For example, aluminum and manganese are more soluble in soil water at a pH low, and when that event occurs, can be absorbed by the roots, being toxic at certain concentrations. On the contrary, certain minerals which are essential for plant development, such as calcium phosphate, are less soluble at a high pH, which has the result that under such conditions are less available in order to be absorbed and nourish plants.
Obviously in nature, there are plant species adapted to extremely acidic and basic environments. But agricultural production cultivars are often based on ionic environments that support soil solutions less extreme. In practice, it is rare to find soils with pH below 3.5 or above 10.
The pH is a measure of the hydrogen concentration expressed in logarithmic terms. The pH values are reduced as the concentration of hydrogen ions increases, ranging from a range of 0-14. The values below 7.0 are acidic, values above 7.0 are alkali and / orbasic, while hovering around 7.0 are called neutral. For every unit change in pH there is a change 10 times in magnitude in the pH (eg pH 6.0 is ten times more acidic than a pH of 7.0, while pH 5.0 is 100 times more acidic than 7.0).
In other words, the acidity of a soil depends therefore on the concentration of hydrogen ions [H+] in the solution of water and is characterized by the value of pH, defined as the negative logarithm base 10 of the concentration H+ : pH-log .=10 [H+]. Is an important diagnostic, with the effect of a series of causes and in turn leads to many agricultural problems.
The letters pH are a mere abbreviation for “pondus hydrogenii”, translated from Latin as potential hydrogen. 1909 Sorensenintroduced the concept referisrse very small concentrations of hydrogen ions. This is because the proponent of the concept of pH. It can be said in very basic terms, that substances capable of releasing hydrogen ions (H +) are acidic and capable of yielding hydroxyl groups (OH-) are basic or alkaline.
Soil pH is generally considered suitable for agriculture if it is between 6 and 7. In some soils, even with a natural pH of 8, can be obtained by good agricultural performance. However, such a threshold from crop production can significantly undermined. In most cases, high pH are indicators of the presence of soluble salts, so that would require resort to the use of crops adapted to saline environments. Similarly, a very acidic pH turns out to be another limiting factor for development of cultivars, which can be corrected by the use of amendments such as lime. Similarly, sometimes applied to sulfur compounds in order to raise the pH of strongly acid soils.
The pH of a soil is the result of multiple factors, which include:
Type of mineral present in soil
Weathering (of such minerals and containing the Roma mother)
Humification at large (decomposition of organic matter)
Nutrient dynamics between the solution and retained by the aggregates
Properties of soil aggregates and in particular what is called ion exchange
When one refers to soil pH, they usually do to solve the ground water at a given moment, but you’ll see that there are other types of estimates. Accordingly, it is estimated the active fraction of hydrogen ions [H+]. Based on the latter soils can be classified according to their degree of acidity in the following categories:
Very acid ? pH.
Acid ? 5.6 • Neutral ? 6.6> pH
Basic or slightly alkaline ?7.6> pH > 8.5
Very alkaline ? pH> 8.6
The conditions of acidity occur with greater frequency:
high rainfall regions
When the bases are displaced by hydrogen or captured by plants
Acid secretion by plant roots
acidic compounds formed in the decomposition of organic matter
young soils developed on highly acidic substrates
Air pollution resulting in the so-called acid rain
Drain some water or waterlogged soil rich in pyrite (acid sulphate soils), like mangroves Etc.
Therefore, in many countries acidity problems are very relevant in regard to agro-pastoral production. The opposite is true, often for alkaline soils. In other words, the conditions of alkalinity are given preference in:
In water-scarce regions (arid and semi-arid)
When the exchange complex (complex colloidal) is saturated with bases
Low soil biological activity (usually due to prolonged water deficits)
When certain circumstances produce cation mineral weathering and not washed or leached (for example, due to the aforementioned arid)
Endorheic basins where leached ions accumulate water draining there
Substrates poorly developed soils rich in salts
Poor water management in irrigation
Factors affecting the pH. Obviously one refers to those that influence the concentration of [H+] on the ground:
Production of CO2 passing H2CO3 to generate hydrogen (the atmosphere of the soil is usually much richer in carbon dioxide than it is on)
Presence in soil of organic acids of low molecular weight such as acetic, citric, oxalic, etc … (the waste of certain types of plants tend to have much to do)
Presence in soil of strong acids such as nitric and sulfuric acid released by microbial activity
Humus containing functional groups such carboxylic, phenolic, enolic, etc … (again the nature of plant residues that contribute to the ground are very important)
Abundance in the soil Fe and Al oxides, which can significantly alter acidic pH
Sales soluble acidic, basic or neutral, which accumulate in the soil either
Weathering of minerals in the soil environment
Mineralization (breakdown) of organic matter incorporated into the soil
Composition of irrigation waters (it is of utmost importance when it is not correct quality on the issue that concerns us here)
Addition of certain types of fertilizer
State oxide reduction in soil types or edafotaxa (ie. Degree of drainage, ponding of water)
As discussed, actually two types of pH can be estimated:
Current pH: It is the derivative of the concentration of [H+] in soil solution
pH change or potential acidity: Consider the concentration and ratio of hydrogen ions that are absorbed or retained by the exchange complex (mainly in soil aggregates).
The optimum pH range on vigorously growing most crops ranges from 6.0 to 7.0. That is talking about moderately acid or neutral soil. This is because that most of the nutrients for plants, present in the soil solution, are easily assimilated or absorbed by the roots in the aforementioned interval.
Finally, just say that soil pH influences the development of plants and vice versa, the acidity also partly the result of leaching and decomposition of litter and the soil biological activity. For example, cation exchange made by plant roots estimated decreases in soil pH, also influence the decomposition of humus, and the respiration of soil organisms.