The soil acidity

The soil acidity is one of the most important soil characteristics. The acid, neutral or alkaline pH of the soil considerably affect factors such as the 

The active acidity is the instantaneous condition of free hydrogen ions in the soil solution. It results most frequently from dissociated free acids or dissociated acid salts occurring in the soil. It is expressed by the pH value, and determined by colorimetric or potentiometric methods. It should be determined in fresh samples as far as possible, preferably in the field or as soon as possible after transporting the sample into the laboratory, to obtain the actual acidity under natural conditions. 

The exchange acidity is defined as the ability of the soil to change the pH value of solutions of neutral salts (electrolytes). This is the acidity resulting from hydrogen ions displaced from the soil into the solution by the action of neutral salts according to the scheme 

The amount of hydrochloric acid formed equals the exchange acidity. It is ascertained either by titration, as the concentration H /100 g of the earth, or by a potentiometric method as a value pH/KCl. 

The hydrolytic acidity is the ability of the soil to change the pH of salts split by hydrolysis. This represents the ability of the soil to bind a strong base and to release an equivalent amount of a weak acid. For determining the hydrolytic acidity, a salt of weak acid and strong base is used (sodium acetate or calcium acetate) the following reaction occurs  

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Sometimes it may be desirable to add nutrients in a form more quickly available or more concentrated than those derived from the above soil-building techniques. It may also be desirable to adjust the soil acidity so the nutrients present are more available to vegetable plants. There are a number of ways to add nutrients and adjust soil pH that are acceptable to organic gardeners. 

Studies have shown that adding bioenhancement agents to oil spilled on land can enhance the removal rate of the saturate and some of the aromatic fraction of the oil, so that as much as 40% of the oil is degraded in time periods from one month to a year. It has been found that the agents are most effective when added at an oil-to-nitrogen-to-phosphorus ratio of 100 10 1. Fertilizers that maintain the soil at a more neutral level are best for degrading oil. Fertilizers that make the soil acidic usually slow biodegradation. Fertilizers that are more oil-soluble and less water-soluble are most effective as they are not as likely to be washed away. 

A second noteworthy flooded soil is acid sulfate soil. Sediments along tropical and subtropical coastlines and river deltas may contain significant quantities of Fe(II) sulfides. When dr ained, these sulfides oxidize to H2SO4 and the acidic Fe3+ ion. The soil acidity can increase to pH 2. Such conditions are highly phytotoxic and can be remedied under aerobic conditions only by extensive leaching and lime applications. If resubmerged, acid sulfate soils revert rapidly to near neutrality as the Fe(III) and sulfate are reduced back to Fe(II) sulfides. 

If the soil is leached with buffer solution until the soil pH equals that of the buffer, titrating the remaining buffer capacity of the solution measures the soil acidity that must be neutralized to produce a soil pH equal to that of the buffer. A more rapid method is add buffer solution to soil without attempting to bring the final pH of the... 

Many plants grow poorly in acid soils. Early workers supposed that this was a consequence either of hydrogen ion toxicity or of Ca and Mg deficiencies. The soil acidity must be greater than about pH 3, however, before the H+ concentration itself is toxic to most plant species. Although the components of acidity are emphasized in acid soils, the major exchangeable cations are Ca, Mg, and to a lesser extent K in soils of pH > 4,5 to 5. 

Soil acidification affects extensive areas in many countries of the region, caused by both natural factors and human interventions. Assessment of natural aspects of soil acidification was shown in Section 2. The anthropogenic problems of soil degradation due to acidification are related to acid rains (mainly forest soils, see more details in Chapter 4) and application of mineral fertilizers, most of which are physiologically acid and increase the soil acidity during perennial application, especially application of nitrogen and phosphorus fertilizers. 

Pollutant deposition may increase soil acidity, decrease nutrient availability, and increase the solubility of toxic ions. In central and northern Europe, increased pollutant deposition over several decades has induced soil acidification. In the soil, acid deposition undergoes many reactions, and this leads to a reduced alkalinity and increased aluminum content in the soil solution. The exchange complex of the soil becomes dominated by aluminum, the exchange acidity increases, bases are leached in association with acid anions due... 

The soil acidity results from mutual effects of several factors, particularly the soil composition, moisture and biological activity. The soil reaction... 

Plants usually require weakly acid to neutral soils, in which most biogenic elements occur in utilizable forms. Soil bacteria prefer neutral soils, and weakly acid soils are advantageous for fungi. The soil acidity is usually adjusted by soil liming, which should be accompanied by the application of organic fertilizers and suitable industrial fertilizers [3-6]. 

Toxic metals associated with wetland soils are present in various forms dissolved, adsorbed, bound to carbonates, to Fe and Mn oxides, to sulfides, and insoluble organic matter forms, and within the crystalline structure of primary minerals (Shannon and White, 1991). The amount of organic matter and clay minerals, the soil acidity (pH), and the sediment oxidation-reduction status (Eh) of soils are very important physicochemical properties influencing the mobility of toxic metals. 

He then links soil acidity (and hence aluminium) and acid precipitation by stating air pollutants considerably increased the soil acidity in Soiling between 1966 and 1973. ... 

Heavy applications up to 25 t/ha of ground limestone may be required to reduce the soil acidity. Sometimes a very acid reddish brown layer ( drummy layer) builds up between the organic topsoil and the imderlying elay subsoil. Peat soils... 

Cation exchange in soil is the mechanism by which potassium, calcium, magnesium, and essential trace-level metals are made available to plants. When nutrient metal ions are taken up by plant roots, hydrogen ion is exchanged for the metal ions. This process, plus the leaching of calcium, magnesium, and other metal ions from the soil by water containing carbonic acid, tends to make the soil acidic ... 

Soil pH is a measure of the soil acidity or soil alkahnity. Because pH can affect the availability of nutrients, it is an important consideration for soil quality. Most crops prefer a neutral or slightly acidic soil. The pH is controlled by the addition of amendments to the soil the addition of minerals such as hme (calcium hydroxide Ca(OH)j) can increase the soil pH, whereas nitrogen-containing amendments lead to increased acidification. Fertilizer can also be added to the soil to ensure the availability of nutrients, specifically the macronutiients nitrogen, phosphorus, and potassium. 

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