ed production will become a serious issue. However, if mismanaged, the use of sodic soils could aggravate salinity and sodicity problems. Sodic soils are ameliorated by providing a readily available source of calcium (Ca2+), to replace excess Na+ on the cation exchange complex. The displaced Na+ is leached from the root zone through the application of excess irrigation water. This requires adequate amounts of water and an unimpeded flow through the soil profile. Over the past 100 years, several different site-specific approaches-involving the use of chemical amendments, tillage, crop diversification, water, and electrical currents-have been used to ameliorate sodic soils. Of these, chemical amendments have been used most extensively (Oster et al., 1999).
Saline-sodic soils have characteristics intermediate between those of saline and sodic soils. Like saline soils, they contain appreciable levels of natural soluble salts, as shown by ECe levels of more than 4 dS m-1. But they have higher ESP levels (greater than 15) and higher SAR values (at least 13). Crop growth can be adversely affected by both excess salts and excess sodium levels. The physic-chemical conditions of saline-sodic soils are similar to those of saline soils.
The problems of salt-affected soils have serious implications in the semi-arid region where both soil and land were prone to different levels of salinity. Salinity-induced land degradation is a major issue in Iran. In addition, sodicity-induced land degradation and microelement salinity such as boron salinity have been developed in some of its areas. The salinization of land resources in Iran has been the consequence of both naturally occurring phenomena (causing primary or fossil salinity and/or sodicity) and anthropogenic activities (causing secondary salinity and/or sodicity) (FAO, 2000).
In Iran more than 25 million ha (over 15%) from total land have associated with saline and sodic characteristics (Mahler, 1979; Barzgar 2002) and most of the land are found in the Central Plateau, the Khuzestan Plain and the northwest regions of the country.
In western-Azerbaijan province, about 24,500 hectares of the studied land have the quality of salt-affected soils (Samadi, 1963) and the most of these areas are located at the margins of Urmia Lake. In the past, most of these lands were used as natural pasture but in recent years due to the development of cultivation, the use of such land has expanded rapidly for cropping.
In view of the fact that salt-affected soils are very sensitive to degradative aspects, it is essential to first determine the basic characteristics of the soils before improvement any management strategies. To examine this hypothesis, the combination of environmental, morphological, physicochemical, and mineralogical properties was investigated in a salt-affected plain from Miandoab region (with area of 16,503 ha) in western-Azerbaijan province (36° 59′ to 37° 16′ N and 45° 40′ to 46° 0′ E).
The main aims of the research are as following:
To evaluate the basic characteristics of the examined region using a combination of morphological and physiochemical attributes.
To determinate the clay mineral types and iron oxides forms as well as their genetics in soils of the region.
To investigate the potential and limitations soils of the region and provide some data that focus on employing more sustainable management systems for soil reclamation and improvement strategies.

Chapter two
Overviews
2.1 Overview of Salt-Affected Soils
While salinity is often thought of as a soil issue characterized by the accumulation of high total soluble salt concentrations, it encompasses plant responses as well as effects of topically applied saline irrigation water (USDA-ARS, 2008). Thus, “salinity” is not a single stress or problem, but there are four major salinity issues that are the primary problems that require intensive site-specific attention on an individual basis in response to each stress (Qadir and Oster, 2004; Carrow and Duncan, 2010). These four primary salinity stresses are as follows:
Excessive levels of total soluble salts, that is, total salinity causing salt-induced drought stress.
Na+ permeability hazard: excessive levels of Na on the soil cation exchange sites (cation exchange capacity or CEC) and precipitated as Na carbonates causing soil structure degradation.
Ions that are toxic to roots or shoots or may cause other problem ion issues as they accumulate.
Actual nutrition concentrations, interactions, and imbalances caused by nutrients and other ions in the water or soil.
2.2 Salts
Salts are composed of positively charged ions (cations) and negatively charged ions
(anions). They can be dissolved in water (soluble salts) or be present as solids. Salts in soil can originate from soil parent material; from irrigation water; or from fertilizers, manures, composts, or other amendments. The predominant salts that accumulate in soils are calcium, magnesium, sodium, potassium, sulfate, chloride, carbonate, and bicarbonate. Any salt that accumulates in excessive amounts in soil can cause plant growth problems. Too much sodium causes problems related to soil structure. As sodium percentage increases, so does the risk of dispersion of soil aggregates. (Horneck et al. 2007)
2.3 Classifying Salt-Affected Soils
Salt-affected soils are very diverse in characteristics and exhibit a combination of the salinity stresses previously noted (Rengasamy, 2010a). Classification of a salt-affected soil is based on two major stresses, namely, (a) total soluble salt concentration, which relates to the potential for salt-induced drought stress and subsequent osmotic adjustments internally in the plant; and (b) the quantity of exchangeable Na+, which relates to the potential for deterioration of soil physical conditions from accumulated Na and for subsequent ion toxicities from excess Na. Soil pH is also often listed, but is generally not used in the U.S. Salinity Lab (USSL, 1954) classification scheme, which is the most prevalent classification.
Traditionally, the main types of salt-affected soils are broadly classified as saline, sodic, and saline-sodic soils. (Robert et al. 2012).
2.3.1 Saline Soils
The predominant exchangeable cations in saline soils are calcium and magnesium. Saline soils commonly have visible salt deposits on the surface and are sometimes called “white alkali” soils. Most salts in soil solution have a positive effect on soil structure and water infiltration. Therefore, water penetration is not a major concern with saline soils. Salts in the root zone can reduce crop yield by making it difficult for roots to extract water from the soil. Salts increase soil osmotic potential, causing water to move from areas of lower salt concentration (plant tissue) into the soil where the salt concentration is higher. High salt concentration in the soil can cause plants to wilt even when soil moisture is adequate.
Saline soils vary considerably in their salt content, type of salt, structure and ease to be reclaimed. Dominant anions are chlorides, sulphates and carbonates, sometimes nitrates and bicarbonates. Sodium salts occur most frequently, but calcium and magnesium compounds are common as well, while mixtures of various salts and complex minerals are not exceptional. The non-salt solution contains mainly calcium salts (50-80%); magnesium (15-35%), potassium (2-5%) and sodium (1-5%) make up the remaining cations. In saline soils, however, the percentage of Ca2+ is lower, and the values of K+, Mg2+ and Na+ is higher. Saline soils are often recognized in the field by the presence of a white surface crust, by damp oily-looking surfaces devoid of vegetation, stunted plant growth, and sometimes by tip burn and firing of leaf margins. Soil analysis rather than visual observations are nevertheless needed to properly assess salinity.
2.3.2 Sodic Soils
Sodic soils are high in exchangeable sodium compared to calcium and magnesium. EC is less than 4 dS/m and often less than 2 dS/m. Soil pH usually is greater than 8.5 and can be as high as 10 or even 11 in extreme cases. High exchangeable sodium, high pH, and low calcium and magnesium combine to cause the soil to disperse, meaning that individual soil particles act independently. The dispersion of soil particles destroys soil structure and prevents water movement into and through the soil by clogging pore spaces.
Sodic soils often have a black color due to dispersion of organic matter and a greasy or oily-looking surface with little or no vegetative growth. These soils have been called “black alkali” or “slick spots.”
2.3.3 Saline-Sodic Soils
Saline-sodic soils are high in sodium and other salts. They typically have EC greater than 4 dS/m (mmhos/cm), SAR greater than 13, and/or ESP greater than 15. Soil pH can be above or below 8.5.
Saline-sodic soils generally have good soil structure and adequate water movement through the soil profile. They can have the characteristics of either a saline or sodic soil, depending on whether sodium or calcium dominates. (Managing salt-affected soil of crop production, D.A Horneck et al. 2007). Table 2.1 show the physicochemical properties of salt-affected soil.
Salt- affected
Soil classification
Electrical conductivity
(EC)

این مطلب رو هم توصیه می کنم بخونین:   پایان نامه رایگان با موضوع گندم و برنج

Sodium adsorption ratio
(SAR)

Exchangeable sodium percentage
(ESP)

Typical soil physical condition
(Soil structure)

None below 4 below 13 below 15 flocculated
Saline above 4 below 13 below 15

دسته‌ها: No category

دیدگاهتان را بنویسید