The alkaline soils they are soils that have a high pH value (greater than 8.5). The pH is a measure of the degree of acidity or alkalinity of an aqueous solution and its value indicates the concentration of H ions+ present.
Soil pH is one of the most important indices in soil analysis, since it decisively influences the biological processes that occur in this matrix, including the development of plants..
The pH of extreme acidic or basic values, generate adverse conditions for the development of all forms of life in the soil (plants and animals).
Mathematically the pH is expressed as:
pH = -log [H+]
where [H+] is the molar concentration of H ions+ or hydrogen ions.
The use of pH is very practical, as it avoids handling long figures. In aqueous solutions, the pH scale varies between 0 and 14. Acidic solutions, where the concentration of H ions+ is high and greater than that of OH ions- (oxyhydrile), have a pH lower than 7. In alkaline solutions where the concentrations of OH ions- are the dominant ones, the pH has values greater than 7.
Pure water at 25orC, has a concentration of H ions+ equal to the OH ion concentration- and therefore its pH is equal to 7. This pH value is considered neutral.
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Among the characteristics of alkaline soils we can mention:
They are soils with a very poor structure and very low stability, not very fertile and problematic for agriculture. Have a characteristic surface seal.
They frequently present a hard and compact calcareous layer between 0.5 and 1 meter deep and various types of compactions in the form of crusts and flats..
This leads to a high mechanical resistance to the penetration of the roots of the plants, and problems of reduced aeration and hypoxia (low concentration of available oxygen).
They have a dominant presence of sodium carbonate NatwoCO3. They are clayey soils, where the majority presence of clay causes the expansion of the soil by swelling in the presence of water..
Some ions that are present in excess are toxic to plants.
They have poor water collection and storage.
They have low infiltration capacity and low permeability, therefore, poor drainage. This leads to rain or irrigation water being retained on the surface, also generating low solubility and mobility of the scarce available nutrients, which ends up translating into nutrient deficiencies..
They are generally located in semi-arid and arid regions, where rainfall is scarce and alkaline cations are not leached from the soil..
As clayey soils with a predominance of clays in their composition, they have aggregates of hydrated aluminum silicates that can exhibit various colors (red, orange, white), due to the presence of particular impurities..
Excessive concentrations of aluminum ions are toxic to plants (phytotoxic), and therefore, are a problem for crops.
The alkaline condition of the soil generates a characteristic chemical composition with factors such as:
This condition reduces the transpiration of the plants and the absorption of water by the roots, due to the osmotic pressure that it generates..
High sodicity reduces the hydraulic conductivity of the soil, decreases the water storage capacity and the transport of oxygen and nutrients..
Boron being toxic to plants (phytotoxic).
High pH values associated with alkaline soils, with predominant concentrations of OH ions-, limit the availability of plant nutrients.
Bicarbonate is also phytotoxic, since it inhibits root growth and plant respiration..
Aluminum is another phytotoxic metal that has effects similar to the excessive presence of bicarbonates.
In general, alkaline soils present phytotoxic concentrations of chloride ions (Cl-), sodium (Na+), boron (B3+), bicarbonate (HCO3-) and aluminum (Al3+).
Alkaline soils also have reduced solubility of plant nutrients, particularly of macronutrients such as phosphorus (P), nitrogen (N), sulfur (S) and potassium (K) and of micronutrients such as zinc (Zn), copper (Cu), manganese ( Mn) and molybdenum (Mo).
The production of vegetable crops in arid and semi-arid environments is limited by the restrictions imposed by low and variable rainfall, the existing infertility and the physical and chemical limitations of alkaline soil..
There is a growing interest in incorporating alkaline soils into agricultural production through the implementation of methods to correct and improve their conditions..
The management of alkaline soils includes three main strategies to increase their productivity:
For the improvement of transient salinity conditions (salinity not associated with groundwater surges), the only practical method is to maintain a flow of water towards the interior through the soil profile..
This practice could include the application of plaster (CaSO4) to increase the fraction of salt leachate from the root development zone. In sodium subsoils, in contrast, the application of appropriate amendments is required in addition to the leaching or washing of the sodium ions..
Soluble boron can also be removed by washing. After sodium and boron leaching, nutrient deficiencies are corrected.
Subsoil plowing, or deep subsoiling, consists of removing the matrix from the subsoil to break compacted hardened layers and improve fertility and moisture by adding water..
This technique improves soil productivity, but its effects are not maintained in the long term..
Correction of the sodicity of the soil (or excess of sodium ion, Na+) with deep subsoiling, it only has positive long-term effects if the soil structure is stabilized with the addition of chemical improvers, such as calcium in the form of gypsum (CaSO4) or organic matter, in addition to controlling the traffic or passage of people, livestock and vehicles, to reduce soil compaction.
Gypsum as a source of calcium ions (Catwo+) to replace sodium ions (Na+) of the soil, has been used extensively with variable success, with the aim of improving the structural problems in soda soils.
Gypsum correction prevents excessive swelling and dispersion of clay particles, increases porosity, permeability and reduces the mechanical resistance of the soil..
There are also research works that report an increase in the leachate of salts, sodium and toxic elements, with the use of gypsum as a correction of alkaline soils..
There are recently developed techniques for the improvement of sodium soils, which include the use of various polymers of polyacrylamide (PAM for its acronym in English).
PAMs are effective in increasing hydraulic conductivity in sodium soils.
Surface padding (or mulchs in English) have several favorable effects: they reduce the evaporation of surface water, improve infiltration and decrease the movement of water and salts to the outside.
The superficial application of organic waste in the form of compost, results in a decrease of Na ions+, possibly due to the fact that some soluble organic compounds in the compost material can trap the sodium ion through the formation of complex chemical compounds.
Additionally, the organic matter of the compost contributes macronutrients (carbon, nitrogen, phosphorus, sulfur) and micronutrients to the soil and promotes the activity of microorganisms..
The correction with organic matter is also carried out in deep layers of the soil, in the form of beds, with the same benefits as the superficial application.
The application of chemical fertilizer beds in the subsoil is also a correction practice for alkaline soils that improves agricultural productivity, since it corrects the deficiency of macro and micronutrients.
Several studies have examined the practice of first-use crops as a mechanism to modify soil structure, creating pores that allow roots to develop in hostile soils..
Perennial woody native species have been used to produce pores in impermeable clay subsoils, whose first-use cultivation favorably modifies the structure and hydraulic properties of the soil..
The use of selective breeding to improve the adaptation of crops to the restrictive conditions of alkaline soils has been highly questioned, but it is the most effective long-term and most economical method to improve crop productivity in these hostile soils..
The principle of avoidance practices is based on the maximum use of resources from the relatively benign alkaline soil surface, for the growth and yield of vegetable crops..
The use of this strategy implies using early maturing crops, less dependent on subsoil humidity and less affected by its adverse factors, that is, with the ability to avoid the adverse conditions present in alkaline soil..
Simple agronomic practices, such as early harvest and increased nutrient input, increase localized root development and thus also allow an increase in the volume of surface soil exploited in the crop..
The retention of pruning and stubble are also agronomic techniques to improve cultivation conditions in alkaline soils..
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