Presentation on theme: "The Punic Wars –Case for Salt in Soils “After the third punic war, the Romans stormed the town and the army went from house to house slaughtering the inhabitants."— Presentation transcript:
The Punic Wars –Case for Salt in Soils “After the third punic war, the Romans stormed the town and the army went from house to house slaughtering the inhabitants in what is perhaps the greatest systematic execution of non- combatants before World War II. Carthaginians who weren't killed were sold into slavery. The harbor and the city was demolished, and all the surrounding countryside was sown with salt in order to render it uninhabitable ” Richard Hooker (The Conquest of the Hellenistic Empires)
History of Western Civilization By E.L. Skip Knox
ALKALINE AND SALT AFFECTED SOILS Soils of Dry Regions (Alkaline Soils) Causes of Alkalinity Characteristics of Alkaline Soils Classes of Salt Affected Soils Management of Soil Affected soils
Soils of Dry Regions (Alkaline Soils) The accumulation of salt is one of several characteristics of dry region soils. Soluble salts are minerals dissolved in water Accumulation of salts occur where evapotranspiration is greater than precipitation Low precipitation means less leaching and more accumulation of cations released from weathering. Soils in these regions are generally in the alkaline range ~pH 7 or above Dry region soils can be defined as follows: – High pH (>7) (alkaline soils) – High saline content (saline soils) –eg, NaCl, Na 2 CO3, NaHCO 3 – High Sodium content (sodic soils)
Causes of alkalinity (High Soil pH) Sources of alkalinity The basic, OH - generating anions are carbonates (CO 3 2- ) and bicarbonates (HCO 3- ) Cations in soil solution and colloid surface (Ca 2+, Mg 2+, K +, and Na + ) do not produce acid nor alkaline in reaction with water (Their effect in water is neutral (pH ~ 7) 1)Carbon dioxide and carbonates
Causes of alkalinity (Contd.) 2)Role of individual cations (eg. Na + or Ca 2 + ) The particular cation associated with carbonate and bicarbonate anion influences the pH. If Na + is prominent in soil, more OH - will be produced because Na is more highly water soluble than Ca 2+. Na 2 CO 3 2Na + + CO 3 2- 3)Role of soluble salts High levels of neutral salts (CaSO 4, Na 2 SO 4, NaCl, and CaCl 2 ) lowers the pH by common ion effect. E.g., increasing the concentration of Ca 2+ or Na + ions on the right side of reaction drives the reaction to the left.
Characteristics of Alkaline Soils 1)Nutrient deficiencies Availability of micronutrients such as Cu, Zn, Fe, Mn, and B are influenced by soil pH. The lower the pH the more soluble and available they are. So in alkaline conditions, plant growth may be limited by deficiencies of these metals. Phosphorus is commonly deficient in alkaline soils because it is tied up with Ca or Mg phosphate. Only few micronutrients like Mo are more available in alkaline conditions.
2)Cation exchange capacity Alkaline soils have higher cation exchange capacity because at low pH, soil charge becomes more positive, whereas at high pH, negative charge increases. (High pH stimulates pH dependent charge on the soil colloids). 3)Calcium accumulation in subsurface layers. Alkaline soils in low rainfall regions commonly have layers of CaCO 3, CaSO 4 or other such layers that can inhibit plant growth. 4)Soil water supply The subsoil layers are always drier than in humid regions resulting in much competition for water by native plants. These soils therefore require greater water management if cultivation is a priority.
Where do saline and sodic soils occur 1)They occur naturally in areas where evaporation is more than precipitation 2)They occur in low flat areas with high water table that may be subject to seepage from higher elevations 3)They occur in areas where there is irrigation
Measuring Salinity and Sodicity Salinity is measured as: – total dissolved solids, TDS (mg/L), or – electrical conductivity, EC (dS/m) Sodicity is measured as – exchangeable sodium percent (ESP), or – Sodium adsorption ratio (SAR).
Classification of Salt-Affected Soils Classification of Salt-Affected Soils ( Classification is based on Ec, ESP, pH) 1)Saline Soils – high dissolved salts in soil solution (affects plant growth) – EC ≥ 4 dS/m (saturated paste) – Exch. Na < 15% – pH < 8.5 2)Saline-Sodic Soils – Same as saline soils except Exch. Na > 15%
3)Sodic Soils – do not have excess salts (EC < 4 dS/m) – Exch. Na >15 – pH > 8.5 – dispersion of soil colloids------->poor permeability/drainage
Plants respond to these soils in various ways. The two common mechanisms are: Osmotic effects – Salts lower osmotic potential of soil water Specific ion effects – The kind of salt makes a difference in how plants respond to salininty. – Some ions eg. Na +, Cl -, HCO 3 - are toxic to some plants. – Also high intake of Na can cause imbalance in uptake of other cations (cations compete among themselves for uptake). Effects on soil physical properties – Breakdown in structure (oxygen depletion) – Breakdown in structure (poor water infiltration and percolation) How salts accumulation affect plant growth
Management of Salt Affected Soils (Soil Reclamation) 1)Saline soils – Establish drainage – Leach with water (Leaching Requirement) 2)Saline-sodic soils – Add Ca (usually gypsum CaSO 4 ) to remove exchangeable Na – Leach with high quality water to remove excess salts – Leach with high Ca irrigation water 3)Sodic soils – Add gypsum – Deep tillage – Leach with high quality water when possible
In Summary Alkaline and salt affected soils occur in arid and semiarid regions. They typically have high pH values and require different management practices The three classes of soils are saline, saline- sodic, sodic soils. They can be managed by leaching of excess salts with good water, replacement of excess ions, and sometimes using deep rooted vegetation