Growth is suppressed when a threshold value of salinity is exceeded

By Dr. S.M. ALAM and M.U. SHIRAZI, NIA, Tandojam
Apr 21 - 27, 2003

It is a well-documented fact that irrigated agriculture, like in Pakistan has faced the challenge of sustaining its productivity for centuries. Because of natural hydrological and several geo-chemical internal and external factors happening on and inside the earth, as well as irrigation induced activities, soil and water salinity and associated drainage problems continue to plague agriculture tremendously. Salinity is the concentration of dissolved mineral salts present in waters and soils on a unit volume or weight basis. The major dissolved solutes comprising mineral salts are the cations of sodium, calcium, magnesium and potassium and the anions chloride, sulphate, bicarbonate, carbonate and nitrate. The criterion used for salinity hazard depends mainly on the use of water or soil media. For instance, an irrigation water of EC > 0.7 dSm-1 posses little or no threat to most crops, while EC > 3.0 dSm-1 may restrict the growth of most field crops. A soil with an EC > 4dSm-1 in the soil saturation extract is traditionally characteristics as saline soil.

Sodium detrimentally affects the soil's physical properties, such as its infiltration rate. The problem of salinity manifests itself in the environment in a number of ways: saline irrigation and drainage waters, saline and sodic soils, saline groundwater, seawater intrusion and brines from natural salt deposits. The primary source of salts in water and soils is the chemical weathering of earth materials, i.e. minerals that are constituents of rocks and soils. Evaporative salinization, e.g. evaporation of water and transpiration by plants, and rainfall, snowmelt waters and irrigation waters, affect the concentration of dissolved mineral salts.

Natural secondary sources of salts include atmospheric deposit of oceanic salts along coastal areas, seawater intrusion into estuaries due to tidal events, seawater intrusion into groundwater basins in coastal areas due to overdraft, saline water from rising groundwater. Anthropogenic sources of salts include irrigation and drainage waters, soil and water amendments, animal manure and waste, chemical fertilizer, sewage sludge and effluent and oil and gas field components.

Most of the water in the hydrosphere in salty and much of the fresh water is frozen. The ocean contain about 97% of the water, continents about 2.8%, and the atmosphere about 0.001%. About 72% of the water associated with land is found in ice caps and glaciers and about 22% in groundwater, much of which are economically irretrievable. This leaves only a small percentage of readily manageable fresh water as a resource of water supply. The world's land surface occupies about 13.2 x 109 ha. of which are arable and only 1.5 x 109 ha. of which are cultivated. Of the cultivated lands, about 0.34 x 109 ha. are saline and another 0.56 x 109 ha. are sodic.

The accumulation of salt in soils and the frequently accompanying problem of drainage has plagued irrigated agriculture for centuries. Such accumulation results when plants transpire waters, but leave most of the salts in the soil solution. In the context of salinity responses of plants, salinity refers to concentration of soluble salts so high as to affect significantly the collective properties of the solution to which the roots are exposed, specifically by reducing its osmotic potential. In the agricultural context, a soil is considered saline if the electrical conductivity of the saturation extract exceeds 4 dS/m at 25C, and the percentage of the cation exchange capacity of the soil occupied by sodium is less than 15. The value of 4 dS/m corresponds to about 40 meq/l salt.

Sodicity refers to Na+ ions in particular in the absence of the osmotic component characteristics of salinity. Sodicity is estimated by several criteria. Excess salinity within root zone reduces plant growth rate. The hypothesis that seems to fit observations best asserts that excess salt reduces plant growth, primarily because it increases the energy that the plant must expend to acquire water from the soil and make the biochemical adjustments necessary to survive. This energy is diverted from the processes that lead to growth and yield, including cell enlargement and the synthesis of metabolites and structural compounds.

Typically, growth is suppressed when a threshold value of salinity is exceeded. The threshold value depends on the crop, external environmental factors such as temperature, relative humidity, or wind speed, and the water supplying potential of the root zone. Suppression of growth increases as salinity increases until the plant dies. Most plants are relatively salt-tolerant during germination and more sensitive during seedling emergence and early stages of seedling growth. Hence, it is imperative to keep salinity in the seedbed low after germination.

Visual observations of soils, crops, topography, elevation of water in surface drainage, and plants are rarely sufficient to diagnose a salinity problem conclusively. For example, salinity may reduce the yields of crops by as much as 25% without visible symptoms. Moreover, visual observations may lead to a false diagnosis. For example, the minerals calcite and gypsum are essentially harmless to plants and soil properties, but they can form a white crust on the surface of soils that may be confused with potentially harmful soluble salts. Still, visual information can be useful. In measurements of soil electrical conductivity and on the spot measurements of the salinity of irrigation water and drainage water will help to narrow the possibilities and pinpoint the areas to sample.

Saline fields often can be identified by the presence of spotty white patches of precipitated salt. Such precipitates usually occur on slightly elevated positions or in unvegetated areas, where water evaporates and leave the salts behind. Visible salt crusts obviously indicate surface salt accumulation, but they do not provide reliable evidence of high salinity in the root zone.