Abstract
The sustainability of irrigated agriculture depends on the quality of irrigation water used. The electrolyte concentration (EC) of irrigation water may lead to the accumulation of salts in the root zone layers and affect the physiological functions of the crop by osmotic and ion toxicity effects. Further, the cationic and anionic composition of the water may alter the exchangeable cation composition of the soil as well as its pH. Because of the dominance of sodium salts in many sources of irrigation water, parameters related to sodium such as exchangeable sodium percentage (ESP) of soils and sodium adsorption ratio (SAR) of soil solutions have been commonly used to study the effects of sodium in irrigation water on soil structural stability. Quirk and Schofield’s concept of ‘threshold electrolyte concentration’ (TEC) has shown the importance of electrolytes in preventing the effects of sodium on soil structure. Based on this concept, several models have been proposed to relate ESP or SAR with EC to predict the possible impacts of irrigation water on soil structural stability. However, many research reports indicate that this relationship varies with soils, and a given model is not suitable for all types of soils. Further, the effects of potassium and magnesium in the processes leading to clay dispersion are disregarded in these models. This essay analyses all the factors involved in the structural failure of soils with different cationic composition, identifies the defects in these TEC models, and re-defines TEC on the basis of new insights on dispersive and flocculating charges of soils. This review does not deal with EC effects on crops nor the role of contaminant ions not involved with soil structural stability.
Highlights
Irrigation of soils is an important component of productive agriculture, in regions where rainfall is not sufficient to provide enough water to be used by crops during their growth and yield cycles
Marchuk and Rengasamy [25] hypothesized that, because of both increasing charge and increasing size of a clay particle, the electron cloud in the bond formation will be less influenced by the clay anion, and the covalency or ionicity index of a cation alone will indicate the ionic character of a clay-cation bond
Classifying irrigation water in relation to the impact on soil structural stability only on the basis of its ionic composition is not realistic, because the characteristics of the soil used for irrigation are critical in the effects
Summary
Irrigation of soils is an important component of productive agriculture, in regions where rainfall is not sufficient to provide enough water to be used by crops during their growth and yield cycles. Quirk and Schofield [4], in their widely cited ‘Landmark Paper’ [5], proposed the concept of ‘threshold electrolyte concentration’ (TEC), which is the electrolyte concentration (EC) in soil water above which the physical properties of a soil at a given ESP are not affected by sodicity Based on this principle, several models have been proposed (e.g., [6,7,8,9,10,11,12], among others) relating ESP and EC of soils to either clay dispersivity or changes in hydraulic conductivity, and identifying the TEC to maintain soil structural stability. The role of toxic contaminants not involved in soil structural processes is not discussed
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