**Table 7.**

*Content of various heavy metals in rice fields in Sidoarjo, East Java [9].*

*c [58].*

The relationship between soil Pb and time follows a logarithmic pattern, where the content increases with land use. The mathematical equation for soil Pb content as a function of time is Y = 11.88 + 2.02 Ln (x) and R2 = 0.956, where Y = Pb content in soils (mg kg−1) and x = age of land use (year).

### **5.2 Cd content in soil**

The soil Cd analysis results based on paddy fields can be seen in **Table 8**. The table shows that the highest and lowest Cd content was found in 80- and 20-yearold rice fields, namely, 0.72 and 0.26 mg kg−1. Compared with 20-year-old paddy fields, the highest soil Cd increase was in 80-year-old fields by 1340%. Pradika et al. [64] also reported that P fertilization could add Cd metal to agricultural land. This is because the raw material for making P fertilizer comes from phosphate rock, which naturally contains Cd metal. The concentrations in surface soils range from 0.06 to 1.10 mg kg−1 with an average of 0.41 mg kg−1 [65]. Satpathy et al. [19] reported that Cd content in Indian paddy soils ranged from 0.02 to 0.6 mg kg−1.

The relationship between soil Cd content and time follows an exponential pattern, where the content increases with the age of land use. The mathematical equations as a function of time are Ln (Y) = 0.17 + 0.02 x and R2 = 0.913, where Y = Cd content in soil (mg kg−1) and x = age of land use (year). The presence of heavy metals Pb and Cd in the soil can be caused by the intensive use of P fertilizer and exceeding the recommended dose. Heavy metals Pb and Cd can increase in line with the age of paddy fields.
