Clay is one of the smallest rock-bit types found in soil, and soil scientists know that those tiny particles attract and hold plant nutrients. This experiment makes that invisible attraction visible. You mix clayey soil into a jar of water and let the suspended particles settle for ten minutes. Then you connect two copper wires to a 6-volt lantern battery and lower the bare ends into the murky water. After 10 to 15 minutes, one wire comes out with clay clinging to it while the other stays mostly clean. Which terminal attracted the clay reveals whether the particles carry a positive or negative charge — and helps explain how soil composition affects a plant's access to nutrients.

Soil composition varies depending on where you collect samples, and farming practices can shift that composition significantly. You gather surface samples from three sites — a forest, a conventionally farmed field, and a field treated with compost — and observe structure, color, texture, and earthworm activity in the field. Back in the lab, you dry and sieve each sample, then test pH, nitrogen, phosphorus, and potassium. The forested soil comes out darkest with the most earthworm activity. The conventionally farmed soil is lighter, shows surface crusting, and registers higher nitrogen from added fertilizer. The differences across these three sites map directly onto how farming changes the rock bits, organic matter, and biological life that make up the soil.

Soil is a mix of rock bits, dead plant matter, water, and air. Organic material in soil changes how much water the mix holds. You test different ratios of sand and peat moss to measure water absorption.