## 2 Materials and Methods
### Calculating Procedures
#### 2.5.1 Effect of Water Quality Constituents on Soil Quality  
The effect irrigation water quality constituents have on soil quality is modified by the fact that soil acts as a temporary store of water for plant use and that crops extract almost pure water, leaving most constituents within the soil. The degree to which these constituents accumulate and interact with soil components determines how they affect soil quality. IriggWQ considers the following suitability indicators to assess the effects irrigation water constituents have on soil quality:  
1. Soil profile salinity  
Soil salinity serves as a qualitative indicator of the suitability (fitness) of soil to support plant growth and of the range of crops that can be expected to be successfully cultivated under specific conditions. IriggWQ uses the United States Salinity Laboratory Staff criteria to assess the FFU of soil profile salinity for crop production [1]. The effects on crop production of soil saturation extract levels below 200 mS/m are considered to be negligible, while only tolerant crops yield satisfactorily at levels above 800 mS/m. Soil profile salinity is evaluated on the annual mean value as calculated with the modified SWB model. 2. Soil permeability  
The maintenance of sufficient permeability is a prerequisite to ensure that it is possible to supply crop water requirements through irrigation and to facilitate salinity control and  
Figure 2: The IriggWQ screen for capturing water quality parameters, with shaded fields calculated by IriggWQ and not entered by the user. Note that calculators assist with the conversion of units for ion concentrations not reported in mg/L and the facility to check for apparent reliability of the water analysis. the reclamation of salt-affected soils. Two processes are at play to determine the permeability of soil. Hydraulic conductivity operates below the surface within the bulk of the soil, unaffected by the mechanical disturbance of surface soil structure, to determine permeability within the root zone. Infiltrability, on the other hand, is a surface phenomenon largely affected by any mechanical disturbance of surface soil structure [19]. Both phenomena are largely affected by soilodicity and the EC of either the percolating soil water or infiltrating rain and irrigation water. Soils differ too much in their response to different sodicity/EC combinations to develop a universal response curve that separates favourable from unfavourable conditions for use in IrrigWQ. Following a literature review [10], sodicity/EC combinations were defined to qualitatively describe the degree to which both hydraulic conductivity and infiltrability of sensitive soils are affected by these variables. IrrigWQ reports the degree to which hydraulic conductivity is reduced for the soil layer that is most affected and how infiltrability is affected by surface crust formation. 3. Oxidisable carbon loading. Although organic matter is a minor soil constituent, it plays a major role in determining soil physical conditions, soil chemistry, and soil biology. Adding organic material to soil through irrigation would, thus, mostly be considered beneficial. The diversity of the soil microorganism population enables microbes to decompose organic materials from many different sources. Oxygen is consumed during this decomposition process. Should the organic loading become so high that microorganisms consume oxygen at a rate higher than the gaseous exchange capacity of soil, oxygen becomes depleted and anoxic conditions are established. Since plant roots require oxygen for their metabolic functions, most plants display reduced growth, or die, when their roots are exposed to oxygen stress. Anoxic conditions can also give rise to unpleasant odours. From a literature review [10], it appears important to restrict both the short- and longer-term organic load that is applied to soil.