Gather different types of soil. Put some of each type in a cup and check out the pH. Then add water to the cups, and mix. Wait for the soil to settle and measure the pH of the water. Be sure you use water from the same source for each soil. Find out more about soil.
Fill boxes with different types of soils and plant the same crop in all the boxes.
You could measure height, width, number of leaves, how fast the plants grow, number of flowers, or yield of seeds or fruits. But water running over soil can cause it to wear away, or erode. A simple experiment in soil erosion . Here's how to test for vitamin C content. Iron is sprayed on the outside of cereal flakes.