending on the form and concentration of calcium carbonate they may result in well structured soils which may however have depth limitations when the calcium carbonate hardens in an impermeable layer and chemically forms less available carbonates affecting nutrient availability (Phosphorus, Iron). pH > 8.5 Indicates alkaline soils often highly sodic (Na reaching toxic levels), badly structured (columnar structure) and easily dispersed surface clays.

ISSOIL (Flag for non-soil units). Field indicating if the soil mapping unit is a soil or a non-soil. CODE ISSOIL 0 Non-soil unit 1 Soil

Percentage silt respectively in the subsoil Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. Silt size is between 0.002 and 0.050 mm (USDA classification) and between 0.002 and 0.0625mm (ISO and FAO classification). In the database no difference is made between the two, but reported figures are used, whatever the source.