| Soils, Crops and Fertilizer Use |
|Chapter 12: Salinity and alkalinity problems|
Salinity and alkalinity problems are most likely to occur under 2 conditions:
• Irrigated soils in semi-arid and arid regions (less than 500 mm annual rainfall) where rainfall or irrigation isn't sufficient to leach accumulated salts out of the root zone. The salts are released by decomposing rock and other parent material below the subsoil and are also brought in by irrigation water and additions of chemical fertilizers and manure.
• Intrusion of salt water into low-lying areas near oceans and seas.
In humid regions, there's usually enough rainfall to flush the salts downward out of the root zone. In low-rainfall areas, irrigation may move salts downward, but they move back up again as the soil dries out between irrigations unless enough extra water is applied. The very high evaporation rates common to these drier regions aggravate this tendency. In many cases, subsurface drainage is also poor, which makes matters worse. Bringing land under irrigation may raise the water table to within a meter or so of the surface, enabling salts to move upwards by capillary action the same way kerosene travels up a lamp wick.
Saline and alkali (sodic) soils fall into 3 classes according to the amount of soluble salts and adsorbed (held by clay and humus particles) sodium they contain: (These soils are also referred to as halomorphic soils)
• SALINE SOILS: These contain enough neutral soluble salts to harm plant growth much like fertilizer burn does. The salts are mainly chlorides and sulfates of sodium, calcium and magnesium. Less than 15% of the soil's exchange capacity (see Chapter 6) is occupied by adsorbed sodium ions, and the pH is usually below 8.5. Saline soils are also called white alkali soils, because the salts tend to accumulate on the soil surface. The usual causes are lack of enough water for adequate leaching, poor drainage, or both.
• SALINE-ALKALI SOILS (Saline-Sodic Soils): These soils not only contain excessive anount of soluble salts, but also harmful amounts of adsorbed sodium (i.e. plus-charged sodium ions that adhere to the negatively-charged clay and humus particles). More than 15% of the soil's exchange capacity is occupied by sodium ions. Although sodium is strongly basic, the pH of these soils is usually below 8.5 due to the buffering influence of the the neutral soluble salts.
• NON-SALINE ALKALI SOILS (Sodic Soils): These soils contain only low levels of soluble salts but have excessive amounts of adsorbed sodium. More than 15% of the soil's exchange capacity is occupied by adsorbed sodium ions held by clay and humus particles. The pH is above 8.5 and often as high as 10, because there aren't enough soluble salts to exert a buffering effect. Sodic soils have very poor physical condition due to their high sodium content; it disperses and puddles (breaks down) soil aggregates (crumbs and clumps of soil particles), making the soil rather impervious to water. Sodic soils are also called black alkali soils, since their surfaces are often black due to the accumulation of dispersed humus brought to the surface by the upward capillary movement of water (from a high water table) and by evaporation.