**5. Liming and gypsum: Chemical soil management aimed at mitigating toxic aluminum and improving the water status of plants**

Agricultural production can be limited by physical and chemical factors in the soil, which reduce root growth and restrict the uptake of mineral nutrients and water. In this context, toxic Al plays an important role in tropical regions where soil pH is not corrected because growth, development, and, consequently, agricultural production are negatively affected. This effect on plants is aggravated when there are dry spells or periods of water deficiency, an increasingly recurrent phenomenon in the context of global climate change.

The water conduction mechanisms in the soil-plant system represented by hydraulic conductivity and stomatal conductance are affected by the increase in the cellular concentration of abscisic acid induced by Al toxicity [31]. In addition, cellular and structural damage to the root system caused by toxic Al reduces cell turgor [49] and decreases plant root growth [31]. The negative effects of toxic Al on plants are potentiated by the action of water deficiency which, together, reduce plant growth [7].

A soil chemical management strategy aimed at neutralizing toxic Al consists of the adoption of liming based on soil chemical analysis. Liming raises the pH and reduces Al availability in the layers where limestone is applied [50], increases the availability of calcium and magnesium in the soil, and the assimilation of nutrients such as nitrogen, phosphorus, potassium, and sulfur by plants [51]. The increase in pH occurs through the exchange of H+ for Ca2+ in the soil colloids and its neutralization (**Figure 6**). Similarly, the Al3+ is exchanged for Ca2+ in the soil colloids forming a little toxic compound (AlOH3) that rushes into the soil (**Figure 7**).

Another strategy to neutralize toxic Al, but in subsurface soil layers, is plastering. Gypsum renders Al insoluble causing it to be leached into deep soil layers. In addition, *Toxic Aluminum and Water Deficit Interaction in Plants: Physiological Aspects and Chemical… DOI: http://dx.doi.org/10.5772/intechopen.111418*

#### **Figure 6.**

*Effect of limestone on the neutralization of soil acidity. Ca2+ displaces H+ from soil colloids, neutralizing it and increasing soil pH. Source: Author.*

#### **Figure 7.**

*Chemical mechanism of action of limestone in reducing soil aluminum saturation. Ca2+ displaces Al3+ from soil colloids, reducing its solubilization and causing precipitation. Source: Author.*

gypsum improves the physical and chemical quality of the soil, creating better conditions for root development and growth. These changes promoted by gypsum nullify the effects of Al toxicity, increase root growth and attenuate the effects of water deficiency in plants [52]. These effects of gypsum indicate that the practice of gypsum can mitigate the effects of global climate change on water deficiency because there is greater root growth of plants in depth in the soil. Agricultural gypsum provides calcium and sulfur that improve soil fertility. In addition, the gypsum insolubilizes toxic aluminium, allowing its leaching to the subsurface layers of the soil (**Figure 8**).

#### **Figure 8.**

*Gypsum action on subsurface toxic aluminium. Gypsum supplies calcium and sulfur to the soil. Ca2+ from gypsum displaces Al3+. This will form with sulfate ions a poorly soluble compound, Al2(SO4), which is leached to deeper layers of the soil. Source: Author.*

The benefit of gypsum is related to the greater root growth of plants. For example, the mean percentage distribution of the root system of sugarcane plants in the 0–20 cm, 20–40 cm, and 40–60 cm layers after gypsum soil application was 51.80, 29.72, and 18.64%, respectively. Furthermore, gypsum shows a positive effect on the root density of sugarcane plants [53]. Gypsum also increases the percentage of water absorbed by the root system at depths greater than 40 cm (**Figure 9**) because gypsum improves the physical and chemical attributes of the soil, allowing greater root growth in depth [53]. Gypsum improves the physical properties of the soil because it has a flocculating action on soil particles. This favors the aggregation of clay, reducing its dispersion. Thus, there is greater soil porosity, increased permeability, and water retention capacity in the soil. These changes promoted by gypsum create a favorable environment for the root growth of crops.

The interaction between limestone and gypsum in the soil shows a positive effect on root growth and agricultural production. For example, the application of gypsum and limestone in the soil shows positive effects on root growth because it improves the relative root distribution in depth [53], in addition to increasing the production of soybeans by 11.4% in conditions of water deficit [54]. Under conditions of water deficit, gypsum is shown to be efficient in increasing grain production in grasses such as maize and wheat [55].

In an oat cultivation area, liming and plastering increase grain production under water deficit [50]. These positive effects of gypsum, together or not with liming, result in a better edaphic environment that favors greater root growth with a positive impact on the absorption of water and mineral nutrients, two essential factors for plant production. Thus, the mechanism of action of gypsum and limestone in the soil suggests that these two agricultural inputs can be used to mitigate the effects of global *Toxic Aluminum and Water Deficit Interaction in Plants: Physiological Aspects and Chemical… DOI: http://dx.doi.org/10.5772/intechopen.111418*

#### **Figure 9.**

*Root growth and percentage of water use by plants in response to the presence or absence of gypsum. In the presence of gypsum, there is greater root growth in depth and, therefore, better use of water by the root system up to 100 cm deep in the soil. Source: Figure adapted from Sousa et al. [54].*

climate change, particularly in tropical regions where the natural acidity of the soil is a limiting factor for plant growth and agricultural production [52].

### **6. Conclusion**

In the context of global climate change, water deficiency is an increasingly frequent phenomenon in many regions of the world, directly impacting plant and animal production, which have water as a vital input. In tropical regions in particular, water deficiency is particularly serious because in these regions many soils are acidic and with high Aluminum saturation. This metal compromises plant growth and production because its target is the root system, the organ responsible for absorbing nutrients and water from the soil. Thus, water deficiency and Al toxicity together potentiate the reduction of plant growth and production, with considerable social and economic impacts. The increasingly frequent periods of water deficiency restrict soil water availability, inducing less water absorption and cellular water influx. These events culminate in the loss of cell turgor, lower cell expansion, and, consequently, lower plant growth with a negative impact on gas exchange, and mineral and antioxidant metabolism. These effects of water deficiency are potentiated by the action of toxic Al present in the soil because the primary site of Al action is the root system. Al reduces the fluidity of cell membranes, affecting their capacities related to the absorption of mineral nutrients and water, inducing nutritional and water deficiency in plants. Although the genetic improvement of plants aimed at tolerance to Al toxicity is an important tool to increase agricultural productivity in Al-affected soils, it occurs slowly and in few areas. A strategy that can be adopted to overcome the problem of acidity and toxic Al is the use of gypsum and agricultural limestone as soil amendments. Together, the use of gypsum and limestone reduces toxic Al in depth

and surface, increases soil pH, and thus creates an edaphic environment favorable to greater root growth in volume and depth. Thus, crops will be more tolerant to periods of water deficit in soils treated with gypsum and limestone. However, the physiological and biochemical mechanisms of plant responses to liming and soil gypsum need further studies, since these two agricultural inputs, when applied to the soil, improve the absorption mechanisms of mineral nutrients and water, gas exchange, and the production of cultures.
