Geochemical study to determine what elements are most influential in soil fertility in the southern part of Lampung region

Lampung is one of the best provinces in Indonesia in the agricultural sector. Research on agriculture in Lampung continues to develop over time. To expand land, research on agricultural land is needed. Agricultural soil contains chemical elements that support soil fertility. The aim of this research is to find out which chemical elements have the best correlation with soil fertility. The samples used were agricultural soil samples from the South Lampung and Pesawaran areas. The method used is a geochemical method with pH measurements and X-Ray Fluorescence testing. The results obtained were that the Mg element had a very strong correlation with soil fertility so that in the future the Mg element could be used as an indicator of soil fertility levels.


Introduction
Soil is an important growing medium for plants.Plants need nutrients in the soil for their growth.Soil that has good nutrition for plants is fertile soil.Fertile soil usually comes from the weathering of volcanic rocks.One area that has soil derived from weathering volcanic products is Lampung.Lampung in the south has two areas whose rocks are dominated by volcanic products, namely South Lampung with the Lampung Formation (QTl) which is dominated by volcanic tuff and Pesawaran with the Young Volcanic Deposit Formation (Qhvp) which is dominated by volcanic deposits.The soil in these two areas comes from weathering volcanic products so the soil is probably fertile soil.
Research that has been carried out to look at soil fertility is Santoso et al [1] conducting research on pH and magnetic susceptibility studies to look at the fertility level of agricultural land around the Sumatra Institute of Technology.This research found that the magnetic susceptibility value was correlated with the level of soil fertility.To continue this research, it is necessary to carry out further research, namely research on the relationship between fertility levels and chemical elements.Research on soil fertility levels is often carried out [2,3,4] but finding new indicators of chemical elements has never been carried out.

Geological setting
Geologically, the Pesawaran area around Mount Betung and South Lampung around ITERA have significant geological differences.The Pesawaran area around Mount Betung has Young Volcanic 2 Deposit Formations consisting of andesite-basalt, breccia and tuff.Meanwhile, the South Lampung area around ITERA has the Lampung Formation which consists of pumice tuff, rhyolitic tuff, tuff solid tuff, tuffaceous mudstone and tuffaceous sandstone.The tuff on Mount Betung is blackish grey which indicates a lot of iron content [5,6,7].Meanwhile, ITERA tuff is white which indicates a lot of silica content [8].The location of soil sampling and the geology of the research area can be seen in Figure 1.

Material and Methods
The samples were agricultural land in Lampung which consisted of 5 samples from the land around Mount Betung and 5 samples from the land around the Sumatra Institute of Technology.Samples were taken from a depth of 5 -25 cm.For details of soil sampling locations, see Table 1

Results and Discussion
The agricultural land classification used is based on [1], this classification has changed due to the addition of land suitability factors for plants [9].Based on Figure 2, it can be seen that the soil fertility level around ITERA is fertile to very fertile.Meanwhile, the level of soil fertility around Mount Betung is fertile.To strengthen the results of the pH classification, magnetic susceptibility values were measured, based on the magnetic susceptibility plot with frequency-based magnetic susceptibility, the results showed in Figure 3 that the soil fertility level of Mount Betung was fertile and the ITERA soil was between fertile to very fertile.This result is the same as the pH classification.
Soil contains macro and micro nutrients [10].Macro nutrients are nutrients needed in large quantities by plants for growth consist of K, Ca, Mg, S. Meanwhile, micronutrients are nutrients needed in small amounts by plants for growth consist of Fe, Cl, Mn, Mo, Zn.It can be seen in Figure 2 that the amount of macro nutrients is less than micro nutrients, therefore if macro nutrients run out it is necessary to fertilize.Micro nutrients from the soil around Mount Betung are more abundant than the soil around ITERA.This is because the original rock differs from soil weathering.The soil around Mount Betung comes from young volcanic deposits of andesite-basalt which contain many magnetic minerals.
Meanwhile, the soil around ITERA comes from rhyolite tuff which contains a lot of silica minerals.
The macro nutrients found in agricultural soil in the southern part of Lampung are K, Ca, Mg, S.
Meanwhile, the micro nutrients are Fe, Cl, Mn, Mo, Zn.These nutritional elements are correlated with pH to find out which elements are most correlated with soil fertility levels.The correlation can be seen in Figure 4.This correlation is based on previous research [11] which classified that the more neutral the pH, the less element is Mg.The more acidic or wet the soil is, the more nutrients it lacks.Previous research did not show the level of correlation of the elements, so this research was carried out.Based on Table 2, it is found that nutritional elements have a strong to very strong correlation with the level of soil fertility.Elements that have a very strong correlation with the level of soil fertility are elements that have an R value above 0.8 such as Mg, S, Ci, Mn, Mo, K. Meanwhile elements that have a strong correlation with the level of soil fertility are elements that have a correlation between 0 .6 to 0.8 such as Zn, Fe, Ca.The nutritional element that has the highest correlation is the element Mg.This element can later be recommended as an indicator of soil fertility levels.Analysis and control of soil pH becomes more important because this is the initial method that can be used to monitor soil fertility levels before checking the Mg element.

Conclusion
Based on geochemical studies carried out on nutritional elements in soil, it was found that nutritional elements have a strong to very strong correlation with the level of soil fertility.The nutritional element that has the highest correlation is the element Mg.This Mg element has the potential to be a new indicator in determining soil fertility levels especially on volcanic soil.

Figure 1 .
Figure1.Location and geology of the research area[5]

Figure 2 .
Figure 2. Fertility level of agricultural land around ITERA and Mount Betung.pH vs macro nutrients (top), pH vs micro nutrients (bottom).

Figure 3 .
Figure 3. Correlation plot between magnetic susceptibility values and frequency-dependent magnetic susceptibility percentages.Betung volcano soil (green circle), and ITERA soil (orange circle).

Figure 4 .
Figure 4. Correlation of pH with nutritional elements in the soil

Table 1 .
. Details of agricultural soil sampling locations The method used in this research is a geochemical method consisting of pH measurements and X-Ray Fluorescence (XRF) tests obtains the composition of nutritional elements such as macro nutrients (K, Ca, Mg, S) and micro nutrients (Fe, Cl, Mn, Mo, Zn).pH measurements are carried out directly in the field.Meanwhile, the XRF test was carried out at the Research Centre for Nanoscience and Nanotechnology (RCNN), Bandung Institute of Technology