1. LWR 107 Soils in Dry Regions SOIL ALKALINITY

2. Causes of Alkalinity: Natural Vs Anthropogenic Characteristics and Problems of Alkaline Soils Development of Salt-Affected Soils Measuring Salinity and Sodicity Classes of Salt-Affected Soils Reclamation of Saline-Sodic and Sodic Soils

3. Alkaline soils?  pH greater than 7.5  Presence of large quantities of Calcium, magnesium, potassium and sodium  Typical in the low rainfall areas- low leaching, and deserted areas. Physical weathering dominates  In some areas, we can find alkaline soils in small areas within production fields  Poor management may cause zones of alkalinity  Typical alkaline soils contains Ca/Mg- and Na- carbonates

4. How the pH increases? Clay-Ca+2 + H2O --------  Clay-H+ Ca+2 + 2OH- Clay-Ca+2 + H2O --------  Clay-H+ Ca+2 + 2OH- OH release in this reaction increases OH in the soil, so if pH=14-(pOH) so pH increases!  Usually in this kind of soil, the %BS is dominated by Ca/Mg in saline soils, and Na in sodic soils.  Usually %BS is very high (close to 100%)  Consequences of high availability of calcium  P deficiency  Micronutrient deficiency  High osmotic tension, water stress!

5. Soil Alkalinity  Soil physical conditions can be severely affected  Sodic soils, dominated by sodium are structure less and very disperse.  Lack of porosity and difficult root penetration  Water stress in most plants, due to an increase in soil osmotic potential

6. Soil alkalinity  Basic definitions  Electrical conductivity: the presence of salts in the soil generates electrical flux of current when an anode and cathode are connected.  Electrical conductivity (mmhos/cm-1) correlate electricity Vs salt content in a soil extract.  Sodicity: Measured as %Na in exchangeable sites (CEC). ESP =exchangeable Na %  ESP = [Na]/CEC x 100  Sodium activity ratio = Na/ 0.5 (Ca) + 0.5 Mg^1/2

7. Types of Alkaline soils TYPE Soil EC (mmhos/cm) ESP (%) SAR SALINE<40-150-12 SODIC0-4>15>12 SALINE- SODIC >4>15>12 No saline no sodic 0-40-150-12

8. How to manage alkaline soils  Difficult to manage  Before identify the type of management we have to identify the type of alkaline soil  Every type (saline, sodic, saline-sodic) has a different management.  In general any practice that reduces salt content will be needed.  Adding amendment (gypsum, OM)  Leaching salts with clean water  Irrigation techniques  Utilizing salt tolerant crops

9. Types of Alkaline soils TYPE Soil pH Typical ions Management SALINE<8.5 Ca, Mg, K (chlorides and sulfates) IrrigationGypsum SODIC >>8.5 can reach 10 >>Sodium Very diff. Add water SALINE- SODIC >8.5 Ca, Mg, K and >>> Na Gypsum No saline no sodic >7<8.5 Ca. Mg, K none

10. Things to remember:  Three types of salt-affected soils are saline, sodic and saline-sodic soils.  Salt-affected soils may inhibit seed germination, retard plant growth, decrease soil physical properties and/or cause irrigation difficulties.  Saline soils often can be reclaimed by leaching salts from the plant root zone.  Sodic soils often can be reclaimed by replacing soil sodium with calcium by adding a calcium-based soil amendment (gypsum).  Sodic soils respond to continued use of good irrigation water, good irrigation methods and good cropping practices.

11. Alkaline soils can be black, why? Because the high pH levels and high salt levels the organic matter becomes very disperse. The disperse humus moves upward in the capillary water flow, and when the water evaporates leaves a very intense black color on the soil surface. So, not always black soils are good soils for food production.

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