Arid Region Soils ( High pH, Sodic, Saline Soils)
What is a halophyte ?
A plant that is salt-tolerant
What is a common development of low-rainfall areas?
Accumulation of calcium carbonate that forms a calcic horizon because there is no rainfall that leeches these cations away
Why is it that salts become a problem with too much water rather than too little ?
As water would move from high elevations to low elevations is will eventually evaporate leaving dissolved salts
What are other nutrient problems in arid regions with alkaline soils ?
Boron deficiency is common, plants tend to have higher requirements for boron if calcium is abundant Phosphorus will react with the abundant Calcium to form calcium-phosphate compounds that become increasingly insoluble over time
How would application of sulfur/sulfuric acid benefit a sodic soil ?
By changing sodium bicarbonate to the less harmful and more leachable sodium sulfate (also decreases pH)
What are the common layers that form in calcareous soils ?
Concrete-like horizons called petrocalcic or duripans Also they may have a horizon rich in Gypsum (calcium sulfate)
What is the opposite of flocculation ? (the rise of tiny clumps[floccules] that create pores between them and promote formation of larger aggregates)
Dispersion; in dispersed soils clay particles seperate from one another, creating a gel-like condition
What classifies a sodic soil ? (black alkali)
EC < 4dS/m ESP > 15 pH generally excess 8.5, can be 10 or higher loss of macroporosity and degradation of aggregate structure leads to slow permeability to water/air (almost to zero, causing puddles) (similar to how rice paddies use soil) The extremely high pH can cause organic matter to dissolve, this O.M can move upward in capillary water flow, when the water evaporates, can give the soil surface a black color
What classifies a saline soil ? (white alkali)
EC > 4dS/m ESP < 15 The exchange complex is dominated by calcium and magnesium, not sodium Usually below 8.5 AKA : White alkali (due to accumulation of neutral salt which creates a crust when water evaporates)
What classifies a saline-sodic soil ?
EC > 4dS/m ESP > 15 Excess salts and excess sodium
What are the causes of low permeability in sodic conditions ?
Exchangeable sodium increases the tendency of aggregates to break up (slake) upon wetness. The clay/silt particles released by slaking clog soil pores as they wash down profile Expanding clay-types' degree of swelling increases in the presence of Na+ As a result larger pores responsible for drainage are squeezed shut Combination of high sodium and low ionic strength(dissolved salt concentrations) lead to dispersion
How would application of gypsum help reclaim saline-sodic soil ?
Gypsum has a high solubility which will increase ionic strength, thus further helping flocculate the soil (soil conditioner)
What are the causes of dispersion ?
High Sodium Na+ is only weakly attracted to soil colloids and have a large hydrated size, so they form a outer-sphere complex around colloids Low Salt Concentration the ionic concentration is slow causing diffusion of exchangeable cations away from clay surfaces, causing an ionic thick/swarmed layer of absorbed cations
What are CEC characteristics of a highly alkaline soil?
High clay that posses high amounts of permanent charge The high pH stimulate high levels of pH-dependent charges on the soil colloids (especially humus)
Low salt (ion) concentrations and weakly attracted ions (sodium) encourage soil dispersion and puddling
High salt concentrations and strongly attracted ions (calcium) promote clay flocculation and soil permeability
If you were dealing with a saline-sodic soil, and you did not take into account the water quality being used for irrigation, what could happen ?
If the water being used is too low in salts, it can hasten the change from saline-sodic to sodic conditions Not only does rainwater leech salts but it can promote dispersion from its impact
The reclamation of saline-sodic and sodic soils requires an additional process before leaching of excess salinity can be achieved
In order to make soils permeable enough for leaching to take place, the excess exchangeable sodium ions must first be removed from the exchange complex
What are some ways of managing soil salinity ?
Irrigation timing, salt can stall germination so to there needs to be a low salinity-zone created by drip-irrigation (precision ag. tech.) which is dependent upon the rate of water app. and texture of soil
what are the essential components of an irrigation system ?
Leaching and drainage
What are the ions that cause alkalinity ?
Na+ and Ca+
Are micronutrient metals readily available in alkaline soils ?
No, micronutrient metals (Zn,Cu,Fe,Mn) are less soluble at pH levels above 7 Plant growth is commonly limited by deficiencies of these elements
What are biological impacts of salt-affected soils ?
Osmotic effects soluble salts lower osmotic potential, making it more difficult for roots to remove water from soil. Conditions require expenditure on osmotic adjustments Salinity may delay/prevent seed germination Na+ competes with K+ in the process of transport across cell membrane during uptake
How do plants support the ecosystem in arid regions ?
Plants litter, host macro- and microorganisms, trap windblown particles and redistribute the limited supplies of water
What is salinization?
Result of accumulation of neutral soluble salts
What are plant symptoms of excessive salinity and sodicity ?
Salinity : severely stunted, small dark-bluish green leaves w/ dull surfaces Sodicity : scorching or necrosis of leaf marigins and tips These symptoms appear first in older leaves because they have been transpiring the longest
What are salinity and sodicity measurements ?
Salinity is the concentration of salts in soil solution Sodicity is the relative concentrations of specific ions that cause deterioration of soil physical properties
What are the methods of measuring sodicity and salinity ?
Sodicity is measure by exchangable sodium %, or sodium absorption ratio (ESP & SAR) Salinity is measure through electroconductivity (EC) (pure water is not highly conductive of electricity, but increases as salt does)
Why are effects of salinity and alkalinity in irrigation accentuated in arid regions ?
Water available from rivers or groundwater in this region are relatively high in salts because it has flowed through its soils, which typically have high levels of weatherable minerals The dry climate creates a relatively high evaporative demand, so large amounts are needed for irrigation. Even the best quality water contains some dissolved salts, a farmer will likely dump 3 tons/acre of salt annually from irrigation
What effects do gypsum and sulfur on the physical condition of sodic soils ? (perhaps better than the chemical)
When Na+ is replaced by Ca+ or H+, soil aggregation and water infiltration are improved The neutral exchange can then leach sodium salts from the soil, thereby reducing salinity and sodicity
When is a soil termed sodic ?
When the Na+ saturation exceeds 15%
Where do saline seeps occur ?
near fossil deposits of salts in lakes or underground pools, which can dissolve in groundwater moving horizontally and ultimately rising to the surface of soil in low-lying areas (creating a saline soil) When native perennial vegetation is removed, to cultivate crop species, decreasing evapotranspiration causing more water to flow towards lower elevations and with the calcic crusts it can further this process, possibly raise the water table to within a meter or less of soil surface, accumulating great amounts of salt to eventually inhibit plant growth
What are the physical effects of sodicity ?
oxygen becomes deficient due to breakdown of soil structure and very limited air movement water relations are poor due to slow infiltration and percolation rates
What are the classifications of salt affected soils ?
saline, saline-sodic, and sodic
What is the result of the negligence to provide adequate drainage ?
salinization
