Effects of Liming on Soil Physical Attributes: A Review

The objective of the present study was to gather information on the effects of liming on changes in soil physical attributes. Soil acidity, caused by natural ways, such as rain, weathering and decomposition of organic matter and by human interference, by the use of nitrogen fertilizer mainly ammonia and urea fertilizer contribute to the acidification of the soil. In this context liming is perform to correct soil pH and neutralize the effect of toxic elements. Numerous benefits of liming are known, but their influence on soil physical attributes is poorly studied. Liming directly affects some physical properties of the soil, such as flocculation, aggregates, density and porosity. Flocculation of soil particles initially is smallest, which promotes greater particle dispersion. However, it changes over time, since H and Al ions tend to be subsumed by Ca and Mg increasing particle flocculation power which favors its approximation and aggregate formation. For soil aggregates, surface liming in improves soil aggregation by increasing the mean aggregate diameter with positive responses as dose increases. Density and porosity of soil also undergo changes. It is important to point out that liming also has the potential to alter the porous structure of the soil. Porosity liming promotes soil density reduction and increase influenced reducing macroporosity values and increase of total porosity and micropores values. The results of studies carried out present divergent and similar results according to the evaluated physical attributes. This is possibly due to the dynamics between liming under different edaphoclimatic conditions.


Introduction
The great demand in food production worldwide demands more and more from soil management to obtain high agricultural productivity in order to ensure food security. The growing acidification of soils because of agricultural activity has become a major environmental concern in recent years (Kryzevicius et al., 2019). Faced with the necessity of intensive agricultural production and together with sustainable management it is necessary to build the soil fertility. This construction seeks chemical, physical and biological improvements.
In this context, the liming technique is a fundamental tool. It is a technique in which calcium and magnesium are applied, mainly carbonates, oxides, hydroxides or a mixture of these, in addition to silicates that can also be used, but less frequently. The objective of this technique is to reduce the protons concentration in acidic soils (Bortolanza & Klein, 2016;Miller et al., 1995). The liming requirement can be defined as the amount of corrective material necessary to obtain the maximum economic efficiency of a crop, which would mean having defined quantities of Ca 2+ and Mg 2+ available in soil and adequate pH conditions to have a good availability of nutrients (Dechen et al., 2006). Soils can be naturally acidic due to the poverty of the source material in calcium, magnesium, potassium and sodium, which are the exchangeable bases of soil or the intensity of weathering processes resulting in higher levels of hydrogen and aluminum in the soil exchange complex and, consequently, also in the soil solution (Sousa et al., 2018). However, the process of agricultural exploitation is also a factor that generates soil acidity, either through exportation, leaching of soil nutrients (exchangeable bases) and intensification of organic matter cycle in soil; in addition, the application of nitrogen fertilizers with an acidifying effect. This form of soil acidification is call anthropic influence (Raij, 2011).
The liming is an agricultural practice capable of altering chemical, physical and biological soil, providing a number of benefits. It promotes a decrease in soil acidity, insolubilization of toxic elements, mainly aluminum and manganese; increase calcium and magnesium levels and phosphorus and molybdenum availability (Ronquin, 2010). Besides, it favors the development of microorganisms present in soil and enables greater root system development, expanding the plants' capacity to absorb water and nutrients from soil.
However, in addition to changes in the chemical and biological attributes of soil, the practice of liming also results changes in the physical attributes. These effects depend on the interaction of several factor such as climate, soil class and intrinsic characteristics of each soil. Several researches have already studied the physical changes resulting from use of limestone demonstrating its influence on flocculation of soil particles (Spera et al., 2008), formation and stability of aggregates (Ferreira et al., 2019), density (Spera et al., 2008;Auler et al., 2017), porosity (Anikwe et al., 2016;Ferreira et al., 2019) and penetration resistance (Stenberg et al., 2000). The results of these changes reflects on other factors, presenting indirect effects: root development (Bomfim-Silva et al., 2019), microbiological activity (Stenberg et al., 2000;Børja & Nielsen, 2009), nutrients availability (Malavolta, 1980;Cahyono et al., 2019), crop yield (Zandoná et al., 2015) and content of organic matter of soil (Passos et al., 2019).
Due to the diversified dynamics that liming can present in different edaphoclimatic conditions, this review article aims to contribute with information about the liming effects on physical properties of soil.

Soil Acidity
Soil acidity is one of the main factors that limit the growth and productivity of many crops, especially in most of tropical and subtropical regions. In this context, it is important to understand its nature in order to perform the correct soil management. It should be considered different forms of acidity: active and potential.
The active acidity related to amount of H + dissolved in soil solution. The activity of this ion is express by pH, defined as a cologarithm of H + activity in solution. A correlation is observed between the concentration of H + in soil solution and its pH, the higher H + content the lower pH.
Potential acidity is related to soil resilience in having its pH changed when basic compounds are added. This is divided into exchangeable and non-exchangeable acidity. The exchangeable acidity refers to ions Al 3+ and H + retained on the surface of soil colloids. This amount of exchangeable H + is small. Therefore, only exchangeable Al 3+ is allow. The non-exchangeable acidity is the H + ion of covalent bond associated to negatively charged colloids and aluminum compounds. The acidity soils show when pH is below 5.5. Above pH 5.5, there is no more exchangeable Al 3+ .
The potential acidity is the sum of exchangeable and non-exchangeable acidity. It limits the roots growth and occupies spaces in colloids, allowing free nutrients in soil solution to be leached (Bissani et al., 2008).

Natural Causes
Some factors affecting soil pH such as climate, mineral content and soil texture cannot be changed. Soils may naturally have acidity depending on the source material and climatic conditions to which it is exposed. Generally, the acidification started or get emphasis from the removal of basic elements (Ca 2+ , Mg 2+ , K + and Na + ) of colloids soil combined with a reduce rate of release by weathering.
The in rainwater contributes to the input of H + ions, which occurs by the partial dissolution of carbonic gas from atmosphere: CO 2 + H 2 O ⇔ HCO 3 -+ H + , releasing HCO 3 and H + . In turn, H + releases an exchangeable cation in solid phase of soil. This exchangeable cation is leach with HCO 3 . This contributes to increasing the acidity of soil. The lower pH of soil the greater concentration of H + ions (Gliński et al., 2011;Bissani et al., 2008).
The decomposition of organic matter by microorganisms also supports the increase in soil acidity due to organic and inorganic acids formed during the decomposition of soil organic matter, plant residues and soil biota (Gliński et al., 2011;Bissani et al., 2008).
Another factor that contributes to acidification of soils is the weathering of minerals and the dissociation of organic acids (Van Breemen et al., 1983;Fujii et al., 2012).

Human Causes
In addition to environmental causes, human actions can also contribute to soil acidification. The application of nitrogen fertilizer contributes to soil acidification, since the use of ammoniacal fertilizer and urea requires a process of nitrogen transformation, called nitrification, which is carry out through the action of bacteria (Nitrosomonas; Nitrobacter). In this process, hydrogen ions are release, which contribute to reducing the soil pH. Another source of rising soil acidity is the occurrence of acid rain, resulting from anthropic interference. Due to the gases emission, mainly SO 2 and NO x from the combustion of fossil fuels, nitric and sulfuric acids are formed that reach the soil through precipitation (Gliński et al., 2011).

Limestone Reactions in Soil
After application, the limestone applied undergoes reactions in soil. The applied calcium and magnesium oxides, carbonates and silicates promote an increase in soil pH, by reducing acidity. A Ca 2+ ion from limestone replaces two H + ions at the exchange sites present in soil. Carbon dioxide (CO 2 ) can react with water and form carbon acid (H 2 CO 3 ). Protons are neutralized by hydroxyls OHand water is formed. The alteration in soil pH occur due to the transformation of hydrogen ions into water and carbon dioxide (CO 2 ) and insolubilization of aluminum since it is transform into hydroxide (Gliński et al., 2011) as can be observed through the reactions present below.
(a) Solubilization and dissociation of limestone: Neutralization of acids in soil: (c) Insolubilization of toxic aluminum exchangeable in soil: With the application of limestone, the exchangeable aluminum is neutralized, increasing base saturation (V%).
With the neutralization of part of adsorbed hydrogen, the pH of soil increases. In order to occur the limestone reaction, it must be mix well with the substrate, remaining in contact with all particles. In addition, it is important that soil is moist in order to solubilize the limestone (Ronquin, 2010).

Flocculation
The liming has ability to change the chemical composition of soil. Depending the dose used and its characteristics, it may change the physical aspects. Among the modified properties, the particle flocculation should be highlight since it is influence by the changes in concentration of ions present in soil.
Flocculation is the process that soil clay particles come together in stable granules. Unlike crystallization, where atoms or molecules form strong structures, in the flocculation the bonds between the particles are weak, due to the compounds that intermediate the bond between the molecules (Benoot et al., 2019).
Due to the flocculation process, the clays treated with limestone have different granulometric distributions, considerably reducing the amount of fine particles, as these interact with the limestone and flocculate, being able to aggregate into larger particles, which are reasonably stable even under subsequent flooding (Gupta et al., 2019).
The importance of lipotropic series must be highlight. The exchangeable elements in soil present different binding energies when it interacts with colloids according to the charge and the size of hydrated ion (Yamada and Roberts, 2005). The lyotropic series in a simplified way is a logical sequence, in which there is an interaction between element and soil, following a relationship from highest to lowest energy: H + >>>> Al +3 > Ca +2 > Mg +2 > NH +4 ~ K + > Na + .
After the solubilization and dissolution of limestone, occurs a precipitation of Al 3+ and neutralization of H + , in addition to an increase in the content of Ca 2+ causing alteration of lyotropic sequence. Therefore, different increases the content of soil organic matter and microbial activity. These factors favor soil aggregation (Corrêa et al., 2009). Changes in soil aggregation due to liming, in turn, modify other physical properties of soil, such as density and porosity.

Density
Soil density is defined as the relationship between the mass of a soil sample dried at 105 ºC and the sum of volumes occupied by particles and pores. Depending on the physical and chemical changes resulting from limestone use, density can also change.
According to Auler et al. (2017) by means of linear correlation for analysis for superficial application of limestone without overturning in relation to the control, in treatments on layer of 0 to 0.10 m, was detected significant interaction, demonstrating an increase in soil pH, clay dispersed in water, contents of Ca 2+ and Mg 2+ promoting a reduction in soil density.
In contrast, Spera et al. (2008)  According to Ribeiro et al. (2007), changes in density and porosity of soil are related. Therefore, it is important to note that liming also has a potential for altering the porosity of soil.

Porosity
The soil pores are represented by cavities of different sizes and shapes, determined by the arrangement of solid particles and constitute the volumetric fraction of soil occupied with air and solution (water and nutrients).
The quantity and quality of soil porosity represents great importance in the development of crops. This physical attribute interferes with aeration, infiltration, drainage and water retention and soil temperature, directly related to the development and productivity of crops (Hiel et al., 2016).
The effects of liming on soil porosity are also the subject of this review. According to Auler et al. (2017) the linear correlation analysis for surface application of limestone without inversion and the control, in layer of 0 to 0.10 m showed significant interaction. The pH increased in soil, dispersed clay in water and levels of Ca 2+ and Mg 2+ , as well as promote a reduction on soil density, also influenced by reducing values of macroporosity and an increase in values of total porosity and micropores.
Alternatively, Spera et al. (2008), when studying the effects of liming in columns with samples of a dystrophic Red Ferralsols observed that there was a difference in the dynamics between used limestone doses and macro and micropores. In addition, a difference was verified in the depths evaluated (0-0.05 m) and (0.015-0.020 m). According to the study, from the dose of 7.6 Mg ha -1 was observed stabilization of change on different physical attributes. In relation to micropores, the reduction was observed in both studied layers. However, in relation to macropores, a significant reduction was found in layer of 0.015-0.020 m, while in the layer of 0-0.05 m no changes were observed.
Liming might affect the soil structure in positive or negative ways (Haynes and Naidu 1998;Bronick and Lal, 2005). However, according to the studies shown in this review, liming normally improve some physical properties of soils, as porosity.
Soil acidity is one of the most important constraints in crop production in temperate as well as tropical regions of the world. Liming is an important practice to correct soil acidity in order to maintain or increase soil fertility within sustainable management in the context of current agriculture. The effects on the physical attributes of soil are diverse and, in some situations, present contradictory results according to the characteristics of each study. However, it is also observed significant lack of studies on the impact of setting the physical properties and structural soil condition. Future studies need to be conducted under different edaphoclimatic conditions in order to contribute with such information. In addition, it is important to consider recent changes in the rotations of the main crops. In general, the effects of liming can be explained by the flocculation and cementing action of Ca 2+ are probably the dominant mechanisms in the short term. In the long term, increases in productivity induced by liming, result in greater input of organic material, accumulation of organic matter and biological activity of soil, both favoring the stability of aggregates and improvement of other physical attributes.