Heavy Metals in Indonesian Paddy Soils

*Dedik Budianta, Adipati Napoleon and Nanthi Bolan*

### **Abstract**

Long-term cultivation of paddy soils has resulted in Pb and Cd accumulation that exceeds the WHO tolerance levels of 2 mg kg−1 and 0.24 mg kg−1 in food. In Musi Rawas, South Sumatra, Indonesia, the paddy soils with the greatest levels of Pb and Cd were those that had been intensively farmed for 80 years, reaching the concentrations of 20.56 mg kg−1 Pb and 0.72 mg kg−1 Cd for soil, and 3.11 mg kg−1 Pb and 0.29 mg kg−1 Cd for rice. The lowest concentrations were obtained with 20 years of cultivation at 17.82 mg kg−1 and 0.26 mg kg−1, for Pb and Cd in soils, respectively. The Pb content in the paddy fields in Pati, Central Java, ranged from 0.23 to 2.55 mg kg−1, while the Pb content in the lowland watershed of Solo Hilir ranged from 0.20 to 2.94 mg kg−1. The highest concentration of Pb and Cd in rice was found at 80 years old in paddy soils with the value of 3.11 mg kg−1 and 0.29 mg kg−1, respectively. The lowest concentrations were found at 20 years old of soils with a value of 2.35 mg kg−1 Pb and 0.15 mg kg−1 Cd, respectively.

**Keywords:** cadmium, intensive farming, lead, paddy soil, P fertilizer, rice intensification

### **1. Introduction**

Rice is a staple food that globally provides calories to more than 3.5 billion people. It has contributed almost 19% of global human per capita energy and 13% of per capita protein [1]. Paddy soils used for rice growth are contained by embankments, called galengan in Indonesia, or canal to hold water. Indonesian farmers have per capita paddy fields of only <0.5 ha, which decreases over time due to population growth and conversion to non-agricultural activities. The total area of rice fields in Indonesia is around 7,483,948 ha [2]. The average production is around 7–8 tons ha−1 when the soil is relatively fertile. However, when the soil is less fertile, rice production is very low, below 4 tons ha−1. The fertility of paddy soils has decreased, as indicated by the decreasing availability of macroand micro-nutrients, low organic matter content, and slightly low pH (**Table 1**) [3, 4].

The fertility of paddy fields continuously decreases with the time of land use due to harvest and irrigation.

According to **Table 2**, paddy soils in Musi Rawas, South Sumatra, have a pH ranging from 5.40 to 5.56 with a low organic C content of 1.75–1.85%, moderate to low soil CEC ranging from 15.31 to 19.58 cmol(+)kg−1, moderately available P between 14.10 and 20.80 mg kg−1, medium K-exchangeable of 0.58 cmol(+)kg−1, exchangeable Na between 0.33 and 0.70 cmol(+)kg−1, exchangeable Ca ranging from 2.10 to 6.48


### **Table 1.**

*Some characteristics of paddy soils in Sidoarjo (East Java) near the industrial area [3].*


### **Table 2.**

*Characteristics of soils based on the age of use of paddy soils [5].*

cmol(+)kg−1, and low exchangeable Mg of 0.35–0.68 cmol(+)kg−1. The low organic C content of 1.75 to 1.85% can increase the solubility of Pb and Cd in paddy soil while increasing the uptake by plant roots. Organic matter is vital as a regulating agent for heavy metal mobility in the soil [6]. Furthermore, Pb and Cd can form complex and chelate compounds with organic materials [7]. The complex form is a reaction between metal ions and ligands through electron pairs [8]. Paddy soils have low pH ranging from 5.56 to 5.4. The high soil acidity or low pH can increase the solubility of Pb and Cd in the soil with the uptake by plant roots [9]. Soil acidity is an essential factor that determines metal transformation and controls the chemical properties of Pb and Cd and other processes in the soil. The decrease in pH increases the availability of heavy metals except for Mo and Se. At low pH, the availability of Pb and Cd increases, and the more acidic the soils, the greater the heavy metal affects the rice [1, 10]. To increase the fertility of paddy soils, farmers intensively apply inorganic fertilizers such as urea, SP-36/TSP, and KCl. These inorganic fertilizers are essential to provide adequate nutrients for crop growth and ensure successful harvests [11]. This is supported by the data in **Table 3**, which indicate that the average lowland rice

*Heavy Metals in Indonesian Paddy Soils DOI: http://dx.doi.org/10.5772/intechopen.109027*


### **Table 3.**

*Average fertilizer usage in five sub-districts of Musi Rawas Regency, Indonesia [5].*

farmer uses around 150.26 kg ha−1 P fertilizer in each growing season, exceeding the recommended dose of 100 kg ha−1 [5].

Paddy soil is not a typical soil classification term but indicates how to manage various soil types for rice cultivation. There are four paddy soil ecosystems: (a) flood-prone rice ecosystem, characterized by a flat to slightly wavy or basin surface; it is flooded due to high tides for more than 10 consecutive days as deep as 50–300 cm during plant growth; (b) aerobic to anaerobic and rice cultivation is carried out by transferring or spreading seeds on dry plowed soil; (c) rainfed lowland rice ecosystem, characterized by a flat to the slightly wavy land surface, bordered by bunds, and inundated due to discontinuous tides with varying depths and periods; and (d) aerobic–anaerobic soil alternating with varying frequency and period, where rice planting is carried out by transferring seeds to silted soil [12]. Intensive management of paddy soils in the long term can reduce soil productivity and environmental quality. High inputs of agrochemicals can deplete nutrients in the soil and cause negative impacts in the form of increased residues of materials. Additionally, consumer demands for food or agricultural products that are safe and hygienic, have a high nutritional value, and are free of contamination are a public concern for the quality of the environment and human health [13]. Furthermore, [13] reported that around 21–40% of paddy soils in the Pantura of West Java were contaminated with these two types of heavy metals; even 4–7% of them were contaminated in the heavy metals category, which was Pb > 1.0 mg kg−1 and Cd > 0.24 mg kg−1.

### **2. Heavy metals**

Various sources and causes of contamination of paddy fields that can lead to soil degradation include agrochemicals, industrial waste, mining activities, and household waste. The two sources of heavy metals are natural and anthropogenic [1]. The use of synthetic fertilizers (inorganic fertilizers) and industrial activities play an important role as a source of pollution in rice fields [14]. There are many reports of contamination of rice fields, especially areas adjacent to factories [15–17]. The amount of waste generated from industrial processes causes water sources to be polluted. Furthermore, materials consisting of toxic compounds can settle in the rice soil. This process is repeated over time, accumulating these materials and heavy metals in the

soil. Therefore, there will be undesirable changes in the physical, chemical, and biological properties of the soil. Productivity decreases with the ability to support plant growth [18]. Heavy metal contents in agricultural soils can directly affect human health by consuming crops grown in contaminated soils [17]. These metals are nonessential elements but can accumulate in plants and adversely affect human health [19]. Contaminated soil adversely affects the whole ecosystem when these toxic metals migrate into groundwater or are taken up by plants, which may threaten ecosystems [20]. In general, the metals are accumulated mostly in the root compared to the stem, leaf, and grain [1]. The occurrence of these metals in paddy field soils ranks in the order Mn > Zn > Pb > Cr > Cu > Cd [19]. Heavy metals are potentially toxic to crop plants, animals, and humans when contaminated soils are used for crop production [21]. Environmental contamination of the biosphere due to intensive agricultural and other anthropogenic activities poses severe problems for the safe use of agricultural land [22]. Heavy metals such as Cd and Pb are of primary concern in soil and food contamination because of their toxicity, particularly in the rice cropping system [23]. These toxic elements accumulate in the soils, contaminating the food chain, endangering the ecosystem's safety, and causing soil degradation.

Degraded soil will have properties that do not support rice growth. It will lose the topsoil or arable layer, lose nutrients needed by rice plants, and result in reduced levels of organic carbon. In addition to these observable characteristics of degraded soils, it can also be distinguished by plants that typically do not thrive in such conditions. The performance of plants is reduced when planted in soil with degraded physical, chemical, and biological qualities. The parameters used to evaluate the level of soil degradation are decreasing base saturation, available nutrients including N, P, K and trace elements, bulk density, soil permeability, and organic carbon [24].

Soil properties influence rice growth and development. The characteristics supporting plant growth should be maintained, one of which is soil conservation measures to prevent chemical damage/degradation. Degraded soil can also lose the top layer, impacting the loss of nutrients needed by plants, changes in soil structure, and reduced levels of organic carbon. The organic carbon has a major role in improving the physical, chemical, and biological properties of the soil [25]. It can also be identified by using plants with poor growth performance. In this regard, the plant can be used as an indicator of soil degradation. Many definitions of soil degradation have been reported, showing a decrease in soil chemical properties compared to nondegraded soil. Land degradation results from one or more processes that decrease the actual or potential ability to produce food and fiber and provide ecosystem services. This definition shows a general understanding of agriculture's broad scope [26]. Land or chemical degradation is often associated with a use that does not follow the aspects of the balance of inputs and outputs. Inputs are related to soil improvement or fertilization in cultivation activities. In contrast, the output is associated with plant nutrient uptake and the possibility of leaching through erosion mechanisms. The phenomenon of land degradation is found in areas of land that promote agricultural activities. Land degradation can be indicated by symptoms of poor plant growth or the growth of weeds on the soil. The marginalization will continue with low inputs for farming and dry land management technology, which ultimately causes physical and chemical degradation. On sloping land, land degradation will occur quickly due to erosion, which reduces the quality of the physical and chemical properties of the soil. Consequently, the soil will be damaged or degraded due to acidification, accumulation of salts (salinization), and contamination of heavy metals, organic compounds, and xenobiotics such as pesticides or oil spills.
