4.2 Physical Parameters4.2.1 Texture
Soil texture refers to the particle size distribution and to particle-size groupings within specific ranges. Textural classes are defined by the relative contents of the three major soil separates, sand, silt and clay. Texture is
considered as one of the most important characteristics with regard to physical soil qualities. It influences such important soil properties as soil water availability, infiltration rate, drainage, tillage conditions and capacity
to retain nutrients. The effect of texture on those properties may be modified by structure, nature of clay minerals, organic matter content, and lime content.Texture is commonly determined by the hydrometer or pipette methods in the laboratory or by the hand-feel method in the field.
4.2.2 Structure
Soil structure refers to the aggregation of primary soil particles (sand, silt, clay) into compound soil particles or clusters of primary particles which are separated from the adjoining aggregates by cracks or surface of weakness. Soil structure exerts a dominant influence on oil's air and moisture regime, on its hydraulic conductivity and consequently, on the root growth and micro)biological activity that occurs within the soil. It is therefore, an important factor in soil productivity and soil genesis. Structure is commonly described in the field under three criteria, namely:
grade which refers to the distinctiveness and durability;
size of aggregate; and,
shape of aggregates.4.2.3 Coarse Fragments or Stoniness
Surface coarse fragments in the top 20 cm will influence tillage conditions as well as the capacity to retain nutrients and water. Coarse fragments can limit the use of agricultural implements and optimum growth of roots.
Coarse fragments with a diameter between 2-75 mm are termed gravel; those between 75-250 mm are called cobbles and those more than 250 mm are called stones (Sys et al,1991).
Coarse fragments are commonly quantified on volume or weight percentage basis.
4.2.4 Rooting Depth
Rooting depth is a crucial parameter in soil productivity because it determines soil reserves of water and nutrients. The relationship between rooting depth and productivity is commonly described according to law of diminishing returns. It is generally defined as the thickness of loose soil above a limiting layer (if any). Limiting layer is impermeable for roots and percolating water. Soil depth is vital for the anchoring of plants and provision of a favourable environment for plant root growth.
Soil depth parameter is commonly quantified by direct depth (length) measurements.
4.2.5 Water Holding Capacity (WHC) or Available Moisture Content (AMC)
This is the amount of water which a given soil horizon can store and is estimated from the difference between field capacity and the lower limit of plant available water (wilting point).
The field capacity (-1/3 bar) and wilting point (-15 bar) are commonly determined in the laboratory by the pressure plate method. The field capacity value could also be determined in the field by the ponding method.
4.2.6 Drainage and Depth to Water-table
Drainage and depth to water-table are vital parameters. The suitability for upland crops decreases when drainage conditions become impeded. Tree crops with a deep root system are more sensitive to poorly drained conditions than annual crops with shallower root systems. Crops like paddy rice react quite differently to drainage conditions; their suitability decreases when drainage conditions improve. For irrigated agriculture, drainage, depth to ground water- table and salinity status are critical evaluation parameters.
Drainage classes are normally described in the field; depth to water-table is also measured in the field.
4.2.7 Slope
Slope angle and length are critical parameters for the assessment of erosion potential. It also influences water movement and distribution within the soil profile.
Slope angles or percentages are determined in the field.
4.2.8 Infiltration
Infiltration is a very important parameter irrigated farming systems. Infiltration is the entry of water into the soil through the soil surface. The rate is dependent on the antecedent moisture, soil structure, pore sizes and their distribution. It is an important parameter in evaluating compacted (physically degraded) soils, for example, degraded rangelands. Rate of water entry is generally very low in degraded areas and most of the water ends up as run-off; causing considerable soil erosion and siltation problems.
Infiltration rate is commonly determined in the field using a double cylinder infiltrometer (Bouwer, 1986).
4.2.9 Bulk Density
This parameter largely depends on the porosity of the soil and is commonly used to evaluate compaction. Loose and porous soils have low values while compacted or physically degraded soils have high values.
Bulk density parameter is conveniently determined by the core or clod method (Anderson and Ingram, 1993).
4.2.10 Total Porosity
Total porosity is the fraction of the soil mass that is occupied by the pores. The pore space is largely determined by the arrangement of the individual solid particles of the soil.The pore space in the soil is partially occupied by the liquid (water) and partly by air.
Porosity is commonly computed from the relationship between bulk density and particle density (Anderson and Ingram, 1993
4.2.11 Flooding
Flooding is considered as a serious limitation for most crops apart from paddy rice. Flooding interferes with the air entry into the soil.
For paddy rice cultivation flood evaluation is based on duration and depth of flooding. Optimal duration of flooding is 110 to 160 days; marginal situations are 90-110 days and more than 180 days. The optimal depth can be considered as 10-30 cm (Sys et al. 1991).