**3. Soil geochemistry**

Volcanic glass, mica, and feldspar, as the main components of the rock source, are mainly submerged in clay minerals [34]. Based on XRD analysis, clay minerals are made up of kaolinite, dickite, nacrite, halloysite, illite, montmorillonite, and chlorite. Other minerals, including quartz, hematite, magnetite, and cristobalite, were present (**Figure 6**).

Kaolinite, dickite, nacrite, and halloysite are kaolinite groups {Al2Si2O5 (OH)4} [31]. Hunt and Yuan et al. [35, 36] noted that the occurrence of kaolinite and halloysite indicates soil extracted from volcanic rocks with a felsic composition. They described the forming climate at tropical temperatures, neutral to acidic pH conditions, free drainage, and porous rocks. Kaolinite and halloysite are secondary minerals commonly present in Andisols [37]. Both minerals are derived from feldspar and chlorite weathering [38]. Nacrite and dickite are clay minerals from the rarest group of kaolinites. The mineral is a transition from an illite mineral. Chen et al. [39] revealed that the presence of nacrite and dickite was associated with tuff deposition. These minerals are produced in the hydrothermal alteration environment.

The illite mineral {(K, H3O)(Al, Mg, Fe)2(Si, Al)4O10[(OH)2, (H2O)} is defined as clay-sized mica contained in clay rocks. Illite is a clay mineral formed in areas with mild climate characteristics or in mountain tropics. This kind of clay is made from weathering rocks rich in K and Al under high pH conditions. Generally,

#### *Tropical Volcanic Residual Soil DOI: http://dx.doi.org/10.5772/intechopen.98285*

#### **Figure 6.**

*Soil composition of mineralogy.*

these minerals are formed by weathering mica minerals such as muscovite and feldspar. Chlorite {(Mg, Fe)3 (Si, Al)4O10(OH)2 • (Mg, Fe)3 (OH)6} is a secondary mineral, typically found in igneous rock, formed from primary Fe-Mg minerals.

Montmorillonite {(Na; Ca)0:3(Al; Mg)2Si4O10(OH)2. NH2O} is a mineral subclass of the smectite group, formed from the intemperate mica, feldspar, and volcanic ash. The mechanism that acts on the shape of these minerals is the process of neoformation. This phase is typical in source rocks of volcanic glass composition [40, 41]. Ryan and Huertas [42] have proposed that the occurrence of kaolinitemontmorillonite in soil suggests that the soil parent material is andesitic that influenced by hydrothermal alteration.

Hematite (Fe2O3) is an iron (III) oxide mineral. Hematite minerals of the clay size can also occur as secondary minerals produced by soil weathering processes and iron oxides or other oxyhydroxides, such as goethite, responsible for the red color of the soil in the tropics [43].

Magnetite (Fe3O4) is a mineral and one of the three most common iron oxides in nature. This mineral has magnetic properties. Magnetite, black, a relatively common metallic mineral, is also one of the most important iron ores of modern society, occurring in various igneous, pegmatite, contact metamorphic rocks hydrothermal veins [44].

Cristobalite (SiO2) is the stable form of silica. This mineral is formed at very high temperatures in medium-acid volcanic rocks [45, 46]. Despujolsite {Ca3Mn4 + (SO4) 2 (OH) 6 • 3H2O} is a mineral formed in hydrothermal manganese deposits [47].

### **4. Characteristics**

Centered on Hardjowigeno [48]; Subardja et al. [49], the West Lampung Semi-Detailed Soil Map on a scale of 1:50,000 in 2016 released by the Agricultural Research and Development Agency, and WRB map (https://www.isric.org/explore/ wrb accessed on January, 01st, 2021) the research area are made up of Andosol-type soil. The soil is acidic, PH <6, CEC is medium - Low, storage capacity and water absorption are very high; N, P, K, Ca, Mo, Mg content and microbial activity lowvery low, and moderate to good drainage conditions/well-drained soils.

**Figure 7.** *Soil cracking.*

Based on observations in the field, generally, the soil in the research area has a brownish-red color, the characteristics of it loose when it is dry and sticky when it is wet, groove erosion can be observed, and it is easy to break off if there is additional water in the rainy season. The soil surface can be seen as the soil severely degraded with very large angular or platy aggregates and restricted pore space (see **Figures 3** and **7**).

Based on laboratory analysis (**Table 1**), the soil of the research area is included in the high plasticity silt type (MH) [50]; has characteristics low to medium plasticity characteristics (LL Brine 32.12% - 68.66%) [51, 52]. This soil has a specific gravity of 2.41–3.03. According to Bowles [53], the soil of the research area consists of mica and iron. Next, the soil has a wet unit weight of 1.27 g/cm3 –2.15 g/cm3 and porosity 35.25% to 67.00%, based on de Castro et al. [54] these characteristics belonged to the soil which has clayey silt and uniform grain size, and inorganic soil type. The soil has a liquid limit >50%, has a plasticity index>17%. Prakash and Jain [55] categorize this type of soil as soil with high plasticity. The permeability of this soil is 4.63E-11 to 5.54E-05 m/s (soil with small to sufficient permeability) [56]. The engineering characteristics of this soil are cohesion values of 0.008 kg/cm2 to 1,675 kg/ cm2 and internal shear angles 18.36o to 39.26o . These values represent that the soil of the research area is in a very loose to solid state/good and slide hazard [57].
