A very common occurrence in Brazilian lands is the serious problems with soil acidity. This problem occurs mainly in tropical and subtropical environments, which characterize most of our country's climate.
In Brazil, about 70% of arable land is considered acidic, having low pH, low cation exchange capacity and commonly high concentrations of toxic aluminum, which can harm plant development.
For this reason, it is worth raising the discussion on some of the aspects related to soil acidity, its causes and especially the strategies to control and correct this serious problem in our agriculture.
Understand what soil acidity is:
By definition, all acid, basically, is a substance that in aqueous media releases hydrogen ions (H +). In the case of agricultural activity, Tales Tiecher, professor at UFRGS explains that soil acidity is divided into two main compartments, active acidity and potential acidity, which are in balance in the soil.
According to the professor, the active acidity is the concentration of the hydronium ion (H3O +, but generally referred to only as H +) present in the soil solution. It is estimated from the pH of the soil equilibrium solution with H2O or with saline solution diluted with CaCl2 or KCl, which works as a “thermometer” of the soil acidity.
However, this cannot be used to indicate the dose of corrective to be applied, as it is infinitely less than the potential acidity. “To correct the active acidity, only a few kilograms per hectare of limestone are needed, while to correct the potential acidity, a few tons of limestone are usually needed,” he explains.
Therefore, the potential acidity, which is the sum of H + and Al3 + present in the solid phase, is the compartment indicated by the teacher that defines the limestone dose necessary to increase the pH of the soil.
According to the professor, there are situations in which two soils have identical pH values, but require totally different amounts of corrective material due to the difference in potential acidity.
"More buffered soils with a higher content of clay and organic material require higher doses of corrective to reach the same pH compared to sandy soils and with a low content of organic matter", he says.
Main causes of soil acidity:
The natural source of acidity in tropical and subtropical soils in Brazil is rainwater. For this same reason, in arid and semi-arid regions there is a predominance of soils with a higher pH, tending to alkalinity.
“With the action of rain for many years, the dissolution of some minerals releases Al3 + in the soil solution, which in turn hydrolyzes water molecules and generates more acidity, in a cascade effect, resulting in naturally acidic soils with high levels of Al, ”explains Tiecher.
According to the UFRGS professor, soil acidification can also result from agricultural activities. “Nitrogen fertilization has great potential for acidifying the soil when N is not absorbed by plants and lost through leaching,” he says.
In addition, crops export large amounts of Ca and Mg in grains, fibers and other agricultural products, increasing the acidification rate of the soil proportional to the amount produced and exported.
Finally, it is also important to note that legumes tend to acidify the soil more than grasses, as they absorb more cations and excrete more H + in the rhizosphere.
Impacts of soil acidity and strategies to correct this problem
Soil acidity acts as a limiting factor for agricultural production on many occasions. Thus, the main limiting factor for soil acidity is the phytotoxicity of Al3 +, which exists in the soil only at a pH below 5.5.
But in addition, the availability of various essential nutrients for plants is also affected by the pH of the soil. “Acid soils have less availability of P, K, S, Ca, and Mg. But soils with very high pH have low availability of cationic micronutrients such as Fe, Mn, Cu and Zn ”, says the professor.
Therefore, the ideal range for most crops (with some exceptions) is between 5.5 and 6.5, where Al3 + does not exist and a good availability of all nutrients is observed.
It is also important to highlight that the acidity of the soil directly affects the symbiotic N fixation in soy, the main crop grown in Brazil, a fact that can compromise the productivity and sustainability of this crop.
For this reason, the correction of soil acidity, through soil analysis, can be done using any material that when solubilizing generates hydroxyls or consumes protons in the soil solution.
“The main product used for this purpose is agricultural limestone, due to its low cost-benefit and availability”, guides Tiecher.
It is important to highlight that neutral salts such as agricultural plaster do not alter the pH, that is, they do not correct the acidity of the soil, but they can benefit the crops by increasing the Ca content and decreasing the saturation by Al.
The dose to be applied can be calculated in different ways, depending on the official recommendations of each region of Brazil.
In RS and SC, the dose is defined according to the SMP-Index, which is directly related to the potential acidity of the soil (H + Al) which defines the required dose of acidity corrector. In other states / regions, the calculation is made based on the desired base saturation.
"As liming increases both pH and base saturation, both methods can be used to correct the acidity of the soil", says the professor.
However, the relationship between pH and base saturation may not be linear, or it may be different for each type of soil depending on its mineralogy. For this reason, the best method for calculating the need for correction will always be the one calibrated for each region.