4.1 Chemical Parameters4.1.1 Soil Acidity (pH)
The parameter generally denotes soil reaction which expresses the degree of "acidity" or "alkalinity". The pH value equals the negative logarithm of the H + ion concentration (C H+ ). Conventionally, the soil pH is measured in a soil - water suspension 1:2.5 (10 g soil in 25 ml water) and is designated pH (water). It could also be determined in suspensions of 1:1 or 1:5.
For the measurement of exchange (reserve or potential) acidity of acid soil a 1:2.5 suspension is used to which a neutral salt (KCl) has been added, in order to bring exchangeable H-ions into solution. It is designated pH (KCl). The pH values of a soil is most accurately measured with a pH meter in the laboratory.
4.1.2 Organic matter
Organic matter, because of its colloidal nature, contributes to the cation exchange capacity, (CEC) and therefore, the nutrient retention capability of the soil. Organic matter improves the physical characteristics of the soil through its enhancement of water permeability and retention. Soil organic matter is high in organic carbon and serves as a source of energy for soil micro-organisms.Organic carbon is commonly determined by the modified Walkley and Black method (Nelson, D.W. and Sommers,
L.E.)4.1.3 Sodicity (ESP)
Normal soils usually have an exchange complex that is dominated by Ca and Mg and has only minor amounts of K and Na. When excess soluble salts accumulate in such soils, Na frequently becomes the dominant cation in the soil solution, a part of the original Ca and Mg is replaced by the cation. In general, physical properties become increasingly unfavourable with increasing levels of exchangeable Na.
4.1.4 Salinity (Ec)
Saline soils contain soluble salts in concentrations that impair crop growth. Although weathering of primary minerals is the source of nearly all soluble salts, accumulation of these on the spot are seldom concentrated enough to form a saline soil. Invariably, strong salinity is found under semi-arid climatic conditions in soils where salts from other locations have accumulated through the inflow and subsequent concentrations of salt-bearing waters. Most saline soils are characterised by a low humus content, no differentiation into horizons and poor structure.
A generally accepted parameter of salinity is the electrical conductivity (Ec) at 25°C. The Ec can be determined according to the saturated paste extract method and measured with a conductivity bridge.
4.1.5 Cation Exchange Capacity (CEC)
The CEC of a soil often indicates its natural fertility and its Development and ability to supply Ca, Mg, and K for plant growth. It is also a measure of the ability of the soil to store added nutrients (fertilizers). Soils which have a low CEC cannot store large amounts of plant nutrients and must be replenished more regularly.
In the inorganic part of the soil complex only clay particles play a decisive role, since the active total internal surface of silt and sand particles in comparison to that of clay is very small. The CEC of clay depends on the type of clay mineral. The organic matter complex (the humus colloids) has a much higher CEC than clay.
The CEC of a soil is determined, in the laboratory, either in an exchange medium with pH = 8.2 or in exchange medium with pH = 7.0. Some times the expression T value is used instead of CEC value.
4.1.6 Exchangeable Bases
This is restricted to the cations Ca, Mg, K, and Na. The total quantity of these four exchangeable cations (S value) can be related to the CEC value and expressed as the base saturation percentage (BSP). The individual values for exchangeable Ca, Mg, and K give certain indicators of the fertility status of the soil (Macro-nutrients). The exchangeable Na percentage (ESP) is an important criterion for sodic conditions.
Exchangeable cations are determined in the laboratory by flame photometry for K and Na, and by atomic absorption spectrophotometry (AAS) for Ca and Mg (Anderson and Ingram, 1993).
4.1.7 Phosphorous (P)
Compounds of P (ADP and ATP) act as energy sources in plants. Energy from photosynthesis and metabolism of carbohydrates is stored in these compounds for subsequent use in growth and reproductive processes. The role of P as a structural component of a wide variety of biochemical and seed formation are also important.
Phosphorous is commonly determined by the calorimetric method (Anderson and Ingram, 1993); Olsen for extractable P and Bray II for available P.
4.1.8 Calcium carbonate
The presence of CaCO 3 affects both the physical and chemical characteristics of a soil. High lime concentrations may not severely restrict water movement but may prevent root penetration. A high CaCO 3 concentration particularly in the very fine fractions brings risks of lime-induced chlorosis for many crops. The physical characteristics of calcareous
soils change when they are irrigated. It is therefore, a vital soil quality parameter under irrigated agriculture.4.1.9 Gypsum (CaSO 4 )
Gypsum indirectly affects soil physical properties and therefore, influences permeability and infiltration rate. It improves the structure and prevents sodium saturation. A small amount of gypsum is favourable for crop growth
because it serves as a source of Ca as a plant nutrient and replaces Na in the exchange complex and thus, acts to soil's chemical and physical characteristics