Electronic Heavy Metal Test for Dry Soil

Many gardeners grow their vegetables in the ground without realizing the harmful effects of doing so. I've seen tons of Instructables and Wikihows about how to grow your vegetables in the ground. Has anyone bothered to test for heavy metals? Probably very few. Heavy metals in the soil have a big effect on your health, especially since those veggies are most likely going to end up on your dinner plate after being plucked. I enjoyed gardening in the ground for many years ever since I was 5 years old. I am 11 years old now. Two years ago, when I was 9 (this happened before my birthday that year), a painter came, painted the house, and dumped the remaining paint in his cans into the ground! That year and last year, I decided not to eat my veggies that I planted. Now let's go, and do some testing!

Supplies

1 voltmeter (or multimeter that can function as one)
1 NiCd or NiMH rechargeable cell
1 battery holder (I used an AA cell, an AA-D adapter, and a D-cell battery holder because that D-cell holder was the most suitable for this project.)
2 keys
Dry soil to be tested

Step 1: Assemble the Power Source

Put the rechargeable battery into its battery holder.

Step 2: Connect

Connect accordingly to the circuit diagram I made. If the voltmeter detects voltage, a metal (likely toxic) exists in the soil.

Step 3: Results

The first result is of the place where I get the soil in my compost from.
The second result is from my latest finished load of compost. (I was planning to use this in my raised bed garden this year, but now that I have test results, I am going to dispose of this stuff in a hazardous waste bag.)
The third result is of the place where I put my potted plants on top of.
The fourth result is of the area where I grew my vegetables last year.